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February 28, 2007

HMT Forecast Wed 2/28/2007

The very long-legged IOP7 is dwindling down to showers today. Guidance suggests that showers up there will continue all day and night, maybe even tomorrow, but no model is predicting more than .15 inch in any 6-hr period.

The 12Z GSF model run develops a very nice atmospheric river that arrives at the ARB on Wednesday, March 7. It's stand-down weather until then.

(AWIPS users: go to NH scale, go to volume browser, select grid=GFS360, field=sfc/2d->misc->precipitable h2o, plane=LYRS->layer. Load as image to enable read-cursor.)

Paul Schultz

February 27, 2007

Long Range Discussion Graphics

Some graphics to accompany Ed Berry's long-range discussion can be obtained at:


Also, Ed's "Atmospheric Insights" blog can be found at:


HMT Forecast Tue 2/27/2007

Kind of strange to be in IOP status with clear sky over Sacramento. From the CalTrans traffic cameras it looks vaguely cloudy to the east, looking toward the ARB, and the satellite imagery supports that, but the DAX radar and the models don't see precipitation going on up there now.

But IOP7 picks up again late this afternoon, let's say 01Z/5pm. We get about two tenths per 6h all night, then that backs off to about one tenth per 6h during the day tomorrow (Wednesday). Total precipitation should be .5-.6 inches, all of it snow up yonder, and this is cold enough to be fluffy dendritic snow with unusually large accumulations, maybe a foot or more in some places. Throughout all this the snow level stays very steady and very low, perhaps down to 1200 ft overnight, maybe up to 1500 during the day tomorrow.

After that it looks like IOP weather is some days away. Ed Berry, who gives occasional discussions on the long-range situation, reviewed the MJO context and concluded that a break in the IOP action of about a week is the forecast.

Paul Schultz

February 26, 2007

HMT Forecast Mon 2/26/2007

I have to hand it to the GFS, it has been doing a very good, very consistent job at forecasting IOP7 since I came on duty last Thursday. I've made nothing but minor changes to the amounts, timing, or snow levels on this event since then.

So: another .7 inch of precip between now and 00Z Tuesday, with rain/snow levels mostly 3400-3600 ft. Then the cold front goes through about sundown, the precipitation increases to .7 inches per 6h all night long starting at 00Z or so, and the snow level drips markedly to below 2000 ft by sunrise, maybe as low as 1200 ft in some places. Precipitation stops right about sunrise and lays off for 12 hrs or so, picking up atgin Tuesday evening around 00Z. Then it looks like pretty healthy precipitation overnight Wednesday into Thursday, maybe another inch overnight, with very low snow levels for the entire time. Note that the GFS starts the precip a few hours earlier than the NAM, ends it sooner, and produces a bit more precip.

Paul Schultz

February 25, 2007

HMT Forecast Sun 2/25/2007

IOP7 is alive and well. Current precipitation rates of .5-.7 in/6hr will taper off after sundown to about half that for about 18 hours through the day tomorrow (Monday). The rain/snow line stays pretty steady at 3500 ft or so through this period, which will produce about an additional inch of precip from 00Z-18Z Monday. Precipitation lightens briefly then shortly before sundown the cold front goes through, the precipitation rates kick up to over .5 in/6hr for 12-18 hrs, while the rain/snow line descends to below 2000 ft. NAM and GFS are in good agreement, and have been, about the first part, but they differ in timing and amount after the cold front. I'll split the difference and predict 1.4 inches between overnight Monday into Tuesday.

Both models taper off the precipitation in the ARB Tuesday afternoon, but after that is in some question. The NAM generates another inch of precipitation in the cold air (sub 530 thickness) from 00Z-18Z Wednesday, but the GFS produces less than half that and ends it by sunrise Wednesday. I prefer the GFS solution because that model seems to be a little better lately, but that's all I'm going on.

Paul Schultz

February 24, 2007

HMT Forecast Sat 2/24/2007

Here is the predicted liquid equivalent precipitation and elevation of the rain/snow line, based mostly on NAM and GFS 12Z runs. (Sorry I couldn't look at more, my basement flooded and I'm darn glad to get done what I did.)

Sun 00Z, T, 3800 ft
Sun 12Z, .80", 3600 ft
Mon 00Z, 1.2", 3400 ft
Mon 12Z, .6", 3400 ft
Tue 00Z, .8", 3400 ft

Until this point we've had onshore flow characterized by good precipitable water and little baroclinic structure. Model thickness hovers around 540 in the general area, but precipitation cooling in this regime causes a 3 kdm depression for precip rates over .5/12hr. I'll be interested to see what this does to the rain/snow line.

Sometime after sundown Monday the cold front goes through, there's nice dynamic lift enhancing the orographic effects, and the rain/snow line really plunges.

Tue 12Z, 1.5", 2300 ft
Wed 00Z, T, 1800 ft
Wed 12Z, T, 1500 ft

After this there is lingering precipitation and cold air, but not IOP weather for a few days. Next chance: early next week.

Paul Schultz

February 23, 2007

HMT Forecast Fri 2/23/2007

The trough responsible for yesterday's IOP6 is leaving behind some lingering snow showers. We decided yesterday to terminate the IOP before midnight based on non-zero but light QPF in the ARB, which is pretty much how it turned out. Hope we didn't miss anything too interesting.

Between Friday's land-falling trough and another one expected on Tuesday there will be almost 48 hours of fairly steady onshore flow with little baroclinic structure to it, resulting in continuous precipitation, sometimes moderate. The flow is at least 30 degrees rotated to the northwest relative to the ideal trajectory for the ARB, and the precipitable water values are healthy, around 1 inch, but not spectacular. Still, accumulations of 1.5 inches and up, most of it during the daylight hours Sunday, will be typical. During this event the 1000-500 mb thickness remains remarkably constant at around 540 dkm, fluctuating 2 dkm or so during the event. (Might be useful to note that after the precip starts, there's evidence of precip cooling keeping the thicknesses that low while points east and west are several dkm higher.) This implies a rain/snow line fluctuating around 3500-4000 ft MSL.

There might be a break in the action for 6-12 hours beginning Monday morning, but the precip probably won't shut off entirely.

Then the trough axis comes in, most ensemble members have the precip picking up in earnest at sundown Monday, and precip amounts around two inches over the next 18 hours or so, ending mid-day Tuesday. Snow levels fall steadily during the event from around 3500 ft MSL to around 2000 ft.

After that there's light precip again starting late Wednesday but at this point it does not look worthy of an IOP.

Paul Schultz

February 22, 2007

HMT Forecast Thu 2/22/2007

Precipitation associated with the current IOP tapers off to one or two tenths per 6h after sundown Thursday, although that light precipitation rate persists throughout the day Friday. The rain/snow line starts today around 4000 ft and descends to 1200 ft or so by sunrise tomorrow/Friday, climbing back up to 2000 ft or so during the day. The precipitation shuts down for good before sundown Friday.

After the current trough pushes through the ARB area today there will be a a 24h period without precipitation, from 00Z Saturday to 00Z Sunday. Then a two-day period (00Z Sun - 00Z Tue) of somewhat direct onshore flow of moderate strength sets up, feeding precipitation rates of remarkably consistent .3-.5 inch per 6h. This is marginal IOP weather by the standards of last winter. The rain/snow line hovers around 1500-2000 ft during this period. This precedes a trough passage that dumps over two inches in the 18h ending 00Z, with snow getting below 1000 ft, and then terminates this precipitation event. This would be IOP weather for sure.

Paul Schultz

February 21, 2007

HMT Forecast 2/21/2007

Radar and satellite shows light showers from mid-level clouds have so far
stayed N of the ARB. The enhanced cloud band remains in NWRN CA at the present
time. Today's 12Z GFS run is showing more digging of the trough and a somewhat
slower solution with minimal pre-frontal precipitation of just 0.1" over
the central ARB by 12Z Thu 22 Feb. The precip then intensifies with a total
of about 1.3" by 03Z Fri. After that we get about .2" additional by 00Z Sat.
The trough maintains a fairly sharp open wave configuration. The PW plume
reflects this by coming in from the SSW with about 1" coming onshore.

The NAM80 has light precip moving in before 00Z Thu and the heavier precip
picking up by 09Z Thu with similar totals as the GFS. HPC is going for a max
precip of 2.5" over the ARB.

850 temps are about 0.5C at 18Z Thu suggesting a snow level of 5000' for the
heaviest precip. By 12Z Fri the 850 temp is -4C suggesting snow levels
dropping to 1000-2000'.

The weekend system is close behind with an open wave passing mainly to the
N of the ARB. There is some precip starting in the GFS at 15Z Sat Feb 24
and totaling 0.6" by 12Z Sun. This system features a PacJet extending back
to about 165W. The PW plume is around .7" coming onshore.

The PacJet sets up a good fetch ahead of the following system that looks
more significant. The GFS gives about 2.6" mainly on Tue Feb 27. GFS Ensemble
members have some timing variation while the ECMWF moves it faster and digs
the trough more. The GFS PW plume looks more impressive with amounts exceeding
1" at SFO.

Steve Albers

February 20, 2007

HMT Forecast 2/20/2007

Today's GFS looks on track for the IOP beginning 08Z Wednesday Feb 21. Some
light orographic precip ahead of the main front should gradually be working
down over the ARB from the northern Sierra between 00Z and 12Z Wed. The heavier
frontal precipitation picks up around 03Z Thu Feb 22. The GFS yields about
1.6" up until a lull in the action on 02Z Fri Feb 23.

The NAM is similar in timing and intensity with a start time of light precip
03Z Wed and the heavier precip picking up around 04Z Thu Feb 22. The 06Z
WRF-NMM highlights the light precip event commencing over the NRN ARB by 00Z
Wed with a max of 1.0" over the NRN ARB by 16Z Wed. The WRF-NMM is consistent
with the other models bringing in the main frontal precip to the WRN ARB
by 02Z Thu. By 06Z Fri the WRF-NMM has widespread areas of 2-3" of precip
with pockets of 5" over the NRN ARB. Snow levels look to be dropping from
5000' to 3000' during the main precip event. The PW plume in the GFS looks
respectable at almost an inch 12Z Thu connecting to the tropics.

The second colder wave cuts off SW of the ARB limiting precip from this event
to just around 0.1" between 06Z Fri and 00Z Sat Feb 24. This cutoff moves
SSE from SFO at about 13Z Fri, so any precipitation after that would have
to battle some NVA and low-level downslope winds. Snow levels could make it
down to 1000' or so early Friday morning.

The weekend system is still below IOP criteria with the GFS having 0.6" on
Sun Feb 25, some increase from yesterday's 12Z run.

The following system for Wed Feb 28 bears watching as it could be strong enough
for a marginal IOP. Confidence is low at this point on the GFS ensembles.

Steve Albers

February 19, 2007

HMT Forecast 2/19/2007

Yesterday's system exited the ARB earlier this morning with LAPS analyses
showing storm total precip ranging from trace amounts up to about 0.35" over
the SE ARB. LAPS is predominantly a radar estimate and the ALERT gauge reports
were generally 0.00 or 0.01 inch in the area.

Today's GFS 12Z analysis shows a positively tilted trough around 150W with some
polar reinforcements located W and N of Alaska, respectively. The GFS shows
an onset time slightly earlier than yesterday's run. Some light precip over
the ARB could begin as early as 23Z Tue Feb 20 and is supported by a heavier
precip max already on the coast at the CA/OR border. This heavier precip moves
in over the ARB around 00Z Thu 22 Feb totaling 1.7" before the first wave
moves east around 00Z Fri 23 Feb. After a lull of up to 6 hours, the second
colder system gives the ARB about 0.4" of precip during the 18 hour period
ending 00Z Sat 24 Feb. The cutting off of the second system just S of the
ARB may shut off the upslope helping to limit precipitation on Friday.

The GFS ensemble as well as the NAM80 have a start time about 06Z Wed Feb 21
with heavier precip commencing just after 00Z Thu Feb 22. The GFS ensemble
gives a total of about 1.3" both systems. Today's 06Z WRF-NMM run shows light
precip beginning over the NRN ARB by 03Z Wed Feb 21. Even this "light" precip
could bring up to 1" over the NRN ARB Sierra crest prior to 00Z Thu in the
WRF-NMM solution. By 06Z Thu the WRF-NMM shows 3" max precip amounts over
the NRN ARB. I suspect subsequent runs will show more as the precip works
southward towards the end of the model integration.

GFS 850mb temps gradually drop during this event with 2C 18Z Wed Feb 21,
1C at 12Z Thu, -1C by 18Z Thu, and -3.5C 12Z Fri. Corresponding snow levels
would be around 6000' on Wed, 4000' Thu, and 2000' Fri.

Thus an IOP is suggested for Wednesday beginning by around 03Z if it is
desired to catch most of the initial precipitation. Since the initial precip
will be light the IOP is actually being called to start at 08Z Wed Feb 21.
Between the two systems the IOP should continue until 00Z Saturday though
Friday's precipitation is somewhat lighter and colder.

Followup system for the weekend still looks to pass N of the ARB with just
a few hundredths of an inch of precip showing up in the GFS run on Sun 25 Feb.

After a break early next week another system bears watching for Wed Feb 28.

Steve Albers

February 18, 2007

HMT Forecast 2/18/2007

LAPS precipitation analysis as of 17Z shows precip so far up to 0.1" in the
SE part of the ARB with lighter amounts to the NW. ESRL WRF-ARW runs all have
a bulls-eye of precip just SW of Lake Tahoe averaging in the 0.5-1.0" range
with the event ending around 03Z Mon Feb 19.

For the mid-week system we see a trough currently near the date line continuing
to progress eastward that starts to phase in at 48hrs (in today's GFS) with a
reinforcing polar wave that starts out near 80N. The 12Z GFS continues to
show an onset time of 06Z Wed Feb 21 with the heaviest precip for the first
impulse between 00Z and 12Z Thu Feb 22, yielding a healthy total of 2.8".
There is a lull Thu afternoon with the second surge picking up about 00Z Fri
giving another 1.0" and ending about 18Z Friday as a developing cutoff low
moves over the ARB from the NW.

The NAM/WRF run has the leading edge of precip on the NRN CA coast by 00Z Wed
Feb 21, so extrapolating beyond that sounds consistent with the GFS. The ECMWF
looks similar for the start of the event but shuts things off earlier in the
day Fri Feb 23. The GFS ensemble mean has a similar start time with lighter
precip amounts compared with the deterministic run.

The PW plume is fairly narrow but well positioned with amounts near 1" coming
into the coast. HPC analogs continue to suggest 3-5" storm total precip. GFS
thicknesses are about 541dm at the onset of the first wave, about 536dm at
the height of the event and dropping to 528dm as the first shot of precip is
tapering off around 12Z Thu Feb 22. Thicknesses average about 528-530dm during
the second event with a drop to about 520dm near the end. Typical 850mb temps
are around 0C during the first event and -2C during the second. Snow levels
should be around 4-5Kft during the first event dropping as low as 3Kft during
the second. We continue to recommend an IOP for Wednesday Feb 21 extending
until the first part of Friday Feb 23.

The following system for Sun 25 Feb still looks somewhat weaker and more of
a WA/OR event. West coast ridging continues to be indicated for the last few
days of Feb.

Steve Albers

February 17, 2007

HMT Forecast 2/17/2007

Today's 12Z GFS run has this weekend's precip event starting around 18Z 18 Feb
(Sunday) as the wave axis comes onshore in NRN CA. Light precip continues
until about 12Z 19 Feb (Monday) as the wave begins to cutoff right over the
ARB. Amounts appear lighter compared with yesterday's run with totals just
about .07". The system continues to cutoff diving south passing the LA area
about 21Z 19 Feb.

The NAM80 grids show even less precip with just a few hundredths. We do have
the WRF-NMM grids showing a basin average to be pretty light with yet some
isolated spots having >0.5" precip on the crest of the Sierra. Overall it
looks like a good decision to forego an IOP on this system.

The mid-late week system currently near 170E is hanging together pretty well
and features two impulses reaching the ARB in the GFS run. Initial precip
could begin as early as 06Z 21 Feb (Wed) with a fairly decent jet coming in
ahead of the developing trough. 6hr precip amounts show a 0.86" bulls-eye
just S of the ARB from 06-12Z 22 Feb (Thu). The first shot brings in about
2" total with a break in the action occuring about 15Z 22 Feb (Thu). Then
the secondary impulse brings in about 0.35" ending about 15Z 23 Feb (Fri).
A moderately strong though narrow PW plume is shown in the GFS extending to
a point N of Hawaii.

The 00Z ECMWF run continues to suggest a slight delay that could prolong the
overall event a little further into Friday. HPC mentions that analog cases show
amounts could total 3-4" in the Wed-Fri period. So an IOP could be indicated
to start anytime Wednesday depending on whether it is desired to capture the
initial light precip before it intensifies later in the day. GFS thicknesses
start out at about 543dm and fall to 528dm dropping the snow levels
significantly below 5000ft by the end of the event.

The third system we've been tracking still passes mainly to the N of the ARB
with a light-moderate precip event peaking at about 06Z 25 Feb (Sun). This
shows a decent PW plume in the GFS and bears watching in case it digs further
south as the 00Z ECMWF run is hinting. Being this far out there is some
spread in the GFS ensemble.

After the third system some west coast ridging could occur during the last few
days of Feb as suggested by the GFS deterministic run and ensemble.

Steve Albers

February 16, 2007

HMT Forecast 2/16/2007

The progressive pattern continues with the wave for this weekend being pretty
much on track in the GFS with precip over the ARB starting 15Z 18 Feb (Sunday).
This system splits a bit with some energy going up through Idaho and the
southern branch digging southward into SRN CA. This helps to limit the precip
total to about .2". The digging aspect of the trough does slow the system a bit
so the end of the event looks to be about 12Z 19 Feb (Monday). The GFS shows
a very weak PW plume. The ECMWF has similar timing to the GFS.

The WRF-NMM run shows the onset time to be a bit later at 22Z 18 Feb and still
in the trailing stages at the end of the model run at 12Z 19 Feb. Precip
amounts vary across the ARB from nothing in the eastern portion to the
0.2"-0.5" range along the crest of the Sierra.

The NAM80-WRF data also shows this later onset of 20Z 18 Feb with 6-hour precip
totals of around 0.2". The run ends at 00Z 19 Feb so past this point the total
could be somewhat more than 0.2". Overall this first system seems to be below
IOP criteria.

The second system for later next week is stronger and has a current position
of 165E. In the GFS forecast, the same splitting tendency limits the precip
amounts to about 0.9" in the GFS. Onset time is consistent with yesterday's
run at about 02Z Thu 22 Feb, lasting until about 06Z Fri 23 Feb. The GFS
PW plume is weak to moderate in intensity. The ECMWF model timing is about
12-18 hours slower with a similar structure to the trough. This may merit an

A third system with a somewhat more zonal character may hit the WA/OR coast
on 00Z Sun Feb 25 with some precip extending as far south as the ARB.

Steve Albers

HMT Forecast 2/15/2007

Fairly progressive pattern showing up in today's 12Z GFS run. We're on track for
a marginal event this weekend with a fairly sharp open wave digging southward
a bit as it approaches the NRN CA coast. Precip onset over the ARB is progged
about 15Z 18 Feb (Sunday) and ending about 03Z 19 Feb (Monday). The GFS run
gives about .25" as a representative total precip amount over the ARB.

A PW plume exists though it is rather weak and narrow with this system.
The GFS ensemble mean is similar with about .2" of precip ove the ARB.
Most ensemble members keep an open wave, though one member has a weak cutoff
developing about 100mi S of the ARB. This could introduce some uncertainty
in how much upslope winds would be expected.

The second larger event continues to look possible later next week with a
fairly sharp full latitude trough. The most concentrated area of precip is
progged to be over SRN CA yet with more than 1" of precip over the ARB.
Onset time is about 05Z Thu 22 Feb. This event could last over 36 hours counting
precipitation coming down with reinforcing shots of energy down the back side
of the trough. This system still has a fairly weak plume of PW though it is
wider than what we had with the previous trough.

Confidence looks good for the overall appearance and timing of the
second system looking at a GFS Dprog/Dt. One thing to consider from the GFS
ensemble is that if any tendency towards the formation of a cut-off low occurs
this could weaken the upslope, conversely a more open wave solution would
strengthen the upslope component.

Steve Albers

February 14, 2007

HMT Forecast 2/14/07

The outlook for any significant weather in the near term remains nil. However, next week has two promising events which might provide grounds for another IOP.

This morning the Pacific high pressure is located at 33N 140 W and directs tropical plumes from a good source at 140E 10N. This is aligned with OR just off the CONUS coast as we have been following over that past few days in these discussions. Today the plume brings some moisture to the ARB by Friday 16utc. A weak atmospheric river with poor roots to the tropics develops at 12 utc 16 Feb and is reinforced by a second moist plume sweeping down from the N. These give rise to a weak precip episode now forecast by the GFS to produce a total of 0.27 inches between 18utc 2/18 and 06 utc 2/20 (last night's run had this 2x higher, about 0.54 inches). Later next week there appears to be a better chance of an event.

By 18 utc 2/21 a better moist tropical flow elongates and sweeps by the ARB. This is oriented E-W and extends back to a very high moisture plume centered over 170 W. The GFS appears to work this into a precipitation event that totals about 2 inches of precip beginning 06 utc Thurs 2/22 and ending around 06utc 2/24 Saturday.

Both events though not ideal atmospheric moist plume events do bear watching with time, especially the second one.

This morning's ensembles underscore the above model indications. The 2/18 storm indicated at 18ut initiation has a high probability. About 50% of the ensembles put the freezing line over the ARB at the onset. After this the second event described above has a better than 50% chance of occurring between 2/22 00ut and should be over by 2/24 12utc. Beyond that there is a low chance (30-40%) of an event about 2/26 and one that might carry higher probability near March 1. These ensemble probabilities have been fairly consistent over the past several days. Beyond March 1, there is poor consensus in the ensembles at this time.

To summarize, I still see the ARB remaining dry until this Sunday that might begin a weak precip event that may not reach IOP criteria. However, I now have more confidence in another possible IOP criteria event occurring about 2/22 (Thursday, a week from tomorrow).

Dan Birkenheuer - ESRL/GSD

February 13, 2007

HMT Forecast 2/13/07

Again, the immediate threat of precipitation over the ARB is minimal. This discussion focuses on the GFS moisture plume forecasts and the GFS ensembles looking for the next possible event, far down the road.

This morning's NAM produces only 0.04 inches of liquid equivalent in the ARB by 00utc 2/17 (Saturday) (total run accumulation). The GFS produces 0.03 inches by 00ut 2/14 and then additional amounts totaling 0.1 inches by 06utc 2/20 (Tuesday of next week, again total run accumulation). So the immediate forecasts are for a mostly dry ARB.

As opposed to yesterday the plume off the Oregon coast is now driven more N to Juneau, AK by 2/18. Later in the run a low spins up at 40N 130W and drives a good part of a plume into S CA. This corresponds to increased ensemble precip probabilities for S CA in this timeframe but only moderate chances exist that this will affect the ARB to any extent. After this the E Pacific remains dry. Monday 2/26 00utc a plume brushes by the coast but does not make it far inland. A more intensive intrusion inland but by a weaker surge of moisture is associated with a potential event on 3/1. It should be noted that NONE of these scenarios appear to be good atmospheric river events.

The ensembles have a low chance of precip 2/18 12utc and given the model forecasts I would anticipate this to be nothing but light showers. A 50% chance is forecast associated with the moisture surge S of the ARB on or about 2/22 12utc through 2/24 00utc. This bears watching especially if the trajectory of the moisture shifts to the N. The potential event on 3/1 appears to approach the area from the N and might have greater chances of producing more than 1.0 inches of precip, but it is too early to tell.

The bottom line is that at least up until Monday 2/19, I expect no precipitation events to reach IOP criteria.

Dan Birkenheuer - ESRL/GSD

February 12, 2007

HMT Forecast 2/12/07

With the end of IOP 5 last evening I will focus on the outlook for the next possible event that looks to be well off into the future.

This morning a large Pacific ridge sits with a central high pressure at 143W 35N and continues to drive moisture plumes over its top. One main plume 150E has merged with another swept north in the E flow at 160W. This moisture will not play a role in the immediate future but may in about a week's time.

Meanwhile a rogue piece of disconnected moisture enters the ARB by 12ut on the 13th. The main moisture remains far offshore west of 130W. This bit of moisture might be responsible for light precip in the ARB forecast area by both the NAM and GFS, progged to occur early this week, but both models keep the total precip well under one inch through 12utc Sunday 2/18.

Meanwhile, the high pressure ridge progresses to 135W 35N on or about 16 Feb teasing the CA coast with some additional moisture but it appears to remain west of the ARB with the main plume intersecting the OR coast and then being driven NE away from CA with time.

Another surge is forecast to brush the coast in the 18utc 2/18 (Sunday) timeframe and this might be associated with the higher ensemble probabilities for precip at about this time.

Ensemble outlook:

Examining the GFS ensembles this morning we see a virtually dry ARB until a 75% chance of precip on or around Sunday 12utc 2/18 through 12utc Tuesday of next week. Then another possible precipitation event occurs around the 24th (Saturday). It is too early to tell whether the upcoming potential Sunday event will be associated with a good upper level moisture river from the tropics. We will keep an eye on this possibility throughout this week.

Dan Birkenheuer - ESRL/GSD

February 11, 2007

HMT Forecast 2/11/07

This very good atmospheric river event is finally coming to a close. This morning's reports are steady rain in Blue Canyon at 36F, 34F at Truckee with moderate snow, 50's at lower elevations mostly cloudy in the ARB with rain still on the coastal areas. The current atmospheric river has moved south of CA and is now impinging on Baja, however lingering moisture remains over the ARB and central CA valley. Out in the Pacific a high pressure center at 33N 155W is now directing the moisture from 2 plume sources and that moisture is just now cresting the ridge. This might be the source of the potential precipitation that we have been watching for later this week.

Near term forecast:


Precipitation in both the GFS and NAM end about the same time, only today the NAM appears to stop the precipitation a bit sooner than yesterday's GFS. The NAM has 0.1 inches liquid equivalent every three hours up until 3utc 2/12 after which precipitation appears scattered. The GFS on the other hand has the event ending somewhere in the same timeframe but reporting 0.01 inches in the 6 h prior to 06utc 2/12. The GFS ensembles on the other hand show an end to the event after 00utc 2/12. So it appears that the precipitation should wind down for sure around 03utc. After this the area goes dry for an extended period that will be discussed in the outlook below.


500 hPa winds remain fairly constant out of the W to SW at 30-50 knots up to and beyond the end of the precipitation with no noted trof passage at upper levels until well past the event at 15 utc 2/13 (Tuesday). Surface winds also remain fairly constant out of the SW at 25 knots up until about 12 utc 2/12 when they back to the S and lighten to 10 knots. This also corresponds to the time when the freezing level stops dropping and might indicate the arrival of the cooler air behind a weak front. As opposed to earlier forecasts that showed a definite fropa, it now appears that this event just sort of runs out of gas.

Freezing Level:

Freezing level begins this morning at about 6600ft which drops to mountain top levels for the duration of the precipitation. Then about 09utc 2/12 it drops to 4600 ft associated with the arrival of cooler air behind the front. This is after the precipitation is indicated to have stopped.


By 2/14 00utc the moisture coming over the ridge center at 30N 145W shows a new high pressure center spinning up at a more favorable location at 35N 130W with moisture making the coast but not inland. By the 15th 18utc 1.8 cm TPW is sown over the ARB with the main plume never making it to shore due to a low at 45N 155W that develops and redirects the main moisture back out to the Pacific. Though the plume "skinnies out" it appears to progress east with time and might eventually make it to the ARB sometime near 2/17.

On the other hand the GFS ensembles show that the current system ends for sure by 00utc 2/12 with 100% chance of precip in the 12h before that time but zero afterward. After that point, the ensembles indicate that the there is virtually no chance of precipitation over the ARB until a possible 50% chance on the 19th at 12utc and a higher (70% chance) on the 20th at 12utc. After this, weak probabilities are indicated for the 23rd and 26th.

Based on these progs, I would forecast that there will be an extensive dry period after the current event ends today. I would call 03utc 2/12 the end of this event.

Dan Birkenheuer - ESRL/GSD

February 10, 2007

HMT Forecast 2/10/07

The event continues as prior forecasts indicated with steady but variable precipitation over the ARB. Today I will mainly focus on guidance looking at the timing of this event's end and prospects of a next possible event.

This morning the progs are consistent with the forecasts seen to date with the exception of maybe a bit warmer air reaching the ARB followed by a short cold burst about 15utc 2/11. The precip is fairly consistent 0.2 to 0.4 inches per 3h up until the temperature swing when there appears to be a pick up in precip intensity to 0.5-0.6 inches in 3h. This occurs prior to 12utc 2/11 (Sunday). Precipitation totals from 12utc this morning until the end of the event appear to be around 3.2 inches liquid equivalent. This essentially means this event produces more moisture overall than originally forecast a few days ago. It appears that frontal passage occurs at 00utc 2/12 (Monday) with just dwindling precipitation by 06utc 2/12. This ends the event somewhat later than the GFS that has it ending about 19utc 2/11.


As prior forecasts, winds at 500 hPa remain westerly 40-50 knots until fropa. Surface winds remain mostly S at 20knots falling to 15 knots with a veering to SW about 00-09utc 2/11 and this appears to be coupled with the pick up in precipitation intensity and the possible shot of warm advection.

Freezing level:

Freezing level begins at mountain top until a warm surge lifts the level to 8800ft at 9utc 2/11. It quickly drops to 6200 ft by 12utc and then further but briefly to 4600 ft at 15utc 2/11. This timing corresponds with an increase in precip rate. After this time it lifts to 6200 ft and remains there until about the end of the event at which time it falls to 5500 ft likely in sync with the cooler air behind the weak front.


The threat of precipitation mentioned yesterday for the 14th does not look so promising today. This will mainly be controlled by the off-shore high pressure directing the moisture plume to the CA coast. Currently the pressure center is forecast to be mainly stationary at about 30N 140W and driving moisture plume near the CA coast from the NW but not getting it on shore. It is very close and some of the ensembles do give a chance of precip in the ARB but they are in the minority. Furthermore, it is disconcerting that this ridge appears to retrograde with time moving the moisture further away from the coast. The only other possibility is some uncertainty in how broad this ridge spreads with time. If it broadens enough, the moisture might make it to the CA area. I think this is the mechanism for the few ensemble members that bring in some moisture. It is unlikely that this would produce over an inch of moisture in any case.

After the current episode, the ensembles look weaker than yesterday for any event out until about the 22nd, as they showed yesterday. So for this forecast I would anticipate nothing of great concern after the current event finishes that would warrant personnel staying deployed.

Dan Birkenheuer - ESRL/GSD

Firehose image


I've attached a picture of the 24 hour accumulation valid at 12 UTC 11 Feb (tomorrow) from the 12 UTC 10 Feb 3-km NMM. The feature of interest is the precipitation streak in the shading of the run-total precipitation from the coast line right up into the North Fork ARB. You can see what appears to be a stream of hourly amounts (the green contours that are aligned along the streak in the shading), maybe call it the firehouse, which is a term I heard at the California conference last June. Large accumulations occur along the Sierra ridges as the firehose moves southward and northward along the Sierras. This sort of detail is lost in the 6-hourly accumulation fields. The effect can be seen in the 06 UTC and 12 UTC NMM forecasts from today.

A couple of notes regarding the mesoscale model outages.

(1) As Wes discovered, the LAPS webpage hosting our graphics from the mesoscale models does not work on Internet Explorer 7.0. It may work for versions prior to IE6.

(2) The mesoscale model outages have two root causes, one that is out of our hands and one that arises from an analysis problem that we discovered during this year's HMT. The first type of outage occurs when our ftp connection to NCEP is problematic and fails to provide us with the complete NAM A212 grids. The second type is a problem in the LAPS initial condition that we are diagnosing and fixing. The LAPS analysis error only affects the NMM simulation. It will not effect the ARW forecasts. The ensemble products can be generated without the NMM. Though the reliability has been confirmed only for the full ensemble, our initial analysis of the ensemble products generated without the NMM has shown only small differences in the ensemble probabilities.

February 9, 2007

HMT Forecast 2/09/07

Things are similar to yesterday but turned around a bit. The observations (GPS) of TPW are higher than yesterday with 2.6cm at both Sloughhouse and Bodega Bay and 0.8cm @ Truckee while the modeled plume is about the same as yesterday, so there seems to be more agreement between the model and the measurements. The area received precipitation on and off overnight and this morning low level surface temps were in the 50's and Truckee was reporting 37F at 16ut. Fog and light drizzle reports were numerous. Radar echoes had been mainly north of the area but a new radar surge looked to be lined up pretty well with the area ARB and will reach the area soon.

Today the models are more in line with each other than yesterday but opposite in other ways. To summarize, the NAM now shows a potential lull in activity (yesterday it was the GFS) around 06utc Sunday while the GFS today has fairly consistent precipitation throughout the weekend. Light amounts of precip in the area are progged to pick up to about 0.2 inches in 3h by 03ut Saturday (tonight). Precipitation remains fairly constant to the end of the episode. The NAM has this ending by 06utc Monday 2/12 and the GFS a bit later (12utc 2/12). Other than the lull in the NAM both models produce comparable precipitation. The NAM renders 2.8-3.0 liq eq. The GFS 2-3.5 inches liq. eq. As far as snow totals, they differ a bit more with the NAM producing 37-39 inches at high terrain and 2-3 inches in the valley. The GFS produces 30 inches in the higher terrain with 1-2 inches in the valley. The high res HMT model runs today have 3.5 inches of liquid in higher terrain (and 1 inch @ lower elevations) by 18utc 2/10. All models seem to put greater amounts north of the ARB however, the satellite shows upstream moisture to be aimed directly at the ARB at the current time.


Winds at 500 hPa remain SW 30-45 knots for most of the episode. Surface winds on mid-slope of the Sierras appears to range from the S to SSE 20-35 knots up until 18utc 2/11 (Sunday) when they go more SW and lighten to 20-15 knots. The upper level trof axis appears to pass the region about 21utc 2/10 (Saturday). Low level winds might show a weak frontal passage about 18utc 2/11, but this is a minor wind feature. As will be seen however, there is a drop in temperature about the same time.

Freezing Level:

Today we begin with a freezing level about 7200 ft that drops to 6200 ft at 18utc today 2/9. It then goes to mountain top level for most of the episode with some minor deviations, rising to 8800 ft 12utc 2/10 then dropping back to mountain top level. By 6utc 2/11 it falls back to 6200ft and stays there until the apparent surface front reaches the region about 18utc 2/11 when it drops to about 5000ft.


This mornings GFS plume forecasts show this to be the best water vapor "river" that we will see for the next week or more and it is actually connected to the tropics. There are some minor surges of moisture reaching the region with no tropical connection later next week, one about the 16th and another perhaps around the 18th. Ensemble GFS forecasts agree with this scenario with a minor chance of precip around the 14th, a slightly more significant one near the 16 and 18th with what looks to be the best system around the 22nd.

Dan Birkenheuer - ESRL/GSD

February 8, 2007

HMT Forecast 2/08/07

Sloughhouse RH has increased to about saturation this morning, Bodega Bay rose to saturation and fell, while Truckee had remained over 80% in the last 12-24 hours. Surface observations and radar show the temperatures this morning are in the upper 40s at lower elevations and about 38F at Truckee after the passage of a few weak showers overnight. Much fog is reported in the area and GPS sites are reporting between 1.9 cm near the coast to 1.5 cm TPW in the ARB region. SSMI imagery shows a moisture plume remaining off-shore associated with a second low spinning up over the E Pacific at about 135W 45N. A more minor plume is associated with the current weather over the ARB with integrated moisture amounts of 2.2 to 2.4 cm TPW as modeled by GFS, a bit higher than what we are observing. The GFS models a moist plume max (about 3.0 cm) at 06 utc Friday 2/9. This slides south but leaves the lower terrain of the ARB with increased moisture until 06ut Monday 2/12. After that, a second moist plume reaches the ARB on or about 06utc 2/15.

This morning the NAM has the following:
Winds: 500 hpa winds remain SW through the event between 30-45 knots until backing more southerly about 21utc Sunday. This is in association with a weak low pressure (almost cut-off) passing north of the area. 700 hPa winds remains about the same direction as 500 with a more pronounced backing as the end of the event nears. Winds on the orographic slope of the Sierra's appear to remain southerly 15-25 knots throughout the entire precip episode.
Precip: Precipitation for this episode remains about 0.3 inches (liq eq.) every three hours with a possible max at 12 utc 2/9 (Friday). The precip core remains to the north of the ARB with the best 3h second surge at 03 utc 2/10 (Sunday) when we get about 0.6 inches at higher terrain and 0.2 inches in the lower parts of the domain in a 3-hr period. Snow totals for the episode appear to render 45 inches at the higher terrain with about 1 inch in the valley.
Freezing level: Stays at mountain top for most of the episode. It drops to about 850 hPa about 3utc 2/11.

The GFS shows a similar forecast:
Winds: This mornings GFS is very similar to the NAM with W-SW winds at 500 at about the same speed until late in the episode, however they veer NW by 12 utc Monday 2/12 with a speed about 25 knots.
Precip: The GFS is not as generous as the NAM in amounts but shows about the same scenario with a few exceptions. Total precip amounts about range about 2.5 inches liq. eq. and 17-20 inches of snow at higher elevations with perhaps more snow (2 inches) in the valley region. Unlike the NAM which keeps the precipitation steady throughout the entire run, the GFS puts a lull in the precip between 09-18 utc Saturday 2/10. Similar to the NAM the GFS has the core of the precipitation north of the ARB for this event.
Freezing level: Again is very similar to the NAM with mountain top level dropping to about 850 hPa late in the event about 00utc Monday 2/12.

The current precipitation event should run until late Sunday evening/early Monday when things tend to wind down. It appears there might be another precipitation event on or about 2/15.

The long range ensembles show favorable precipitation probabilities through early morning Monday with scattered amounts after that time as the episode concludes. The next precip event seems to show a high likelihood of occurring on 2/15. After this, a closed-low event appears to tease the region with precipitation around 2/18.

Dan Birkenheuer - ESRL/GSD

February 7, 2007


SYNOPSIS: Still anticipating 3-5" of pcpn at Blue Canyon (KBLU) between now and midday Sunday 11 Feb, when the welcome wet spell that began this morning is anticipated to be about ended.

Precipitation that began in the ARB this morning continues, although intensity is not heavy at this time. This is ahead of the cold front associated with the first system anticipated yesterday to give pcpn to the ARB today. The surface low-pressure center (~992 mb) well off the OR coast is a bit deeper and northwest of where it was anticipated to be located this time yda. The cold front appears to be within 100km of Cape Mendocino at the moment, and oriented NNE-SSW at the latitude of the ARB. This front is expected to traverse the coast ranges late this afternoon and early this evening and clear the ARB by 06-07Z Th, that is by 11pm PST. John McGinley reports that the WRF-LAPS ensembles from 12Z this mrng are indicating a 60% chance of > 1" pcpn and 20% chc greater than 2" (I think this was for KBLU). I am inclined to be a little more pessimistic on QPF, settling for .75 to 1" as the most likely amount for KBLU by 12Z Th 8 Feb. Snow levels may rise a little this afternoon as the air mass moistens up, but then will drop to 4,500ft or so after tonight's frontal passage.

The next wave is estimated near 34N/147W currently, based on 6.7 micron water-vapor imagery, and progressing E-ENE. It has a large cold cloud shield ahead of it, strongly suggestive of the warm advection property the progs have been indicating it would have when it reaches the coast. There are also 2 ship reports with SE winds of 35 and 40kts at 18Z, supporting the notion that this will be a stronger feature than the one now affecting the ARB. The surface low with this is forecast to be a little deeper and a bit farther NW than the GFS high-res "deterministic" run of 12Z yesterday, with the warm advection a bit weaker over ARB. Nevertheless, I think this still has promise of being a significant event, with 1-2" at KBLU, beginning late Th aftn or early evening, and probably letting up Friday morning after 12Z. Snow levels will rise to perhaps 6,500ft by Friday morning PST, but don't appear likely to rise as much as was anticipated yesterday.

Details after this point are less certain. The GFS and some of its ensembles are suggesting that this second pcpn event will end with some cold advection, laying out a frontal zone south of the ARB. With the main upper trough still offshore on Friday, the GFS is suggesting the focus of the pcpn will shift south with the frontal zone, into the southern Sierra. According to HPC, the SREF WRF runs are keeping the pcpn focused more on the ARB vicinity. GFS ensembles are also a bit scattered by 48h into the forecast, strongly hinting at considerable uncertainty in details of timing and intensity of pcpn over ARB after this point. I think most likely is for there to be a break in the pcpn after 12Z Friday, and that the daylight hours of Saturday (10th) will be wetter than daylight hours of Friday, mainly because Saturday the ARB will likely be in synoptically related ascent as the upper trough axis approaches shore. There are hints in some of the GFS ensemble members of wave development offshore along the front by Saturday morning.

The trough axis is anticipated to cross the West Coast Saturday night or early Sunday. This indicates that the pcpn will become mainly showers by daylight hours of Sunday, although the possibility that the progression of the upper trough could be enough slower than forecast to allow continued steady pcpn into the daylight hours of Sunday can't be ruled out at this point.

Looking farther ahead, the upper ridge is outlooked to slowly amplify over the Eastern Pacific early in the week. Nevertheless, there will be at least one additional significant upper short-wave trough that will approach the west coast early in the week. This is seen both on the GFS and the ECMWF from this morning. Both these models leave open the possibility, however remote, that this wave will be IOP material.

Its been fun...John B.

February 6, 2007


SUMMARY: Major pcpn event (at least relative to what we have had so far this season) on tap for the American River Basin, beginning Wednesday morning (7th) and continuing thru Saturday (10th).

SYNOPSIS and FORECAST: West Coast ridge has shifted inland and is losing amplitude as the first in what appears to be a series of upper air waves and associated surface lows has made it to the Pacific Northwest. The next in the series is near 40N/135W with a decent surface low and front and evidence for a cold-core upper trough based on presence of deep convection near 140W. The surface front has
a good push of NW surface flow behind it. This system has been given successively more emphasis by the numerical guidance over the last 72h and at this point looks likely to give > 1" of pcpn at Blue Canyon by 12Z Thursday. Pcpn (snow above initially 6000 ft, lowering to 5,000 ft or lower as pcpn tapers off early Th) will begin mid-late morning Wednesday and peak before midnight W evening. I expect a detectable frontal passage with this trough, most likely W evening.

Following this, details are more uncertain. It does still appear that there will be a break of at least 6-12h between the end of the postfrontal pcpn with the first system and the onset of pcpn from the next. This next wave to affect the west coast is apparent on water vapor imagery near 30N/165W at midday Tu. This is a well-marked feature with a strong rising/falling vertical motion couplet with it and a pronounced warm-advection bulge ahead of it N of Hawaii. The GFS high-resolution and the NAM runs from 12Z this mrng differed considerably, particularly beyond 36h, with the GFS more progressive with this central Pacific feature, and the NAM suggesting more tendency for this feature to cut off and slow in its eastward progress toward the coast. The result is that the open warm-frontal wave feature that yesterday looked as if it would give pcpn on warm advection on Th is maintained (but is a little slower) in the GFS, but in the NAM travels on a more NNE trajectory, is weaker, and affects mainly the coast ranges near and north of Cape Mendocino. GFS ensembles were a little less optimistic about this warm-frontal wave than the high-resolution run, but there was also quite a bit of spread.

In the end, I leaned toward the GFS, for the reason that the NAM beyond 48h with strong westerly flow is going to be dependent on 06Z GFS-provided lateral boundary conditions. Accordingly, I am sticking with a good warm-advection event beginning near noon on Thursday (8th). During this event, snow levels should rise to at least 7,500ft in the Sierra by early Friday morning. Another 1 to 1.5+" of pcpn should fall at Blue Canyon during this period. If the GFS is correct with its forecast of a surface low moving approaching the Northern California coast and then mvg northward overnight Thursday, there is likely to be a good barrier over the west slopes of the Sierra Thursday night. This is also suggested by this morning's ECMWF.

Looking farther ahead, all the models agree on amplification of a trough in the eastern Pacific by Friday and its progression inland over the weekend. (This trough will be some combination of the upper trough now near 170W, with another wave approaching the dateline.) As this happens, a frontal zone appears likely to remain over northern or central California, gradually settling southward as the trough approaches the coast. I anticipate that there will be frontal waves on this frontal zone that are not currently apparent in the global model forecasts, with the consequence that the continuous pcpn the GFS is forecasting Th thru Saturday over the ARB will actually come mainly as one or 2 more discrete and probably at times more intense 6-12h periods.

The main event looks like it will end sometime between late afternoon Saturday and during the wee hours of Sunday, with a break of at least 24-36h before any further system gives pcpn to ARB. At present the longer range progs suggest a resumption of the Ern Pacific-West Coast ridging pattern at upper levels by late next week. However, earlier in the week it appears likely that one or 2 light or moderate pcpn events should affect the ARB.

February 5, 2007


Good news today for lovers of foul Wx--such is on its way to the American River Basin.
Events are close enough at hand to have high confidence that there will be an IOP before the week is out.

SYNOPSIS and FORECAST DISCUSSION: Overall trend of the last several days is still on track. This is for the W Coast ridge to lose its contiguity with a northern part retrograding to the NW and southern portion retreating southward, allowing a strong branch of the westerlies to develop across the Eastern Pacific. Looking more in detail, there are four identifiable features on Pacific satellite imagery this morning, each having some rational relationship to features identifiable on the initial conditions of the GFS. These are as follows:
1) A swirlie in water vapor imagery associated with the low latitude feature noted E of Hawaii yesterday. This is moving NNE, now W of KSFO near 132W. There is a surface cold front with this feature and a westerly push behind the front at the surface. Ahead of this, winds have turned SSE just ofshr Nrn CA. Snow levels are likely to be 5,000 to 5,500 feet with this.
2) A trough feature near 36/145. This is moving briskly eastward attm, and is beginning to interact with 1). Progs indicate that this will at least partly merge with 1) to give a significant surface development that will bring cooler low-level air onshore south of the low, which will track toward the OR coast. This system will spread rain into far Northern CA by Tu evening and into the Sacramento Valley and HMT by Wednesday morning by present reckoning. However, the main thrust of this system will be into the coastal mountains of Northern California and southern OR, and pcpn is anticipated to be relatively light in the ARB, with Blue Canyon's chances of getting more than 1" by 1200Z Th morning about 50%. By Wednesday evening there likely will be a frontal zone south of the ARB, setting up the potential for warm advection ahead of the next feature:
3) An upper wave near the Dateline with associated bulge and convection near 170W, moving eastward. This feature is trackable on the GFS, approaching the coast late Thursday as a warm-frontal wave. Ahead of it will be a broad pcpn shield. Anticipate that snow levels will rise from near 5,000 ft to near or perhaps above 7,000 feet with this notable warm air advection.
4) The next, and potentially strongest system is most likely to give heaviest pcpn Friday night. Snow levels will drop from 7,000 ft to 4,000ft by the tail end of this event late Saturday or Saturday night.

One of the most uncertain aspects of this period is whether there will be real breaks in the pcpn.
Continuous precipitation from Wednesday morning thru Saturday is a distinct possibility in hier elevations, with periods of 6-18h where the pcpn will be light, perhaps just drizzle. The times of lightest pcpn are anticipated now to most likely be Wednesday night and late night--morning Friday.
It is noteworthy that over the ARB the GFS "deterministic" run from 12Z this morning has in excess of 0.5" in every 6-h period beginning 12Z Thursday the 8th through 12Z Saturday. About 6 inches of pcpn is forecast during this 48h spell.

Looking beyond Saturday, there are indications the subtropical ridge will amplify off the West Coast, leading to flow aloft from north of west and perhaps another less wet and colder system early next week.

John B.

February 4, 2007


Broad trends for increased westerlies into the US W coast still on track, but latest trends in progs suggest for the broader scale (above the scale of individual short waves and jet streaks) more of a tendency for a high over low block to develop in Ern Pacific by next weekend. Details of timing continue uncertain in my view, but it looks as if Th 8 Feb would be the very earliest for an IOP-quality event in the American River Basin.

There are 2 main routes of middle and upper tropospheric tropical moisture into the midlatitudes at the moment. The most persistent has been a weak, semipermanent low-latitude trough off Baja that has been bringing tropical moisture across central Mexico into Texas and the Gulf of Mexico for several weeks now. The other is as pointed out on Friday by Klaus and Ed in the western Pacific where there is a feed from the vicinity of the Phillipines E and ENE toward the general vicinity of Hawaii. This one has been largely lacking through much of the winter so far. Temporarily there is a third, being produced by an upper trough near 30N/140W that is becoming detached from the main polar jet.

This upper trough is forecast by the models to shear apart, with the upper portion propagating eastward to reinforce the low-latitude mean trough near 115-120W (and the northward feed of moisture to its east), and the middle tropospheric portion tracking north to north-northeast with a weak manifestation at the surface moving toward the Oregon coast. The main effect of this latter portion is to flatten out the ridge and set up a baroclinic zone in the eastern Pacific that is oriented SW-NE near the coast. In the progs this then acts as a kind of wave guide for future waves coming out of the central Pacific, undercutting the high latitude portion of the major ridge near the West Coast that has been with us for many days. As well, and perhaps more importantly, it provides a conduit whereby the western Pacific water vapor feed may reach the mid latitudes. The first system of this new regime to affect the HMT domain is due to arrive about Thursday. This almost certainly will be followed by at least one other of more significance.

D(prog)/Dt of GFS ensembles suggests that the Thursday system is not to be written off as an IOP candidate at this point. However, the most promising event still looks to be Friday-Saturday for HMT. Beyond this, details are fuzzy, but there is the distinct possibility of more IOP-candidate material following.

In summary, Thursday (8 Feb) is the earliest possibility for an IOP, and prospects continue good for an IOP in the Friday-Saturday-Sunday range.

February 3, 2007


Ridging surface and aloft continues to keep Northern California unseasonably dry. However, changes toward a wetter pattern, hearalded by the models over the past several days, continue to be indicated by the progs. The overall evolution continues to be that of poleward amplification of the ridge just off the West Coast, to the point that it becomes a high latitude block, with flow breaking underneath it. However, the subtropical portion of the ridge appears likely to hold in place near 120-125 W. This combination suggests that northern California will be more favorably situated with respect to the flow breaking underneath the high latitude ridge, than will areas farther south. Of some concern is the strength of this flow...will the embedded short waves aloft and attendent surface features be sufficient to create a lengthy fetch of onshore-component flow that would enable moist trajectories
from lower latitudes and heavy pcpn.

A low latitude upper trough and surface cyclone presently N of Hawaii will move NEwd and flatten the southern portion of the ridge just off the West Coast, and may get close enough to the coast late Monday and Tuesday to give a little light rain along the Northern California coast, but nothing in the HMT area. The system following this, crossing the HMT area late Wednesday and Th 7-8 Feb is indicated to give some pcpn in the HMT area, but may lack a sufficient onshore geostrophic flow at low levels ahead of it to displace dry air that is likely to be present over the land areas into the middle of next week. It thus may be too weak to produce an IOP-worthy event. This will be watched the
next couple of days.

The more important rain events will come later, as the WSW fetch from the central Pacific becomes better established. The first of these appears most likely late Friday and Saturday, 9-10 Feb. Given that this system will approach shore from the WSW and be embedded in broader-scale flow from the SW, this will be a warm event (and perhaps rather wet also), with snow level probably at or above 7,000feet during at least the first half of the event. Indications are that there may be more storms to follow, so that there is a possibility of 2-3 IOPs in quick succession beginning on the 9th or 10th.

John B.

February 2, 2007

Break through update, then what?

Some models are continuing to advertise the break through of westerlies on the west coast with the PSD ensemble showing a distinct shift toward probability of above normal precipitation in the California region in week 2 (Feb 10-16). Tropical convection is now established over the west Pacific and continues to be active over the Indian Ocean. The tendency for convective forcing to be further west should continue to effect the North Pacific circulation with more retrogression of features. This could eventually (beyond week 2) lead to weakened westerlies in midlatitudes and a return to a trough along the US west coast, especially if a new MJO develops over the Indian Ocean.







Klaus Weickmann and Ed Berry


Overall, trends discussed yesterday appear on track. Early next week, the ridge just off the West Coast amplifies northwestward as the southern branch underneath it deepens down from the tropopause and upper troposphere as systems from the west act to extend the tropospheric baroclinity farther out in the Pacific eastward into the ridge. The first rain-bearing system to reach the West Coast still looks like the one yesterdays progs were indicating for about Wednesday 7 Feb. The consensus of the various models and model runs I looked at (...) is that this will be a weak system, involving 500mb heights mostly at about 552dm and above, with fairly high snow levels and perhaps only about 1" at Blue Canyon. The models also have a rough consensus that this will be followed by another system ~48h later and perhaps a 3rd after that, these later ones having more promise for heavier pcpn. So at least 2 IOPs in quick succession is a distinct psblty.

John B.

ps: I discovered the next day that I had failed to publish and save this...sorry.

February 1, 2007

Waiting for a break through?

The following are a few annotated images to "prep us" for tomorrow's extended discussion. We will probabaly add a few more slides. Tropical forcing remains concentrated around 5-10S/160E (putting the USA in a cold regime) while SPCZ starts to "heat up" again. Meanwhile activity continues to slowly increase from South Africa into the South Indian Ocean. As the anticyclonic anomaly around Alaska continues northwest, increasing subtropical flow looks more probable to impact the ARB region during week 2. If we would have had the "classic El-Nino" response, the polar and subtropical jets would have combined across the east Pacific (see slide 2).

Ed Berry and Klaus Weickmann






When I was a kid, growing up in San Diego and always looking for some sort of Wx, it seemed that the coming of a new month would bring the best chance for a change toward something more interesting. In that spirit, things are looking slightly more favorable for HMT. But, not till next week.

After many tries without success earlier this winter, a low-latitude connection underneath the persistent large-amplitude ridge has finally been established across the eastern Pacific between a mean-trough position north of Hawaii and the persistent large-amplitude trough over North America the past several days. (This can be seen most readily by viewing wind and PV on the 340K isentropic surface.) Southern California and northern Baja have so far been the only beneficiaries of this development. However, the models generally, and the GFS in particular, are indicating that the present low-latitude upper flow south of the ridge will eventually deepen in latitude, as the ridge itself becomes more entrenched at higer latitudes. This will allow the possibility of identifiable surface lows to make their way under the northward-displaced Pacific High to affect California, beginning middle or
late next week.

This morning's GFS ensembles would argue that the first system of substance to make it across the Eastern Pacific will most likely give rain to Southern CA, probably Th 8 Feb. However, last night's GFS and this morning's ECMWF suggest that northern and Central CA have a better chance, and as early as W 7 Feb. (Both models indicate this will be preceeded by a weak system now N of Hawaii that will approach the coast over the weekend, but turn north and weaken west of 130W, bringing no pcpn onshore.) The ECMWF, in my opinion, is the most optimistic for HMT, with the possiblity of 2-3 IOP potential systems over a week period beginning with the one on W 7th.

Ed Berry ventured that he thinks week 2 (i.e., 8-14 Feb) looks pretty good. He and Klaus W. will discuss these possibilities in more detail in tmrw's telcon.

John Brown