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fxus66 ksew 221616 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
915 am PDT Thursday may 22 2008 


Synopsis...increasing north flow from an upper level ridge offshore 
will gradually reduce the clouds over western Washington this 
morning...with partly sunny skies expected by late afternoon. The 
giant upper level low over Nevada will start dominating Friday as 
east flow aloft brings middle level clouds and possibly some showers 
over the area by late in the day. East to northeast flow around the 
low will maintain partly cloudy skies and mainly some mountain 
showers over the area into early next week. 


&& 


Short term...the combination of northwest flow aloft and low level 
onshore flow has buried western Washington under clouds again this 
morning. Tops are lower than yesterday...running 080-100 based on infrared 
cloud top temperatures compared to uil sounding and katx VAD wind profile. 
Problem is that bases are running lower as well with IFR/low MVFR 
around much of Puget Sound sound. A good sign is that the VAD winds and 
acars soundings also show north flow from 3000-4500 feet and above. This 
increasing north flow from the large upper level ridge offshore will 
gradually reduce the moisture through the day with partly sunny 
skies expected by late afternoon. Would expect the middle of the 
Puget Sound sound region to gradually clear from the north first...with 
clouds tending to pile up near the Cascades for much of the day. 


Main forecast problem is now Friday afternoon. Models have remained 
consistent now for about a day and a half in slowly retrograding 
westward the giant low that just formed over the SW U.S. Forecasts 
had already counted on increasing east-NE flow aloft and increasing middle 
level moisture on Friday as the low starts to dominate. However 
precipitation has been a question. Latest models show some light rain 
reaching Puget Sound sound Friday afternoon...which is slightly faster 
than previous runs. The moisture source is apparently all the clouds 
over the intermountain west from Colorado up through SW Montana. The 
forcing apparently will be supplied by a shortwave trough currently 
seen in water vapor imagery over New Mexico that will whip around 
the low reaching West Washington Friday afternoon. 12z NAM and 00z European model (ecmwf) show 
the shortwave trough arriving a little slower than the GFS...but 
will still have to add a chance of showers to the interior lowlands 
to account for this feature. Will make the changes in the afternoon 
package. 


GFS/European model (ecmwf) are in pretty good agreement for Saturday as well...so 
there is more confidence in the details. Looks like a second 
shortwave rotating around the main low will bring another batch of 
moisture and forcing to the area Saturday afternoon. Kam 


Long term...previous discussion from the 3 am afd...the upper low 
that will be moving westward to our south will be the main player 
into the extended period. The longer range models are in agreement 
through Tuesday that the upper low will evolve into a larger trough 
on Sunday and drift west of the coast on Monday and Tuesday. The GFS 
shows a northern branch trough moving onto the British Columbia 
coast on Wednesday. I see no reason to change the current forecast 
for the extended period. It reflects the model guidance well and is 
close to climatology -- generally a good idea when confidence in the 
models is not very high. Mcdonnal 


&& 


Aviation...little change from previous discussion. Onshore flow 
continues this morning with cross Cascade gradient plus 8 mb. 
This combined with low level moisture from a convergence zone is 
keeping ceilings mostly in the 1200 to 2500 foot range around the 
Puget Sound sound this morning. Ceilings 1200-2500 across most of western 
Washington except 4500-6000 feet San Juans and northwest interior. Not much 
change expected through the morning hours. Ceilings should gradually 
lift but expect ceilings to remain below 3000 feet through midday. 
Conditions improving during the afternoon hours for the Puget Sound sound 
and along the coast with ceilings up around 5000 feet by middle 
afternoon. 


Ksea...ceilings in the 1200 to 2000 foot range through the morning 
hours with gradual improvement to near 3000 feet midday and to 5000 
feet middle to late afternoon. Light wind 2-5 knots through the morning 
hours into the early afternoon. Wind becoming northerly 8 knots or 
less late afternoon into the evening hours. Prange/Felton 


&& 


Marine...high pressure over the coastal waters with lower pressure 
over eastern Washington will keep the low level onshore flow going 
through tonight. A thermally induced trough of low pressure will 
develop along the coast and over the coastal waters Friday and remain 
through the weekend with light gradients across the area. Winds will 
ease overnight as the flow along the coast becomes more northerly. 
Strong high pressure centered along 140w will lead to weak onshore flow 
on Thursday. Weak surface low pressure will shift east of the Cascades on Friday 
with light winds expected across the area. Light winds will persist 
through the weekend. Felton 


&& 


Sew watches/warnings/advisories... 
Small Craft Advisory coast...central and eastern Strait. 
Small Craft Advisory Grays Harbor bar for rough bar conditions. 


$$ 


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