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Area forecast discussion 
National Weather Service Seattle Washington 
300 am PST Monday Nov 23 2009 


Synopsis...a warm front will bring some rain to western Washington 
today...mainly along the coast and in the north. A weak cold front 
will move inland tonight. Weak high pressure will move across the 
area Tuesday. A weak warm front will move through the area Tuesday 
night...followed by another weak cold front Wednesday night and 
Thanksgiving day....followed by an upper trough on Friday. 


&& 


Short term...the vigorous system that was moving into the Pacific 
northwest 24 hours ago is now long gone...making its way east of the 
northern rockies. An upper level ridge has moved over the Pacific 
northwest overnight -- the beginning of a trend toward a high 
amplitude...less progressive...and much less eventful weather 
pattern that will continue through the next week. Skies across 
western Washington are generally cloudy this morning with very 
spotty light precipitation...as the warm front of a system 
immediately upstream of the ridge is moving into the forecast area. 
Temperatures are in the lower to middle 40s. 


The upper ridge will progress eastward today across the Cascades and 
almost to the eastern border of the state. This will allow the 
frontal system just offshore to move slowly inland today. It will 
bring fairly significant precipitation amounts -- in the .50 to 1.25 
inch range -- to the coast and olympic peninsula...with somewhat 
less over the north Cascades. But the front will weaken as it moves 
through the area tonight...and precipitation totals for the interior 
lowlands will mostly be less than 0.25 inch. The snow level...which 
is starting out around 2500 feet...will climb to around 6000 feet 
tonight. 


As the front falls apart and moves east of the Cascades Tuesday... 
another stronger upper ridge will move over the Pacific northwest. 
The ridge will send 500 mb heights to around 5700 meters. The warm 
front of another frontal system just upstream of this ridge will 
keep a chance of light rain in the forecast Tuesday. 


By Tuesday night the upper ridge axis will be east of the Cascades 
and it will move slowly eastward to The Rockies on Wednesday night. 
Models show the frontal band translating very slowly eastward as it 
stretches south-southwest from Vancouver Island. It will keep a 
chance of rain in the forecast for western Washington...but 
precipitation will be generally very light where we get any...and 
the main threat of precipitation will be along the coast and over 
the olympic peninsula. The snow level will be 7000-8000 feet. Mcdonnal 


Long term...the models disagree on how the pattern will change in 
the longer range. The GFS shows a much drier solution while the 
European model is wetter. So confidence in the extended forecast 
Thanksgiving day through Sunday is rather low and the approach is a 
broad brush. In a very general sense they agree that Thanksgiving 
day through Friday night will be wetter as the frontal system 
sitting offshore Wednesday moves through the area. And they agree 
that an upper ridge will build over the area over the weekend 
..reducing the threat of wet weather to a chance. Mcdonnal 


&& 


Hydrology...the Skokomish River remains just above flood stage 
early this morning. The Current River model forecast shows it 
dropping below flood stage later today...but the actual result will 
depend heavily upon the operation of Cushman dam #2 on the North 
Fork of the river. A planned release of water from the dam began 
early Saturday and continues early this morning. 


Two frontal systems will move through the area through Wednesday 
night...and the snow level will gradually climb to 7000-8000 feet. 
These systems will not bring enough precipitation for flooding 
concerns on any western Washington rivers...with the possible 
exception of the Skokomish River. 


The forecast for Thanksgiving and beyond is less certain...but at 
this time there does not appear to be any threat of flood-producing 
rains. 


No flooding is expected on the Green River at any time during the 
forecast period. 


&& 


Aviation...the tail end of an upper trough will brush by this 
afternoon...pushing a weakening cold front across western Washington. Fog is 
becoming increasingly unlikely this morning given the thickness of 
middle-level clouds and enough wind to mix the boundary layer. An 
offshore component to the low-level flow in advance of the front 
will limit clouds below 040...except near Hood Canal. The front 
should cross western Washington from 00z-06z this evening...accompanied by rain 
and followed by increasing low-level moisture overnight tonight. 


Ksea...ceilings should improve this morning as an offshore low-level 
flow component strengthens in advance of the approaching front and 
inhibits lower clouds. A cold front will pass ksea between 00z-03z on 
Monday evening. Winds aloft should peak around the time of frontal passage...with 
southwest 850 mb winds near 40 knots around 00z. Low-level moisture 
will increase behind the front with ceilings deteriorating to IFR 
tonight and continuing on Tuesday morning. Haner 


&& 


Marine...southerly flow will increase today in advance of an 
approaching front. The wind at buoy 46036 gusted to gale force in 
advance of the front...so am thinking low-end gales should affect 
the coastal waters today. The front will be weakening quickly as it 
moves inland late this afternoon and evening. As a result...not expecting 
the gales along the coast to translate to the north interior. Small 
craft winds over the inland waters are the most likely outcome at 
this point...but will need to be watched. High pressure will build over 
the waters tonight and Tuesday with light to MDT offshore flow developing. 
Haner 


&& 


Sew watches/warnings/advisories... 
Washington...a Flood Warning remains in effect for the Skokomish River 
in Mason County. 
Pz...Gale Warning coast and West Entrance. 
Small Craft Advisory all other waters except the central 
Strait. 
Small Craft Advisory for rough bar conditions at Grays Harbor 
entrance. 


$$ 


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www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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