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Area forecast discussion 
National Weather Service Seattle Washington 
900 PST Sat Dec 29 2007 


Synopsis...strong moist westerly flow aloft will produce 
significant snowfall in the Cascades and Olympics later tonight 
through Sunday. Showery weather will prevail on Sunday for The 
Lowlands. Look for dry weather on Monday as high pressure aloft 
builds over the region. The ridge will shift east on Tuesday with a 
warm front brushing the north portion Tuesday night. Another front 
will move through western Washington on Wednesday. 


&& 


Short term...snow is starting to pick up in the mountains as 
expected this evening with the front now into eastern Washington. Models show 
the 850 mb flow to pick up substantially over the next few 
hours...possibly as strong as 40-50 knots. At the same time...much 
colder...moist...and unstable air will be moving into western Washington. This 
spells heavy orographically enhanced snowfall over the Cascades. 
Both the NAM and GFS maintain this strong westerly flow through Sunday 
and probably into Sunday night as well...allowing snow amounts to 
really pile up. With mesoscale model liquid quantitative precipitation forecast amounts as high as 3+ 
inches the next 24 hours...2 to 3 feet of snow is likely with 
highest amounts over west facing slopes above the passes. The current 
winter storm statement indicates up to 30 inches which looks good. 


The coldest air aloft will arrive Sunday morning with 1000-500 mb 
thickness values near 5200 M...and 925 mb temperatures near -2c. If the 
flow were offshore or weak...this could support wet snow showers in 
The Lowlands. However...onshore flow is very strong with this system 
and much of the area will be shadowed by the Olympics. Its 
certainly possible some lowland spots could see snow mixed in at 
times tonight and Sunday morning. Already had a few reports of mixed 
precipitation at Port Angeles and Tacoma this evening. One thing to keep in 
mind is the pscz which could form on Sunday. The strength of the 
flow suggests most of convergence will be pushed up into the 
Cascades enhancing snowfall there. Not sure how much of a band will 
remain intact over The Lowlands as it migrates from S Snohomish 
County into north King County during the day. Could see some flakes of 
snow mixed in with this band. Will need to watch this closely. 


Snow will finally wind down Sunday night as westerly flow weakens and 
upper heights begin to build. Expect a brief break in precipitation on 
Monday ahead of the next system. The next front will arrive on 
Tuesday with more rain and mountain snow. Mercer 


Long term...previous discussion follows...extended models 
consistent with the ridge shifting east and the next organized 
system moving into western Washington on Wednesday. Have gone ahead 
and raised the probability of precipitation into the likely category for Wednesday. Some 
differences in the models with regard to surface features Thursday 
through Saturday but there is a general agreement that western 
Washington will be under the influence of moist southwesterly flow 
aloft. Hard to time the individual system as this point so have gone 
ahead with a broadbrush rain likely forecast for the remainder of 
the extended period. Felton 


&& 


Aviation...onshore gradients are quite strong with gales along the 
coast and in the Strait and strong inflow through the Chehalis gap. 
The combination of the flow...stability...and terrain is giving a 
rain shadow to the Seattle area and confining showers to the 
Coast...Mountains...and convergence zone though scattered showers 
are nicking the south sound and areas around the Strait. The air 
mass is cold and unstable enough that heavier showers are providing 
a brief snow flurry or mixed precipitation. The only terminals that 
I see any accumulation of an inch or so at best would be at kpae 
where the convergence zone could linger more effectively. 
However...onshore flow is so strong that the convergence should be 
effectively pushed into the Cascades through Sunday afternoon. 


Layers at 015-025 and around 035-050 should dominate with scattered 
conditions dominating in the shadowed areas and lower ceilings in the 
shower bands. Strongest winds of west 30-40kt will be along the Strait 
of Juan Delaware fuca...though the Puget Sound sound area will see SW 20g35kt. 
Little change until around 00z when conditions slowly improve. 
Thunderstorm potential is low...around 10-20 percent...in this cold 
and unstable regime with the greatest likelihood of a strike being 
over the coastal terrain and in the convergence zone. 


At ksea...sct020 sct050 with infrequent bkn020 over the next 24 
hours. Showers should stay north and S of the terminal and probability 
of snow less than 10 percent. Wind will stay up at 24020g30kt through 
18z before very slowly decreasing. Wind at 040 west-southwest 40-50kt through 
00z Monday. Albrecht 


&& 


Marine...whole gales coast and Strait and strong Small Craft Advisory conditions 
elsewhere will continue through sun as a cold trough and associated 
strong jet stream pushes east and high pressure builds offshore. Combined 
seas 25-28 feet over the coastal waters will develop overnight and 
continue through sun. The high surf warning will be continued along 
the Washington coast. Conditions settle down late Sun night and Monday 
before strong offshore flow develops. Toward midweek a series of 
storm systems swings through the Pacific northwest for another active 
period. Albrecht 


&& 


Avalanche...previous discussion follows...currently have mostly 
small soft slabs overlying buried weak snow layers...which top off 
very large amounts of recent unconsolidated snow that also pose 
suffocation threat. Also some old deeply buried crusts with weak 
layers lurking nearby. Next front and following very strong west 
winds late Saturday and Sunday should force major orographic 
snowfall and build progressively larger and more sensitive wind slab 
over the weakness noted above. This should produce generally 
increasing and mostly high danger Saturday night through Sunday. 
Appropriate watches and warnings have been issued. Expect very 
sensitive snowpack and some large slide releases...some may trigger 
all of the recent snow to the early December crust...though this may 
be primarily confined to East Slope locations as deepening snowpack 
west side and more recent crust development may help to limit such 
large slides west of the crest. Bottom line...travel in avalanche 
terrain not recommended Saturday night through Sunday so you will be 
around to enjoy the New Year. Details at nwac.US. Moore 


&& 


Sew watches/warnings/advisories... 
Gale Warning coast and Strait of Juan Delaware fuca. 
High surf warning coast tonight and Sunday. 
Small Craft Advisory for severe bar conditions on Grays Harbor bar. 
Small Craft Advisory north inland waters...Admiralty Inlet...Puget Sound 
sound and Hood Canal. 
Avalanche warning for Saturday night and Sunday for Washington 
Cascades...Olympics and Montana Hood area. 
Winter Storm Warning Olympics tonight and Sunday. 
Winter Storm Warning Cascades tonight through Sunday night. 


$$ 


Weather.Gov/Seattle