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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