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afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
920 am PDT Sat Sep 6 2008 


Synopsis...an upper level ridge located offshore will amplify 
northward on Saturday and skies will clear in the early afternoon. 
Temperatures are expected to reach near or just above seasonal 
normals. Sunday will be warmer as a thermally induced trough of low 
pressure creeps up the Washington coast and the surface flow turns 
offshore. Monday and Tuesday will be cooler as a dissipating weather 
system brushes the area. High pressure with offshore flow will bring 
warmer temperatures Wednesday and Thursday. 


&& 


Short term...this morning's visible satellite shows areas of low 
cloud and fog on the coast...Strait...and from portions of the Puget Sound 
sound southward. Nice hole of sunshine exists from Edmonds to South 
Seattle. Closer to the Canadian border...two layers of stratus 
around 020 and 060 are in place. Time heights and soundings show 
skies breaking up after 18z. Due to the cloud cover...overnight 
temperatures are running a few degrees warmer then this time 
yesterday. With 500 mb heights around 580 dm...may increase today's 
maximum t's a degree or two with a morning update. 


Also of concern is the possibility for for fog tonight. Thermally 
induced trough of low pressure will build northward up the coast early 
Sunday...with the flow taking on a weak offshore component. Despite 
a decent northerly gradient through the sound...timeheights show the 
lower atmosphere decoupling with the stronger north winds aloft. Fog 
that does form tonight will burn off quickly Sunday. Temperatures 
look to be in good shape on Sunday...so no major changes planned. 


Offshore flow will continue through Monday morning...and then 
onshore flow begins sometime late Monday afternoon. Monday should 
still be a nice day with temperatures just a few degrees cooler due 
to cooling aloft and marine influence only through the coast by 
early Monday afternoon. Weak northwest push will move clouds in 
Monday night...and heights will drop further on Tuesday as a 
shortwave trough digs into southern British Columbia. Damico 


Long term...previous discussion follows...the trough moves east Tuesday 
night with high pressure rebuilding over the area Wednesday through Friday 
(pic5). Thermal low pressure will likely form along the coast with 
another period of light offshore flow. Expect dry weather and warmer 
temperatures. A weak system may brush the area by next weekend bringing 
moderating temperatures. However...no precipitation is anticipated. Mercer 


&& 


Aviation...an upper ridge along about 137w is bringing northwest flow 
aloft. A weak short wave trough sliding southeast through southern British 
Columbia and eastern Washington is bringing some clouds at around 
060 in the interior. In the mean time light pressure gradients and 
radiational cooling overnight is giving abundant stratus and fog to 
The Lowlands with exception to the central Puget Sound sound from about 
north of SeaTac Airport to south of Everett. 


Low level moisture will evaporate from the edges between 18z and 20z 
while the ceilings around 060 slide east into the Cascades and 
dissipate. Just a few cumulus are expected later this afternoon and 
evening. 


Models still show the development of a thermally induced trough of 
low pressure northward from western Oregon into the interior of 
western Washington Sunday into Monday as offshore flow increases. 
Unfortunately...it appears that there will be sufficient time 
tonight in light NE gradients to allow the boundary layer to invert 
for the formation of a shallow but moderately extensive area of fog 
and very low stratus... including some Lake Washington stratus east 
of I-5 moving toward SeaTac Airport. Many of the terminals will 
likely see a 2-4 hour period of stratus and fog Sun morning...and 
some visibilities will likely be 1/2sm or less in fog. 


Because of the increasing offshore gradients Sunday morning... 
insolation should lift out and burn off low level moisture 
early...by 17z. Severe clear will follow. 


Ksea...the terminal is on the edge of a low stratus and fog deck 
with areas just to the south reporting visibilities around 1sm br 
and ceilings below 005. Conditions will burn back from the north by 
terminal issuance time leaving decreasing sct-bkn050. Winds this 
afternoon 34006kt becoming 02005kt 04z Onward. Expect very low 
stratus and fog to form in the valley NE of the terminal around 
12z...waft SW into the terminal between 13z and 17z...then rapidly 
evaporate after 17z. Visibilities between 14z and 17z may 
occasionally drop below 1/2sm fog. Albrecht 


&& 


Marine...strong high pressure along about 137w will give northwest flow to 
the coastal waters through early next week. Winds will range from 10 
to 20 knots but may occasional reach Small Craft Advisory levels in the 
outer waters at times later sun into Monday. In the interior expect 
weak pressure gradients to give light northwest follow today. 


Northerly flow in the interior will increase to 10 to 20 knots and 
possibly to Small Craft Advisory level sun and Sun night as a 
thermally induced trough of low pressure forms over western Oregon then 
develops north into western Washington Sun afternoon into Monday. Winds 
will becoming light in the interior Sun night and Monday as the trough 
shifts east of the Cascades and weakens. Moderate onshore flow will 
develop Monday evening in the wake of the thermal trough. Albrecht. 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...none. 


$$ 
Weather.Gov