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March 17, 2008

HMT forecast discussion for Monday 17 March 2008 (entered 19z Monday)

Summary: Models, including now all the NAEFS ensemble members, continue to agree that the moisture plume currently impinging on the Pacific NW will stall out near the CA/OR border on Tue, with 3+" of precip forecast by the NAM just into nrn CA, but nothing south to the ARB. This leaves the Wed-Thu shortwave to bring in the precip, and pretty much all models do, but with wide variation. It looks though that the main precip falls with cold advection and would likely be over to snow at BLU for the bulk of the precip on Wed night into Thu. The late Friday shortwave is becoming less likely to do anything, and there continues to be lots of differences in something for the weekend and beyond. The 12z models paint a similar picture, so it does not look good for any IOP this week. Details are below, but based on discussion during the telcon, it was decided that the project would end today, and this will be the last formal forecast discussion.

Discussion: Upper low sitting over Alaska with a jet and moisture plume into the Pacific NW. The upper low will remain pretty much in place this week, with shortwaves moving around it and towards the Pacific Nw. The plume extends from the Pacific NW all the way back to the wsw to an unsettled area wnw of Hawaii. As we've been talking about for awhile, the plume will drift down the coast over the next two days, but all models and now all the 00z NAEFS ensemble members stall it near the OR/CA border. This ends up producing a nice precip max in this area with the 12z NAM predicting over 3" by late Tue in sw OR into extreme nw CA; less on the GFS but well over an inch. However, the moisture never gets farther south and the ARB stays dry, although our local hi-res models do put out some light precip in the higher areas of the ARB Tue night. Then a shortwave rotating around the upper low, that is now in good agreement amongst the forecasts, comes across the ARB late Wed, which forces the precip to the south. General timing would be 18z/Wed until 15z or so on Thu. The 06z NAM had 0.84" at BLU but the 12z is down to about 0.55", with less than a tenth at KSAC. Other models have less, with the 00z/ECMWF only predicting a few hundreths. Looking at the BUFR soundings from the 06z NAM, it appeared that most of the precip came very late Wed and then overnight during cold advection with the shortwave, so with good cooling the mild temperatures initially would drop and at BLU it looks like the bulk of the precip would fall as snow Wed night into early Thu. Our local model ensembles do have spots over an inch in the higher areas of the ARB with this system, and individual members areas of 1-1.5" by 12z/Thu. This may be a tad high based on experience so far with these 3 km runs. Looking at cross-sections of precip type and temperature, the local runs confirm the conclusion based on the larger scale models that the bulk of the precip at BLU would be in the form of snow with the freezing leverl lowering. Discussion during the telcon was focused on this feature, with the CRNFC suggesting snow levels in the 4-5 kft range by Wed night. Given then that: 1) the warm rain with the plume never makes it far enough south Tue-Wed, and 2) when the precip does fall it is with the shortwave that brings cooling and likely snow at BLU, the system was not of interest for a potential IOP. Therefore the decision was made to end the conference calls as of today as well as the field program for this season.

To finish out the discussion...the next shortwave would be on track for Friday but all indications are this is weaker than earlier forecasts and most of the models and even the ensembles predict nothing or only light precip with this in the ARB. Thereafter there remains quite a bit of dissagreement on the timing and strength of the next storm, perhaps by Sun/23 Mar, but overall the trend is for a weaker event and not a strong storm Sun-Mon that was indicated in earlier runs.

ed szoke NOAA/CIRA/GSD

March 16, 2008

HMT discussion for Sunday 17 March 2008 (entered 19z Sunday)

Summary: As we saw yesterday the plume stalls north of the ARB Tue and then a shortwave could bring something on Wed into Thu, another wave possible Fri. Both fast-moving, the first warmer but unlikely to produce >0.5" of pcpn at BLU, although some signs noted by HPC that the new ECMWF was stronger with the Wed wave, so maybe more precip is possible. Bigger storm Sun into early next week but lots of timing differences.

Discussion: Big upper low and surface low over the Aleutians with trailing moisture plume and jet back to north of Hawaii. Upper low will remain in the vicinity of Alaska spinning off shortwaves with some timing and strenght differences amongst the models and ensembles for this week. The plume itself, similar to earlier forecasts, drifts down the coast and gradually gets thinner, though still maintains a nice connection all the way back to southern latitudes. It then stalls near the OR/CA border on Tue, then lifts north slightly ahead of the first shortwave. When this wave passes later on Wed this pushes moisture back to the south and this is when the ARB could get some (up to 0.5" in the 00z GFS) and should be warm enough for rain at BLU, at least for a good portion of the event. Other models do not produce as much moisture. A few ensemble members have more, most have something Wed late into Thu (19-20 Mar). The 12z/GFS was weaker with this wave and did not produce much precip at all for the ARB, so way less than the 00z run. HPC noted on the telcon that their quick look at the ECMWF 12z run just in had a stronger wave for late Tue into Wed, so opposite the trend on the GFS. This variability is probably to be expected for small features in the Pacific rotating around an upper low positioned over Alaska.

Another wave in the Fri time frame is stronger in last night's GFS with about an inch near BLU (but over to snow) but this GFS run seems to be high compared to most ensemble members and other models. And in fact the latest 12z GFS is much weaker with this wave with NO precip at all from it. Quite a lot of variability in the ensembles as well. At any rate, any system would likely be fast-moving and probably cool enough for precip to go over to snow after a possible start as rain at BLU.

Lots of timing differences seen in the ensembles and individual model runs for a potential bigger storm late in the weekend into next week with the 00z/ECMWF the slowest. The 12z GFS has reversed its faster solution from yesterday and from 00z and now is more in line with a system midweek (beginning maybe 26 Mar). Have not seen the 12z ECMWF yet.

ed szoke NOAA/CIRA/GSD

March 15, 2008

HMT discussion for Saturday 16 March 2008 (entered 1920z/Sat)

Summary: the plume we've been talking about drifts south but never quite makes
it past nrn CA midweek in the deterministic runs from 00z and those in from 12z.
There is a new shortwave that drops across the ARB perhaps as early as Thu but
maybe Fri that does produce precip, maybe a rain to snow feature though it looks
fast moving. Bigger storm likely Sun-Mon or possibly Mon-Tue timeframe but would
be colder, maybe rain to start but certainly over to snow at BLU.

Discussion: Currently a digging trough is found along the West Coast,
progressing eastward into the Rockies this weekend and hopefully bringing a nice
snow to the Front Range Sunday night. Currently the storm is bringing cold
conditions and occasional snow to BLU at this time. A sharp ridge is found west
of the trough and this will progress eastward as well. A very deep upper low is
currently centered over the western tip of the Aleutians with a colocated strong
surface low. A nice curl of moisture with this system with the lead edge advancing
towards the British Columbia coast, while the southern part extends into a tropical
tap to a source region and weak system wnw of Hawaii. An upper low in the vicinity
of Alaska will dominate the overall pattern this week, with waves rotating to the south
of the upper low sending most of the precip into the Pacific NW. Eventually the
system is forecast to move southeast out of the Gulf of Alaska and be a pretty
good storm in the Pacific NW and south well into CA in the Sun-Tue/23-25 Mar
time frame.

The plume of moisture that we've been noting for something possible midweek is
the one currently wrapped into the strong western Aleutian low. This gets
stretched out this weekend and sinks southward, bringing precip to the Pacific
NW. By Tue/18 Mar a thinning plume of moisture extends from sw OR all the way
back to n and nw of Hawaii. Last night's GFS and the 12z GFS dip precip into
nrn CA but no farther to the south as a shortwave trough swings into OR on
Wed/19 Mar, initially lifting the precip back into OR. The 00z Global Canadian
model was not so far north though and did bring some precip into the ARB on
Wed, but the 12z run is just a tad farther north so the ARB stays dry.
The 00z/NOGAPS also halts the southern progress of the plume near the
CA/OR border, and a similar picture comes from the 00z/ECMWF. The main
difference though with the ECMWF is in the timing of the shortwave midweek.
I noted this in the telcon, and upon further review it is the same one that the GFS
pushes into OR on Wed, but is later in timing in the ECMWF. In fact the GFS
appears at odds with the 12z/UKMET as well, which is very close to the ECMWF.
The 12z/Global Canadian run is closer to the GFS but maybe a tad weaker.
The ensembles from 00z show spread with this feature, which is probably to be
expected as it is a shortwave trough rotating around the main system. Only a
few members though bring significant precip to the ARB with this wave, but
because the ECMWF is stronger and slower, it actually has the most precip.
Just in though is the 12z ECMWF, which for this system is now close to the GFS
and has much less precip. Clearly confidence not great at this point even
for something as close as wed-thu, but hopefully will get clearer over the next
couple of days. At this point would have to say the chance of something worthy
of an IOP is fairly low for this week, but worth watching for a couple more days.

Another shortwave rotates around the Alaskan system and into the Pacific NW on
Fri/21 Mar, with the GFS bringing the southern end of the precip with this
system across the ARB, but probably not a lot. The 00z/NOGAPS is more agresive
and faster with this wave and has perhaps 0.5" of precip into the ARB on Thu.
As might be expected, the ECMWF and other models/ensembles show other
variations, so even less confidence of this timing and strength for anything for
Friday. It appears if there is something it would be a quick-moving system,
perhaps some rain then to snow at BLU.

More confidence actually looking farther ahead, with more of the models and
ensemble members (though certainly not all!) moving the Alaskan upper low to
the southeast, with a pretty significant storm in the Sun-Tue/23-25 Mar timeframe,
but a colder system. Maybe some rain then over to snow at BLU but a greater
chance of significant precip.

ed szoke NOAA/CIRA/GSD

March 14, 2008

HMT Forecast Discussion for Friday 14 Mar 08 (entered 1930z 14 Mar)

Showery precipitation is now over to snow at KBLU and should remain that way as the next trough moves across the region with falling temperatures through Saturday and limited precipitation. This big trough will move slowly across the West over the weekend with a ridge building along the West Coast, downstream of a deep upper low centered near the western Aleutians. There is a strong jet on the south side of the Aleutian low, and a nice plume of moisture south of this jet extending into the tropical Pacific. Over the coming week an upper low will remain positioned over western Alaska, while the current system centered near the western Aleutians slowly works eastward across the Gulf of Alaska. The associated jet stream then is forecast to stretch to the northeast and hit the British Columbia coast by Sunday, with the moisture plume to its south extending into the Pacific Northwest. Then early next week the jet and moisture plume gradually shift southward, reaching northern CA by late Tue/18 Mar, but with the moisture weakening with time. There is decent agreement then that what is left of this plume reaches the ARB by Wed-Thu, but the deterministic forecasts never produce much precip in the ARB. There are some ensemble members from the 00z/NAEFS that have considerably more precip extending in a nice plume fashion to the wsw, but the trend in the latest 12z GFS supports an overall weakening of the plume and not much precip. The most favorable aspect of this whole thing would be mild enough temperatures for rain at BLU, but taking everything into account at this point suggests that it would be a long shot to get an IOP-worthy event midweek out of this. A look at the latest 12z ECMWF run shows a plume working down to the OR/CA border by late Tue, but then any precip waits until an a shortwave passes later on Wed into Thu and by then things cool down so the precip might actually be some snow, so not very favorable. Similar picture from the 12z Global Canadian model; a nice but very narrow plume doesn't quite make it far enough south so just some light precip late Wed with the shortwave. During the telcon it was noted that we will want to watch the trend of this feature and if it still does not look that favorable Monday would likely be the last telcon.

Beyond this feature, the main upper low gradually sinks south from Alaska into the Gulf of Alaska and then possibly farther south into Easter weekend and beyond. This brings heavy precip into the Pacific NW for most of next week, and then perhaps into the ARB by later next weekend and beyond, all after the time frame of the end of the experiment on Friday/21 Mar. There is certainly spread in how far south this system will sink and when the precip would finally get into the ARB, and of course how much. It could be a big event, as early as Sun/23 Mar, or maybe just another rather cold one with any rain going over to snow at BLU with the strong dynamics.

ed szoke NOAA/GSD/CIRA

March 13, 2008

HMT Forecast Discussion for Thursday 13 Mar 08 (entered 19z/Thu)

Actually had a burst of rain last night into the morning hours with the current disorganized system. Rain began according to the KBLU METAR at 0611z, and as of 16z the METAR site recorded 0.68" with a temperature of 40F/~4C. Interesting to compare this to the ESRL gages at BLU: 0.82" for the tipping bucket (TB) and 0.98" for the hot plate (HP). At Alta 0.55"/TB and 0.75"/HP, and at Big Bend 0.59"/TB and 0.63"/HP with a temperature there of 3.6C. Expect off and on generally light rain showers today but could see another 0.10" or so at KBLU (18z total on the KBLU METAR is 0.75"), then gradually turning cooler and would think things should go to snow late today or tonight. Some light precip on Friday will increase with the next wave but then temperatures will be considerably colder with snow off and on at KBLU Friday night into Sat night.

Looking farther ahead there is a system midweek of next week that is fairly weak with a decent moisture plume extending back to nw of Hawaii, but most of the moisture with it is currently forecast to hit north of the ARB in the Pacific NW, so barring a southward shift this system is not expected to do much. It seems a longshot that this system will amount to anything of interest except for the moisture plume, which if compared to today's system is actually predicted by the 12z/GFS to be even a little better. But little precip is actually forecast from the GFS for the ARB with this. This plume will be hitting the Pacific NW mon-tue (17-18 Mar) with some good precip forecast by the GFS. At upper levels at this time the upper low is in the Gulf of Alaska but a strong zonal jet extends to its south from the Pacific NW back to the WSW to near 35N/160W. The jet and moisture plume are forecast by the GFS to drift southward Tue-Wed with the plume gradually falls apart. The main chance then would be that more moisture is left in the plume when it finally drifts into the ARB region around Wed/19 Mar. It seems I recall something like this with a system way back in Dec/07 (?) that ended up having more moisture then expected when it eventually drifted into place, but was not IOP-worthy as it kept moving south. The other item of interest with the feature would be that the snow level would likely be high enough for rain at Blue Canyon. Not a lot of folks on the conference call today but considerable skepticism was expressed by HPC that this could amount to anything of interest. We certainly can watch it, as the next potential would not occur until the following weekend or maybe beyond.

This greater potential appears to lie in a major trough that deepens off the Pacific NW later in the week and brings considerable moisture into that region into the weekend of 22-23 March into the early part of the next week. How far south this goes is the uncertainty at this point and will probably have to be watched if this is of interest. The 12z deterministic GFS does bring some good precip into the area Sun-Mon/23-24 Mar but most of it is farther north. Ensemble set from 00z has overall good agreement on a strong system but variations in position of the plume of moisture/precip into CA and the Pacific NW. A look at the loop of the 12z/GFS on the U of HA site shows that the system for the weekend is a strong upper low that does not have a good moisture tap into higher PW areas. In fact it is not until after this system that one by midweek of the following week (~Wed/26 Mar) that a system comes in on more zonal flow with a much better moisture tap, though in the longer range models remains a fast-mover. It seems the best chance of some big event would be the strong wave for next weekend (22-23 Mar) coming in farther south and tapping into more moisture than it is currently forecast to do. Currently a few members of the 00z GFS and Canadian Global ensembles (the NAEFS) have such a scenario, but not a majority. The latest 12z GFS ensembles paint roughly the same picture, with little chance for the system mid next week but some members quite favorable for the following weekend and beyond. It appears from the telcon that all of this would be too late in the game to be of interest for an IOP, with the current last date of 21 Mar.

ed szoke NOAA/GSD/CIRA

March 12, 2008

HMT Forecast Discussion: 12 March 2008 at 1945 UTC

Today will be the last dry day across the ARB domain until the end of the weekend. IR satellite loops show a minor cluster of enhanced cloud tops moving across CA as of midmorning in association with a transitory vorticity spoke aloft. No significant wx is tied to this feature, although it has flattened the midtrop ridge. Mid/high clouds will thin temporarily today as this feature moves eastward. Meanwhile, the cold-core storm system offshore continues to grind slowly Eward and is now centered at ~45N/150W. A prominent Eward-moving baroclinic cloud leaf is located SE of the cold-core circulation center and represents the ejecting S/W trough that will affect our area tonight and tomorrow. Clouds will thicken again this afternoon as the S/W approaches.

As the ejecting S/W takes aim at CA, zonal (i.e., onshore) flow will increase to 15-25 kt at 850 mb and 35-45 kt at 700 mb late tonight into Thursday with moistening at low levels, although this flow is *not* progged to back around to a more favorable SWerly direction. Hence, orographic forcing will be modest but not strong. Moderate large-scale dynamics with a cold-frontal passage will contribute to atmospheric lift late tonight and Thursday. The SSM/I satellite imagery shows a respectable PW plume with this S/W pointing toward CA, although core values over the eastern Pacific have decreased from ~3.5 cm yesterday to barely 3 cm today. This decreasing trend is consistent with the GFS solutions depicted on the Univ. of HI website, which show core values decreasing to <=2.5 cm upon landfall late tonight and tomorrow. This plume will also promptly sag Sward beyond the ARB during the day Thursday and Thursday night. Hence, we should expect to see measureable, but not copious, precip across the ARB starting late tonight and on Thursday, perhaps in the 0.5-1.0” range. Snow levels will be in the 6-7kft range at the onset of precip, lowering steadily on Thursday and Thursday night in the cold advection, eventually settling in on the 4.0-4.5kft range by early Friday.

Midtrop heights continue to fall toward the end of the week and into the weekend in response to the approaching Gulf-of-Alaska storm system. The lower-trop flow may briefly become more SWerly on Saturday. This could result in a more favorable orographic flow direction, during which time unstable cold-core conditions will encompass the Sierras (500mb temps will be less than -30C). Hence, convective snow showers will likely impact the ARB on Saturday, although the storm system will probably not be a major precip producer given it’s cold-core character and lack of a warm-source moisture feed. Locally moderate snow accumulations may occur in persistent regions of convection. Snow levels will likely descend to 3kft or lower during the weekend. Brrrr!

Dry wx should return to CA on Sunday as Nerly flow takes hold across the state. Low-amplitude ridging is progged to return for early next week, as the deepening trough aloft moves eastward into the central U.S. Moist zonal flow will likely impact the PacNW next week, although the long-range model solutions exhibit a high degree of uncertainty as to how much (if any) of the moist zonal flow will sag far enough S to impact the ARB. It will need to be watched.

Paul Neiman
NOAA/PSD

March 11, 2008

HMT Forecast Discussion: 11 March 2008 at 1945 UTC

As last night’s shortwave trough quickly exits the region without much effect, transient ridging is quickly taking hold across CA for today into tomorrow. Consequently, the string of dry wx will persist for two final days before a cold-core storm system impacts the ARB later in the week and over the weekend.

The current IR satellite imagery shows this cold-core system taking shape over the eastern Pacific. As of midmorning, it’s circulation was centered at ~44N/160W, while a secondary comma-cloud head and baroclinic leaf resided ~1000 km E-SE of the primary circulation center. This latter feature represents a leading shortwave trough that is progged to eject Eward in advance of the trailing cold-core storm system.

The low amplitude ridge over CA promptly flattens Wednesday and Thursday in response to the approach and landfall of this initial ejecting shortwave trough. High clouds are already approaching the OR/CA coast with this S/W, and should overtake the ARB later today and tomorrow. The cloud deck will thicken tomorrow as the S/W nears. Zonal (i.e., onshore) flow will increase to 15-25 kt at 850 mb and 35-45 kt at 700 mb late Wednesday night into Thursday with moistening at low levels, although the flow is *not* progged to back around to SWerly. Hence, orographic forcing will be modest but not strong. Modest large-scale dynamics with a cold-frontal passage will contribute to atmospheric lift late Wednesday night and Thursday. A transient PW plume will accompany the cold fropa, but peak values w/in the plume will likely remain under ~2.5 cm. Hence, we should expect to see measureable, but not copious, precip across the ARB starting late Wednesday night and on Thursday, perhaps in the 0.5-1.0” range (with possibly larger amounts in extreme N CA). Snow levels will be in the 6-7kft range at the onset of precip, lowering steadily on Thursday and Thursday night in the cold advection, eventually settling in on the 4.0-4.5kft range by early Friday.

Midtrop heights continue to fall toward the end of the week and into the weekend in response to the approaching Gulf-of-Alaska storm system. The lower-trop flow may become more SWerly by the weekend. This will result in a more favorable orographic flow direction, during which time unstable cold-core conditions will encompass the Sierras (500mb temps will range between -30 to -35C). Hence, convective snow showers will likely impact the ARB during the weekend, although the storm system will probably not be a major precip producer given it’s cold-core character and lack of a warm-source moisture feed. Snow levels will likely descend to 3kft or lower during the weekend. Brrrr!

West Coast ridging is progged to return by early next week, as the deepening trough aloft moves eastward into the central U.S.

Paul Neiman
NOAA/PSD

March 10, 2008

HMT Forecast Discussion: 10 March 2008 at 1945 UTC

The current satellite loop shows the next progressive shortwave trough approaching the WA/BC coast as of midmorning. A well-defined comma-cloud head is currently centered near 48N/136W and moving steadily ENEward toward Vancouver Island. A long, skinny cold-frontal comma-cloud tail extends Sward from the circulation center to ~30N; it is approaching the CA coast. The models take this shortwave across the PacNW and into the N Rockies late today and tonight. The trailing cold front limps across the ARB tonight, yielding very weak dynamics and orographics with limited moisture. Hence, precip should be less than 0.1-0.2” of liquid equivalent tonight. Snow levels will descend from ~6kft to <5kft in those areas where light precip. might fall. Clearly, this is not IOP material.

Midtrop heights rebound tomorrow (Tue) afternoon into Wednesday in response to transient ridge building along the West Coast. Hence, Tuesday and Wednesday will be dry.

The ridge promptly flattens Wednesday night and Thursday in response to the approach and landfall of a shortwave trough ejecting from a developing cyclonic circulation over the Gulf of Alaska. Zonal (i.e., onshore) flow will increase to 20-25 kt at 850 mb and 35-45 kt at 700 mb late Wednesday night & Thursday with moistening at low levels, although the flow is *not* progged to back around to SWerly. Hence, orographic forcing will be modest but not strong. Modest large-scale dynamics with a cold-frontal passage will contribute to atmospheric lift on Thursday. The Univ. of Hawaii website of the GFS shows a coherent, thin plume of PW (i.e., an atmospheric river) extending from from N of the HI Islands to the SF Bay area early Thu., although the PW values within the plume barely scratch 3 cm and it quickly dies out while moving Sward on Thu. Hence, we should expect to see measureable (but not copious) precip across the ARB on Thursday, perhaps in the 0.5-1.0” range (with possibly larger amounts in extreme N CA).

Cold air filters into the region behind the cold front on Thursday and beyond. In addition, midtrop heights continue to fall toward the end of the week and into the weekend in response to the approaching Gulf-of-Alaska storm system. The lower-trop flow may become more SWerly by the weekend. This will result in a more favorable orographic flow direction, during which time unstable cold-core conditions will encompass the Sierras. Hence, convective snow showers will likely impact the ARB during the weekend, although the storm system will probably not be a major precip producer given it’s cold-core character and lack of a warm-source moisture feed. Snow levels will likely descend to 3kft or lower during the weekend.

West Coast ridging may return by early next week, as the deepening trough aloft moves eastward into the central U.S.

Paul Neiman
NOAA/PSD

March 9, 2008

HMT Forecast Discussion: 9 March 2008 at 1945 UTC

The current satellite loops show mostly sunny skies across CA today, as a ridge amplifies along the West Coast. Ridging and dry wx will persist through tomorrow afternoon, although high clouds may start streaming over the ridge Monday afternoon in advance of an approaching shortwave trough.

The new 12Z NAM has come around to mirror the global GFS and EC solutions for the landfall of the shortwave on Monday night and Tuesday morning. Namely, the large-scale dynamics and orographics primarily impact the PacNW, although the trailing cold front may generate some light precip along the N half of the Sierras Monday night. In addition, 850 mb wind speeds across the ARB briefly ramp up to the 20 kt range in prefrontal SWerly flow, so modest short-lived orographic forcing may briefly aid in precip generation early Monday night. However, subsident NWerly flow with stabilizing lapse rates will quickly take hold in the postfrontal environment on Tuesday, so don’t expect much in the way of postfrontal orographics. Total precip across the ARB should be less than 0.3-0.5” liquid equivalent. Snow levels will likely start out in the ~6.5kft range before dropping to ~5kft toward the end of the brief period of precip.

The ridge amplifies yet again along the West Coast Tuesday night and Wednesday, yielding dry and mild conditions. The ridge flattens in response to the approach and landfall of a shortwave trough from the Gulf of Alaska on Thursday into Friday. However, given that the storm will originate from a cold source region rather than from the tropics, expect relatively low snow levels and only modest amounts of precip. Low heights and unstable conditions may persist into next weekend, with the possibility of a little cold-core precip.

Paul Neiman
NOAA/PSD

March 8, 2008

HMT Forecast Discussion: 8 March 2008 at 1945 UTC

The next in the series of weak, dry shortwave troughs has moved inland across the Great Basin. Midtrop heights will promptly rebound this afternoon and Sunday across CA in response to ridge building along the West Coast. Today’s satellite imagery shows mostly clear skies over CA beneath the amplifying ridge. Dry weather will continue through the weekend across the ARB, although light warm-advection precip may fall across the PacNW later Sunday and Sunday night. Enhanced cold cloud tops in the IR imagery offshore of the PacNW clearly mark this region of warm advection. Perhaps some high clouds will drift across the Sierras tomorrow.

The new GFS and EC solutions are still consistent with the handling of the progressive storm system for early next week. The shortwave is slated to make landfall along the West Coast for Monday evening into Tuesday while remaining mostly N of the ARB. Specifically, the large-scale dynamics and orographics primarily impact the PacNW, although the trailing cold front may generate some light precip along the N half of the Sierras Monday night into Tuesday. It should be noted that today’s 12z NAM solution is quite a bit more aggressive than the GFS/EC with grinding this shortwave through the mean ridge position. Moderate height falls, large-scale dynamics, and orographics affect the ARB, resulting in QPF values between 0.5 and 1.0 inch. I don’t much faith in this outlier solution.

All models then amplify the West Coast ridge yet again for the middle of next week, although unsettled wx may be in the offing toward week’s end and beyond.

Paul Neiman
NOAA/PSD

March 7, 2008

HMT Forecast Discussion: 7 March 2008 at 1945 UTC

Mostly clear skies graced the Sierras this morning under transient ridge conditions. Meanwhile, the next progressive shortwave trough is clearly evident in the IR imagery making landfall in the PacNW and N CA. High and middle clouds will increase this afternoon and tonight across the ARB, as the tail end of the shortwave skirts the region and flattens the ridge aloft. However, no significant precip is expected locally.

Midtrop heights will promptly rebound tomorrow and Sunday in response to ridge building along the West Coast and weak Great Basin cyclogenesis downstream. Consequently, the dry wx will persist through the weekend across the ARB, although light warm-advection precip may fall across the PacNW and over extreme N CA later Sunday and Sunday night.

The progressive storm system slated to make landfall along the West Coast for Monday evening into Tuesday is still progged to remain mostly N of the ARB. Current model solutions are consistent with their predecessors in depicting the large-scale dynamics and orographics primarily impacting the PacNW, although the trailing cold front may generate some light precip along the N half of the Sierras Monday night into Tuesday.

The West Coast ridge amplifies yet again for the middle of next week. Unsettled wx may be in the offing toward week’s end.

Paul Neiman
NOAA/PSD

March 6, 2008

HMT Forecast Discussion: 6 March 2008 at 1945 UTC

The latest IR satellite imagery shows the remnants of the splitting shortwave trough making landfall across CA. Areal coverage of cold cloud tops is decreasing with time, and a dissipating band of weak precip is coming ashore in extreme northern coastal CA. As for the ARB, expect some high clouds to drift overhead today but no rain/snow.

The next progressive shortwave trough is clearly evident in the IR imagery crossing the 145th meridian between 30-50N. It will come ashore across the PacNW and N CA on Friday night into Saturday, temporarily flattening the West Coast ridge. Large-scale dynamics and orographic forcing will remain well N of the ARB, so no significant precip is expected in our area of interest.

All models increase the midtrop heights over CA later Saturday and Sunday, prolonging the dry wx across the ARB. Light warm-advection precip may extend as far S as Shasta in N CA toward the end of the weekend, although any significant rain/snow will likely remain focused across OR/WA.

The storm we have all been anticipating for Mon/Tue continues to look weaker and farther north in each successive GFS and EC solution. The current solutions show a quick-moving, shortwave trough and cold front making landfall across the ARB on Mon/Tue. The core of the dynamics and orographic forcing stays N of the forecast area. Midtrops heights promptly start rising again by later Tuesday, as the shortwave energy drops into the Intermountain West.

The GFS and EC are hinting at a storm toward the end of next week. We shall see!

Paul Neiman
NOAA/PSD

March 5, 2008

HMT Forecast Discussion: 5 March 2008 at 1945 UTC

The latest satellite imagery shows clear skies with ridge conditions firmly entrenched along the West Coast for today, including a sunny bluejay day in store for the ARB. Meanwhile, the first shortwave trough is quite evident in the IR imagery as a N-S band of cold cloud tops moving Eward beyond the 140th meridian. This wave will cut through the ridge Thursday into Friday, splitting in the process. By the time the remnants of this wave reach the ARB, it will be devoid of substantive large-scale dynamics and moisture, and there will be no lower-trop orographic forcing. Hence, expect little more than scattered middle/high clouds and slightly cooler temperatures.

The next progressive shortwave trough rolls into the PacNW and N CA on Friday night into Saturday and should remain somewhat more coherent at landfall than its predecessor. Nevertheless, because it will propagate through the (flattening) ridge along the West Coast, the large-scale dynamics and orographic forcing will be modest at best and should remain well N of the ARB. The best chance for precip will be for extreme northern coastal CA into the PacNW. The ARB should remain mostly dry.

Shortwave #3 originally slated for Sunday into Monday is looking less impressive with each successive U.S. model run, and in line with persistent EC solutions showing ridging over the ARB during this period. Most or all significant precip is progged to remain N of the ARB.

The potentially significant storm for next Mon/Tue is looking less impressive in the latest 00Z and 12Z GFS model solutions, and in line with the EC prognosis for this event. The current solutions show a quick-moving, shortwave trough and well-defined cold front making landfall across the ARB on Mon/Tue. The core of the dynamics stay N of the forecast area. Additionally, the lower-trop onshore flow only briefly goes around to WSW before veering to WNW during fropa, hence the orographic forcing does not appear to be ideal. Given that a prominent 200 mb jet stream (>150kts) presently extends across western Pacific Basin, we should not yet write off this storm for the ARB early next week. However, it must be acknowledged that d(model)/dt is going in the wrong direction.

Paul Neiman
NOAA/PSD

March 4, 2008

HMT Forecast Discussion: 4 March 2008 at 1945 UTC

Satellite loops show the tail end of a transient shortwave trough having passed the ARB overnight. In its wake, midtrop ridge-building, dry N to NE flow, and subsidence is overtaking the region. As a result, expect mostly sunny skies and near seasonal temperatures along the Sierra foothills the next couple of days.

The first in a series of shortwave troughs embedded in strong Pacific zonal flow aloft will make landfall across the PacNW and extreme N CA Thursday into early Friday. This wave will rapidly minor out as it breaks through the W Coast ridge. In addition, this wave will not have significant dynamic/thermodynamic signatures at the surface. Low-level frontal forcing will be weak at best and orographics nil. Consequently, expect little more than light precip along the N CA coast region and perhaps some increase in high/middle clouds across the ARB on Thursday into Friday.

A couple more shortwave troughs are progged to come ashore along the PacNW and/or extreme N CA on Friday night into Saturday and again on Sunday into Monday, based on the GFS. Each successive wave will serve to further erode the midtrop ridge along the West Coast and perhaps yield a bit more precip than its predecessor. Most of the significant rain/snow should remain N of the ARB throughout the weekend, although we will need to keep an eye on the Sun/Mon wave given that we may have a period of favorable orographic forcing. It should be noted that the current EC solution shows transient ridging rather than a shortwave trough over CA for Sun/Mon, so confidence on this precip event is presently low. If/when precip falls with these waves, snow levels will likely be in the ~7kft range.

A stout 200 mb jet stream (>150kts) presently situated across the western Pacific should eventually extend eastward across the Pacific Basin and may yield a storm of IOP caliber for the ARB early next week. This potentially significant landfalling shortwave for the Monday night/Tuesday timeframe that John B. mentioned in previous forecast discussions is still represented in the various models and ensemble spreads, although the intensity of the wave varies from model run to run, across models, and across ensemble members. This storm also looks like it may be a repeat of storms earlier this winter, that is, a dynamically strong but cool event with relatively low snow levels (i.e., <6kft) rather than a warm atmospheric-river tap of subtropical moisture from the SW.

Thereafter, a period of cool, unsettled, onshore flow may be in the offing for the latter half of next week. Time will tell.

Paul Neiman
NOAA/PSD

March 3, 2008

HMT Forecast Discussion 1945z M 3 March 2008

The possibilities of an IOP for next week are looking better this mrng. The GFS hi-res tendency from late last week to not build the upper ridge along and off the West Coast this week as strongly as most of the GFS ensembles and the ECMWF, seems now the more favored scenario for later this week. As a result, the strong westerly flow (up to 190kts in the GFS 250mb analysis) now extending from east Asia to near the dateline appears likely to extend eastward to the West Coast by the end of next weekend (8-9 March). The details of this are still uncertain. The most likely scenario appears to delay serious pcpn over the ARB until Sunday night 9 March. An earlier weaker system, the first one to give pcpn to CA, seems likely to affect mainly the north coast as it weakens into the ridge late Friday or Saturday.

The GFS hi res from 12z this mrng and the ECMWF from last night indicate a strong system affecting northern CA, including the ARB, on Monday. The GFS would argue for 2-3" pcpn with this storm. However, the diversity in the GFS ensembles visible on the Penn State Ewall argue that it is too early to regard this as a sure thing. Further, as pointed out in the discussion, the GFS is a little earlier than the ECMWF with this system. In addition, the GFS often errs on the side of being a bit too early in bringing in pcpn to the Sierra with collapsing upper ridge situations such as this one. Given these considerations, this does not appear to be as predictable (or as strong) a situation overall as gave rise to IOP4 on 4 Jan, but it is certainly worthy of close attention. Snow levels in this situation seem likely to be in the 6000 ft range in the ARB, so there is potential for a weak rain on snow situation in the ARB (it looks to me as if there is not much snow cover below about 4,500 to 5,000 ft in the ARB; the more reliable word from California is that the serious snow pack starts at about 4,800 ft elevation).

Looking farther ahead, Klaus Weickmann thinks that, with convection reorganizing over Indonesia at present, we may see a retrogression of the long-wave pattern later in week 2 and 3 as this convection begins to impact the midlatitudes. A hint of this is seen in the ECMWF 240h, which has the subtropical ridge off the west Coast. This could lead to retrogression of the mean trough to back over the West with an implication of colder storms tracking SEwd from the Gulf of Alaska. Klaus's confidence in this scenario did not seem too high.

John B.

March 1, 2008

HMT Forecast Discussion 1945z 1 March 2008

There was 0.06" between 12 and 18z this mrng at KBLU, and there may be a bit more pcpn this aftn as another weak band of clouds and showers passes thru. However, pressures are rising at the surface over the Northwest, indicating the building ridge sfc and aloft over and just off the West Coast. There will be another weak wave pass to the N of the ARB M-Tu 3-4 March, but this should bring only a few high and middle clouds over the ARB.

As noted in yesterday's discussion, this feature (or part of it) will dig southeastward or southward midweek next week. There remains quite a bit of difference of opinion amongst the different models and ensemble members about whether this will cut off over the Southwest or will remain connected to the main midlatutude waveguide and give a major storm to the East. The hi-res GFS run from this mrng follows the latter scenario. In any case, the only hope for the ARB is for the ridge along and off the Coast to flatten and shift inland to allow Pacific systems to penetrate sufficiently far south to be of interest. The first inkling of that is next weekend when the ECMWF from this morning brings a wave across OR and Nrn CA of sufficient strength for a little pcpn on northern CA. A couple of the Penn State Ewall GFS ensembles are indicating a possibility for a stronger system early the week after next (say, near Tu 11 March). However, there is large ensemble spread by this point. In summary, I consider chances of an IOP candidate to be < 10% before M 10 March and < 30% before Th 13 March.

John B.