March 07, 2006

WX Discussion and Forecast for 7-March-06

Precipitation picked up overnight and this morning a pretty good band of precipitation is moving through the ARB with heavy snow reported at Blue Canyon. Some storm total numbers as of 12z this morning include 2.27" at Blue Canyon, but in the heavy band that set up to the north of the area 4.56" at Yorkville and 5.68" at Venado, both north of San Francisco. Some missing times at Brush Creek and Bucks Lake, both having at least 3". Squaw Valley Ski area reported 12" of snow so far but Sierra-at-Tahoe has received 22" over two days. This morning's heavier precipitation is occuring ahead of the trough axis which will move across the ARB 18-21z today with winds shifting to the NW from the surface through midlevels and drier air pushing in after the trough passage. Precipitation will persist in higher areas for at least 6 hours or so after trough passage then become scattered this evening and end later tonight. The heaviest precipitation should mostly be over by the time of our conference call today.

Looking ahead the overall scenario outlined yesterday still looks good: the next system a fast-moving one arriving late Wed night into Thursday followed by a series of waves with progressively colder conditions before the overall upper low sinks south of the ARB by about Mon/13 Mar bringing an onset of what looks like an extended period of drier weather as the mean ridge moves over the West Coast for next week and the mean trough into the CONUS. The ensembles (00z GFS) are in pretty good agreement with this through day 10 (next Friday/17 Mar, and the mean 8-10 day forecast from the 00z ECMWF also supports this ridge position), then more spread develops with the ridge potentially moving inland, opening the West Coast to possible moist systems again for later this month, though right now not a lot of the ensembles show much heavy precipitation even for days 10-15. The 12z GFS deterministic forecast does have such a moist system by day 15 (23 Mar), a long way off.

As for the upcoming action from Thu-Sun (9-12 Mar), the Thu system looks to be moving in quicker and with somewhat less moisture, so likely Wed night through midday Thu. This is followed by a series of shortwaves as an overall upper low strengthens off the Pacific Northwest Friday and moves slowly southward down the West Coast. Timing is tricky with each wave and precipitation amounts would be less than the current moist system, with progressively colder temperatures. By Sat/12z (11 Mar) thicknesses in the ARB are down to the mid-520's which would drop snow levels quite low, but most of the more significant precipitation may be confined to higher areas. Total precipitation for the whole Wed night through weekend period on the GFS is about 2" in the higher areas, so the amount of moisture certainly the main issue (though, as noted yesterday, there currently is a tie to the Hawaiin moisture as the system is moving across the Gulf of Alaska, so perhaps there could be more). The biggest characteristic of this period would be how cold the conditions become with time.

Decision was made to end the HMT field exercise at the end of this IOP. It's been enjoyable, we look forward to next season! ed szoke ESRL/GSD

March 06, 2006

WX Discussion and Forecast for 6-March-06

The heavy band of precipitation with the main frontal band unfortunately
ended up hanging to the north of the ARB longer than forecast with the
result that the period of heavier precipitation, which is wrapping up
now, was shorter than anticipated. It didn't miss the ARB by much;
looking at the 24-h precip reports there was a band of 3-4" accumulation
about 60 or so miles to the north, from Brush Creek north of Blue Canyon
wsw back to north of San Francisco. It looks like this band will be
passing east of the area the next couple of hours (what is left of the
cold front) and with gradually decreasing temperatures aloft there
should be widespread convection this afternoon, which would decrease
somewhat after dark. In the higher areas (like Blue Canyon) would
expect more or less continuous precipitation with snow levels now around
4000 feet or so gradually dropping. The trough axis still looks to pass
near 18z Tuesday and would expect more steady precipitation to develop
ahead of this beginning around 09z and continuing through 18z Tue, with
the 12z Eta and GFS both having about an inch new of precipitation in
the 24-h ending 00z/Wed. Our latest (12z) 3 km run nicely shows the
more convective nature of the precipitation this afternoon then the
above noted timing for more widespread precipitaiton later tonight.
Following trough passage snow levels would lower to about 2500 ft or so
but precip will be winding down tomorrow afternoon and should mostly end
even in the high country Tue night.

The models seem to be coming into somewhat better agreement on the
overall broad pattern for the next week plus, with less spread than was
shown in yesterday's runs. The general trend is to shift the longwave
trough inland over the next week so that by early next week an upper
level ridge will be near the West Coast which could set up an extended
period of dry weather. There is still some spread in the GFS 00z
ensembles but not as much as yesterday, and the 12z GFS run is also in
agreement in showing dry weather for the most part in the ARB area from
about Sun/12 March through Tue/21 March. The mean 8-10 day ECMWF
pattern is similar to the GFS in showing this trend.

In the interim between the current system and next week there is active
weather as waves continue to affect the region in the NW flow. The
first such wave now appears to be more in the Thursday timeframe with
the bulk of the precipitation from about 12z/Thu through 06z/Fri 10 Mar.
The GFS shows only about an inch or so accumulation but, as noted
yesterday, there is a connection to the tropical plume extending north
from the system west of Hawaii as the next wave rides eastward near the
Aleutians, so there could be more of a heavier burst of precipitation
with this first wave, and, additionally, it is not all that cold to
start. After this relatively fast moving wave timing of any further
identifiable systems gets a little trickier, with the best guess Friday
night into Saturday (11 Mar), as the overall trough tries to close off
and shift south and east, with mostly scattered precipitation around by
about Sunday. Whatever happens from Friday and thereafter it will be
very cold, with thicknesses down into the mid 520s (dam), so very low
snow levels but moisture amounts could be an issue.

March 05, 2006

WX Discussion and Forecast for 5-March-06

Precipitation is moving into the area about as everyone felt yesterday with the heavier precipitation at the moment hanging up a bit north of the ARW area. Latest totals indicate over 2" so far along the coastal regions north of San Francisco and over an inch in the higher areas north of the ARW already. Satellite imagery shows an extensive band of deep cloudiness ahead of the cold front extending from northern California southwest to what appears to be a wave near 135W/30N. There was at least (and may still be) some thin connection with the extensive tropical plume that is found from Hawaii west and northwards but it is tough to say how much has made it into this moisture band. SSMI imagery supports a definite connection in a thin band that still extends all the way back to a piece of the Hawaiin system with PW values of about 1.5 inches. Another separate band of tropical connection is found from Southern California well back to the southwest well to the east of Hawaii but this is aimed more for areas south of the ARW, although there is a chance this could merge with the main plume of moisture later today. Judging by the way things are moving along it seems like the 12z models may be a little slow bringing the onset of heavier precipitation into the ARW, but otherwise paint a picture that is in good agreement with the overall scenario outlined over the last couple of days: increasing precipitation and wind this afternoon with at least a couple of inches of precipitation with the frontal band through about 18z Mon when the front passes, then less organized precipitation with the initial high (5000 ft +) snow levels dropping to 3000-4000 ft, then another onset of heavier precipitation Monday night ahead of the trough passage around 18z Tue (another inch at least in the Eta and GFS latest 12z runs), with the snow levels lowering to near where they were with this last event as the precipitation then becomes spotty by 00z/Wed and ends at higher areas during Tue night. The latest Eta and GFS both agree on about 4" of maximum precipitation but it seems the amounts will depend on how much moisture from the tropical plume made it in, any stalling of the frontal band, and of course the amount of convection after frontal passage. Would expect some areas to come in with 4-6" total by late Tuesday I would think with 3-4 feet or more of total mountain snows. The 3 km LAPS 15z WRF run has local max precip amounts for the 24-h ending 15z/Monday at 3 to 5 inches. South winds are already gusting to 30 mph at Blue Canyon with a temp of 36 and the Sacremento WFO forecast has S winds up to 60 mph in gusts later today and 65 mph tonight.

Beyond the current system things appear somewhat more uncertain today then they did yesterday in some regards. The next system is still on track to quickly move in probably Wed night into midday Thu/9 March with a quick shot of potentially heavy precipitation as it does seem to have a connection to the Hawaiin tropical plume. Snow levels start out around 4000 or so feet then plunge quite low by late Thursday. The main issue after Thursday is how quickly the overall trough position shifts into the Rockies, with the 12z GFS slowing this process down as it deepens an intense closed low off the Pacific Northwest next weekend that then slowly retrogrades slightly and finally moves through Northern California midweek (around 16 March). This is much slower than what looked like an end to precipitation on yesterday's runs (and last night's GFS) more like very early next week. The 12z GFS solution is supported by the 00z Canadian and 12z NOGAPS, but the 00z ECMWF is more progressive. The ensembles from the 00z GFS have some spread though are not as dramatic at holding the trough back as the latest GFS. Either way what all this means is that following the initial next system late Wed-Thu we enter an extensive period, at least through the weekend and possibly well into next week, of waves of precipitation moving in with timing and amount very uncertain. One relative certainty is that at least for later this week snow levels would drop to extremely low values not yet seen, I believe. If the latest GFS is correct they could then rise again and there would be the potential for a more moist episode or two of precipitation from say Sun/12 March through about Wed/15 March. All this may be moot if resources have run out, but wanted to point out that 1) uncertainty in the late Wed-Thu system, 2) potential for VERY cold weather with anything later in the week or weekend.

Ed Szoke - ESRL/GSD

March 04, 2006

2006-03-04 WX Discussion and Forecast

The approaching system still on track to be a big precipitation producer with the first main band dumping heavy rains and strong winds through midday monday with higher freezing levels, then extended precipitation until the trough axis passes midday on Tuesday with a second max falling later Monday night into midday Tuesday ahead of the trough. The 12z Eta and GFS are in pretty good agreement overall and the 00z GFS is consistent with the latest run. There is also good agreement among the GFS ensembles through Tue/7 March with trough passage again around 18z Tue and therefore precipitation ending by later Tuesday (00z/Wed). The beginning of the event still is on track for Sunday morning though the latest Eta and GFS hold off the heavier precipitation until beginning around 21z Sunday, maybe 18z according to the 18z run just coming in. WOuld probably be prudent to begin earlier as this first band does tap into the tropical Pacific moisture and, as noted yesterday, will be accompanied by strong winds (60 mph or so mountain areas and coastal regions) from the south, so a pretty dynamic event. Initial snow levels 4-5 kft then lowering to 4 kft later Sunday night but really lowering after the first band passes, which should be about 18z Monday, and then of course lowering further after the trough passage on Tuesday. Eta and GFS both show about 2" max with this first batch, I imagine more could be more expected in some areas (no 3 km run available today). After this could be a lull in the precipitation a bit though will certainly continue in the higher areas of the HMT and then would expect things to pick up again as the trough axis approaches with the models indicating a second max later Monday night thru about 18z or so on Tue with another 1-2", and colder with this with snow levels probably down to 2500 ft or so. Precipitation then becomes more spotty after trough passage and should slowly diminish after 00z/Wed. Storm totals in Eta and GFS 4-5" and one would expect 4 or more feet of snow in the mountains. Certainly a nice storm.

The next system does come rapidly behind the first one but may be more an extended system and overall colder and not as wet, though there could also be a more distinct first punch to the system and then a longer lull. It looks like precipitation would begin in earnest by 00z/Thu (wed night) and continue for about 15-18 h with dropping thicknesses to the lowest seen so far as this wave dives down the western Rockies. About an inch or so of precipitation forecast by the GFS, though see note below. Then some uncertainty with the latest GFS keeping off and on precipitation and cold temps the rest of the week before the final wave Sat night into midday Sunday (11-12 March), while the 00z GFS had a bit less off and on precip (so more of a distinct break) but still the Sunday system though not quite as organized. Looking at some of the other models (Canadian, ECMWF, NOGAPS) indicates some uncertainty with the timing and strength of the midweek system, though all have something. I think the best that can be said about the midweek system is it might be interesting, it will be colder, could produce a good burst of precipitation say later Wed into midday Thu though not as much as this coming event, and that how it winds down is uncertain, as is whether there is a further distinct system by the weekend. One thing that should be noted is that, examining the nice U of Hawaii web site with its PW forecast in the Pacific from the GFS, there IS a decent connection setting up with the tropical moisture west of Hawaii as the system moves southeast down the west coast of Canada, and this would be part of the precip that hits later Wed into Thu. It is, though, a fairly fast moving system, but the point being there could be a decent burst of precipitation, moreso than might be anticipated by such a system moving from that direction, and a colder system.

Looking further ahead, both GFS runs are consistent with the idea that there will be a shift in the overall longwave trough position by this weekend and this creates a ridge closer to the West COast by early next week that could lead to a drying period for at least a week. The mean 8-10 day 00z ECMWF is also consistent in this idea. But the GFS ensemble shows a lot of spread, especially by day 9 (Mon/13 Mar) and continuing through the 15 day run (Sunday 19/Mar), so this would make one have less confidence in the extended dry forecast of the two GFS deterministic runs. Still, the end of resources might be nicely timed with an extended break in good events.

Ed Szoke - ESRL/GSD

March 03, 2006

2006-03-03 WX Discussion and Forecast

A solid band of rain with a few embedded thunderstorms is moving across
the HMT area this morning just ahead of the trough axis moving
onshore. Trough passage across the HMT is expected around 21z today
with the winds becoming more westerly and the precipitation decreasing
in coverage, though convective showers will remain over higher areas
through most of the night for some additional snows. the snow level is
already quite low, down to around 1500 to 2000 feet, and may lower a
bit more as the trough moves by. The LAPS run has the well-defined main
band of precipitation persisting until about 22z before breaking up into
less organized convection. The 3 km run max precip amounts 15z today
thru 15z tomorrow are up to 1.5 to 2" in the HMT area which could be a
bit high. The ski report this morning from Squaw Valley says they have
had 24" of new snow in the last 2 days.

Saturday should be quiet between systems but as noted yesterday the
next trough now crossing the eastern Aleutians is quickly diving
southeastward and intensifying for the next event. This system will be
stronger than the current one and, as noted yesterday, still looks to
be tapping into some of the moisture associated with the broad trough
near Hawaii. This moisture plume appears to be contained mainly in the
initial strong rainband that will arrive in the HMT area later Sunday
and persist until around 18z on Monday. It looks like todays model
runs are a little faster with the start of this event and significant
precipitation in the HMT area could well be underway by later (~21z)
Sunday, perhaps even 18z, with initial precipitation beginning in the
morning. In addition, strong SW to SSW winds are predicted to accompany
the first main rainband, with speeds forecast by the Eta and GFS to be
about 50 knots sustained near 925 mb and 55 to 60 knots at 850 mb.
This strong southerly flow will push warmer air in with the freezing
level rising to 850 mb so initially snow level will be fairly high
(4500-5500 feet) but eventually lowering to near what is happening now
as the trough passes on Tuesday, which probably happens around 12z or so,
with precipitation likely ending by later in the day on Tuesday. With
the system tapping into more tropical moisture than the current event and
a longer duration the 00z GFS is forecasting about 5" maximum precip
amounts for this event which look reasonable. The 12z GFS is a little
lower but still plenty wet.

Yet another wave closely follows the Sunday-Tuesday storm and this one
at least temporarily shifts the mean trough position over the Rockies
with upper level ridging shifting eastward off the West Coast extending
north to the Aleutians. This system is now forecast to arrive on
Wednesday/8 March and will be the coldest so far with thicknesses
forecast to drop into the lower 520's (they are near 530 dam with the
current event). After the initial Wed-early Thu burst of heavy
precipitation (perhaps aided with at least some contribution from the
same tropical source) there will be an extended showery period likely
perhaps into Friday. Beyond this somewhat uncertain with a number of
the GFS 00z ensembles indicating a deep trough setting up farther off
the West Coast mid-March while others regain something akin to the
current pattern, but there does seem to be a chance of a gap in the
events after the Wed-Friday storm of this coming week.

Ed Szoke - ESRL/GSD

March 02, 2006

2006-03-02 WX Discussion and Forecast - Ed Szoke

The first wave of rain and snow currently pounding the area with
freezing level around 3500 to 4000 feet or so and heavy snows at Blue
Canyon where about an inch of precipitation has fallen thusfar. The
precipitation will become more spotty especially at lower elevations
today and much of tonight before increasing again friday morning
ahead of the trough axis. the forecast soundings for this afternoon are
pretty unstable so would expect some good convection and possible
thunderstorms as well. Latest 12z models are in good agreement that the
trough axis at 500 mb pushing across the HMT area between 18z/Friday and
00z/Sat. the 700 mb trough axis will precede this by 6-9 hours with
winds shifting to southwest by about 18z/friday then more westerly by
00z/Sat. the precipitation by friday night then should become much
more spotty and pretty much be over by saturday morning except for some
lingering mountain snow showers. As the trough axis approaches the
precipitation will increase in coverage and intensity then diminish to
showery after passage but persist at higher elevations well into friday
night at least in showery form. The heaviest precipitation is expected
from about 10z through 21z on Friday. By 00z/Sat the eta generates an
additional 1.5 inches of melted over the higher terrain for storm total
and the gfs about this or a little more. The 12z 3 km LAPS HMT run
generates about 3" max amounts for the 24-h ending 12z Friday, but at
that time still has the heavy second round of precipitation approaching
the HMT area, which may be a bit slow. Not as wet as the last system but
should be good for 2 to 3 feet or more of snow in the mountains. The
snow levels should drop quite low as the trough axis approaches and
passes as thicknesses go down to 528 dm, so would think down to at least
2000 feet or so, maybe lower in some areas.

The active pattern will continue as the mean trough position remains
just off the West Coast. Another strong wave deepens off the West
Coast by Saturday/4 March and will set up what looks like a fairly long
duration event beginning by Sunday/5 March. There is quite a mass of
tropical moisture lurking in the Pacific extending from southwest to
northwest of Hawaii in association with a broad and rather deep
trough near 175 W. Currently this connects northward all the way north
to the Aleutians...at least in the IR imagery...ahead of the next wave
that will be diving southeast and deeping off California. These two
systems don't end up phasing as the sourthern one is left behind...but
it seems apparent that some of this tropical moisture will get caught
up in this next wave and so would expect this next system to be wetter
than the current one. Best guess right now is that this next event
would begin as early as later Sunday afternoon (5 March), with higher
terrain precipitation perhaps as early as 18z but certainly by 00z/6 Mar,
and not end until later tuesday or tuesday night. During this period the
00z GFS generates between 4-5 inches of melted precipitation over
portions of the area and the 12z GFS is just a little less. Initially
snow levels should be rather high on Sunday night but lower to around
4000-5000 feet or so on monday and then possibly quite low by tuesday
when the trough moves inland...although it should be noted the 00z ECMWF
is slower with the end of the event although the latest 12z run has
similar timing to the GFS.

Looking farther ahead yet another wave could be heading down the coast
towards the end of the week for a potentially colder system though the
ECMWF is farther north than the 00z GFS with this one so obviously
uncertainty exists that far out. Nonetheless, the GFS ensembles
generally support a potential late week system, beginning as early as
late Wednesday, and beyond this a pattern that would remain very active.

March 01, 2006

2006-03-01 WX Discussion and Forecast

The weather discussion today will look at the next system that begins later this week and then will attempt to describe the longer range picture for next week and possibly beyond.

Both NAM and GFS are fairly parallel on this next system however the GFS wants to keep the closed low together longer as it moves inland with the NAM opening it up earlier (9z 3/3) and bringing it in as a progressive wave, the GFS holds off on this and finally opens the wave at 00z 3/4. Neither model has a true surface fropa wind signature with this system however both indicate a weak upper level trof passage at about the same time between 21z 3/3 and 00z 3/4. The NAM has this as a stronger feature and like yesterday this corresponds with a drop in the precip intensity. Actually, in the NAM it looks like it might spell the end of the event. The GFS continues precipitation beyond this time. Both models have the start of this event at about 06z 3/2 with the precipitation on shore but not quite to the ARB at 03z 3/2. The precipitation in both models begins light with an early maximum around 12-15z 3/2. The precip in the NAM drops abruptly when the winds back to the SE between 21z 3/2 and 3z 3/3. QPF amounts for this system between the models is again pretty consistent like they were yesterday. The NAM has a mid-storm total eq liq. of 1.22 inches by 18z 3/2 before the lull and then continues through to 21z 3/3 with an ending storm total at that point of 2.06 with 0.84 falling in the latter half of the event between 3z 3/3 and 00z 3/4 when then NAM has the event ending.

The GFS continues precipitating a bit longer but with comparable QPF for the event of 2.09 inches ending by 12z 3/4.

Winds from the NAM show less than optimal direction throughout, with primarily south wind direction (as also progged yesterday) with speeds between 10-30 knots. It is only at the end of the event that winds veer to the SW for a brief time. This occurs at 12z 3/3 and 00z 3/4 and coincides with GFS precip rate of about a quarter inch per six hour period, of course the NAM is ending the event at about this time also.

The thermal profile appears to again be much warmer than progged last week, but maybe a tad cooler than yesterday near the end of the event. The freezing level starts at about 850 hPa and after 09z 3/2 begins falling in elevation, finally reaching abut 900 hPa at 15z 3/3, after that time it climbs back to 850 by 00z 3/5.

Looking at the ensembles for additional guidance on the end of this system and the following events it appears that the GFS ensembles are all dry by 09z 3/4. The normal GFS run has the next system beginning about 6z 3/6 (Monday). The GFS ensembles have the system well underway (about 100% agreement) by 12z 3/6 and the ensembles keep this going until 12z 3/8 (Wednesday) and there appears to actually be a surface fropa in the normal GFS run that occurs about 00z 3/8 which probably coincides with the event end, but the timing of course is the issue, putting all of this together, it could be any time between 00 and 12z on Wednesday.

Moisture plumes are looking a bit anemic in the future. The last vestige of a good tropical fetch is now departing the CONUS well south of the ARB and was associated with the last IOP. There is a minor surge from the NW 06z 3/2 associated with this event of about 2.0cm and then the next plume arrives at about 06z 3/6 and is very narrow extending back to a major N-S oriented plume at about 175w. This mid-pacific plume has 4.9cm of water to about 30N and then it falls off to about 3cm up to 40N. An appendage from the top part of this plume finally shears off and interacts with the ARB around 18h 3/9 (Thursday) with pw values of 2.8cm. After this episode, plumes really don't show much promise of making it to the ARB until 06z 3/16 (Thursday) about one week later.

Dan Birkenheuer - ESRL/GSD

February 28, 2006

2006-02-28 WX Discussion and Forecast

Light echoes in the ARB this morning, 40 dBz tapering down to 20 over the higher terrain. There appear to be numerous echoes still on and off of the coast and these may still portend some continued precip through out the day over the ARB as they progress inland. The models indicate that the event should begin winding down with probably only lingering showers past 06h utc Wednesday.

This morning HMT run shows precipitation occurring up until 06h 3/1 utc after which it falls off rapidly, so this high resolution run also corresponds well to the NAM and GFS.

The forecast question of the day is really the onset of the next event and its forecast duration.

The NAM has the approaching precip band with the progressing closed upper level low moving inland by 06h 3/2 (Thurs). The first precipitation in the ARB from both NAM and GFS appears to begin about 09h 3/2 utc. Of the two, the NAM is slightly stronger with precipitation out to 12h 3/2 with a lull for a few time periods and picks up at the end with an additional 0.2 inches in the final 3 hours. As far as the NAM extends, it shows a total QPF of 1.58 inches between 09h 3/2 and 06h 3/3 utc.

The GFS begins the precipitation about the same time, likewise with a strong surge of moisture. Both models tend to show bands of precipitation that have been entrained in circulation around the low pressure system. As each of these bands moves over the ARB the precipitation appears to rise and fall accordingly. There is an initial strong precip event at 12h 3/2/06 utc much like the NAM followed by a tapering of precip until 9-12h 3/3 when there is another surge. This is then followed by a complete lull until 03-06h 3/4/06 utc afterwhich we see 0.84 inches in a 6 hour period. The event continues until about 09h 3/4 utc (Sunday). It is followed quickly by another system. Total QPF for the GFS for this system through Sunday is 2.82 inches with 1.14 inches falling in the same time frame as the NAM forecast termination (which is comparable).

The wind field is not as strong as this current system with winds staying mostly south at 20-30 kts and an upper level wind shift passing about 12h 3/3 utc possibly associated with the decrease (lull) in precipitation noted in the model. However, this feature has little affect at the surface being evident only above 600 hPa. After this upper wave passes it appears that precip at the surface resumes until the moisture in the system moves through. The GFS never runs far enough out to show a good surface wind shift associated with fropa.

The freezing level with this next system appears to start the episode at 850 hPa just above mountain top level and then drops to mountain top level for the duration of the event. It is definitely not as cold a system as was forecast a week ago, nor does it look as interesting to me as the event that is currently taking place.

Dan Birkenheuer - ESRL/GSD