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fxus66 ksew 221025 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
325 am PDT Monday Mar 22 2010 


Synopsis...scattered showers will continue today in the unstable 
air mass over western Washington. Showers will decrease this evening 
as an upper level ridge builds just offshore tonight. The ridge will 
move over the area Tuesday and over eastern Washington Wednesday 
producing mostly sunny weather and a warming trend. Rain will return 
late Wednesday night and Thursday as a frontal system moves 
northeast across the area. An upper level trough will bring showers 
behind the front Thursday night and Friday. 


&& 


Short term...the upper level trough over the Cascades at 12z will 
not be a factor today as it moves east of the area. Even though an 
upper level ridge is building just offshore...NAM bufr soundings 
show the lower air mass remaining unstable up to about 10000 feet 
today...so there will still be scattered showers around. Latest 06z 
models are a little more stingy with the quantitative precipitation forecast coverage this afternoon 
than the 00z runs...so maybe the shower activity will end up being a 
little less than expected. The surface pattern still supports some 
weak convergence zone activity today...especially this afternoon. 


Have left in chance probability of precipitation over mainly the interior this evening...but 
the showers should diminish quickly around sunset as the upper level 
ridge moves inland and the air mass stabilizes. Enough moisture 
should still be around for some stratus Tuesday morning...both 
NAM/GFS show this on the ksea time-height section. Otherwise mostly 
sunny Tuesday afternoon under the ridge. 


Models agree on moving the ridge and the associated surface high 
east Tuesday night...and bringing an offshore frontal system toward 
the area...so offshore flow will begin increasing. Should be another 
nice sunny warm day Wednesday due to the offshore flow. 850 mb temperatures 
around 8c and MOS guidance support highs in the lower to middle 60s. SW 
flow aloft should bring a lot of cirrus over the area...probably 
overcast but thin...so it will be a filtered sunshine day. Kam 


Long term...GFS/ECMWF remain on track with bringing the next 
frontal system NE over the area late Wednesday night. Models have 
been trending slightly slower...so probability of precipitation were lowered a little over 
the north Cascades at the request of weather forecast office Spokane. Both models have the 
surface front over the interior 12z Thursday...so the precipitation on 
Thursday will be caused by the upper level trough moving north across 
the area. 00z GFS/European model (ecmwf) were in good overall agreement through the 
weekend...although small differences may eventually have impact on 
the chance of precipitation. 


A second shortwave trough moves east across the area Thursday night and 
Friday for showers. An upper level ridge will be building inland 
right behind fridays trough...so expect showers to taper off Friday 
evening similar to this evening. The upper level ridge has good 
potential to produce a dry day Saturday...but the European model (ecmwf) shows a 
little rain over the coast...so no dry day in the forecast yet. Both 
models have another front arriving Sunday...but the European model (ecmwf) is 
stronger and wetter. Kam 


&& 


Aviation...northwesterly flow aloft over western Washington today 
as an upper level trough moves east of the area and an upper level 
ridge builds offshore. Convergence zone over the northern and 
central sound at 10z will dissipate in the late morning hours. 
Ceilings within the convergence zone in the 1000-2500 foot range 
with isolated visibilities 3-5sm in rain showers. North of the 
convergence zone scattered showers with ceilings in the 3000-4500 
foot range until middle to late morning then ceilings lifting and 
shower activity coming to an end. South of the convergence just 7000 
scattered to broken. All areas this afternoon scattered to broken SC 
deck at or above 5000 feet. Skies will clear out tonight as the 
upper level ridge builds inland. 


Ksea...south or southwest wind 8-12 knots through this morning 
decreasing this afternoon. Ksea near the southern end of the 
convergence with a scattered to broken layer around 2000 feet 
through the morning hours. Ceilings at or above 5000 feet this 
afternoon. Felton 


&& 


Marine...weak onshore flow today with surface high pressure over 
the coastal and offshore waters. The high will shift inland tonight 
with flat gradients on Tuesday. Offshore flow will strengthen on 
Wednesday ahead of the next approaching frontal system. That feature 
will move through the forecast area late Wednesday night and 
Thursday with Small Craft Advisory winds expected over the coastal 
waters. High pressure will rebuild behind the front on Friday. Felton 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory tonight and Monday Grays Harbor 
for rough bar conditions. 


$$ 


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For an illustrated version of the forecast discussion please see 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html (all lowercase).