Weather

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afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
300 PM PDT Sat Oct 11 2008 


Synopsis...a strong upper ridge offshore will make its way toward 
western Washington tonight and Sunday...keeping the area dry but 
allowing some high clouds to spread inland. The ridge will move 
inland Sunday night...and a Pacific frontal system will move through 
Monday and Monday night. Westerly flow aloft will bring another weak 
frontal system into the region later in the week. 


&& 


Short term...dry northerly flow aloft continues over the forecast 
area courtesy of the high amplitude upper ridge that extends from 
the offshore waters into British Columbia. Surface high pressure 
over southern British Columbia is producing weak northerly offshore 
flow at low levels. There is a Pacific frontal system immediately 
upstream of the ridge axis...and high cloudiness ahead of this 
feature is already moving onto the north coast. 


The upper ridge will keep western Washington dry tonight and Sunday 
as it continues to move slowly eastward. However high cloudiness 
will gradually spread across the forecast area tonight. Minimum 
temperatures will be cool tonight...but the cloud cover should keep 
temperatures a little milder than last night. I will keep patchy fog 
in the forecast...but it does not look like another frost advisory 
will be necessary. 


The models remain consistent and in broad agreement that the upper 
ridge will progress through western Washington Sunday night and 
across eastern Washington into western Montana on Monday. The weak 
warm front of the system upstream will brush the area Sunday night 
bringing a chance of rain to the coast and Olympics. 


The cold front will move through the forecast area Monday afternoon 
and evening...and it looks somewhat stronger in the 12z suite of 
model runs. So I nudged probability of precipitation upward into the likely to categorical 
range for Monday evening. Precipitation will switch to showers and 
taper off behind the front Monday night. I kept a low end chance of 
showers in the forecast Tuesday...though the threat looks minimal. 
Mcdonnal 


Long term...the longer range models agree on a break in the weather 
Tuesday night and Wednesday. Beyond that though...the solutions 
diverge. The scenario in the GFS has a weak warm front brushing the 
area Wednesday night...a cold front moving through on Thursday and 
Thursday night...with a weak upper trough bringing a chance of 
showers Friday through Saturday. I leaned in this direction but 
undercut the GFS in probability of precipitation to account for drier solutions in the other 
models. In particular the European model does not bring in the 
frontal system until Friday or Friday night. Mcdonnal 


&& 


Aviation...upper ridge offshore with dry northerly flow aloft over 
Washington. Surface high pressure in southern British Columbia with light offshore flow across 
Washington. Air mass dry and stable except increasing high level moisture 
spilling over the upper ridge. 


Satellite imagery shows higher clouds spilling over the offshore 
upper ridge. Expect enough cloud cover overnight to minimize fog on 
Sun morning. Best spot for fog will be the south sound and nearby 
river valleys..again. Otherwise..VFR weather through sun. 


Ksea...fog again quite likely to miss the terminal Sun morning. 
Otherwise..passing higher level clouds tonight and sun. Northerly wind 
6-10 knots this evening to become light by 06z through at least 18z sun. 
Buehner 


&& 


Marine...light offshore flow to ease tonight as the 1032mb surface 
high over southern British Columbia drifts east. A weak warm front will approach the 
coast Sun night and Monday followed by the associated cold front Monday 
night. Expect low level flow to turn onshore Sun night/Monday with 
possible Small Craft Advisory winds later Monday along the coast..parts 
of the Strait and northern inland waters along with building coastal 
water swells to near 10 feet. Buehner 


&& 


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


$$ 


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