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Area forecast discussion 
National Weather Service Seattle Washington 
900 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...a weak warm front clipping the area this morning is 
bringing spotty light rain to much of the area. The precipitation came in 
quicker and further inland than was expected yesterday. Even the new 
12z NAM is dry this morning with the exception of the coast. The 
GFS/European model (ecmwf) appear to have a better handle on this showing some light 
quantitative precipitation forecast this morning...intensifying during the afternoon as the main 
front approaches. Quantitative precipitation forecast amounts appear highest on the coast/olympic 
peninsula where up to 1 inch of rain could fall. Much less amounts 
expected elsewhere with snow levels expected to rise through the 
day..rising to over 5000 feet by this evening. Snow accumulations in 
the passes should remain below advisory criteria...maybe up to a few 
inches this evening before changing to rain. 


The front dissipates over western Washington under a building upper ridge 
tonight. There could be some residual light rain/drizzle and low 
clouds hanging in through Tuesday morning. Could end up being a 
rather murky day Tuesday with subsidence aloft and residual moisture 
trapped in the lower levels. Quantitative precipitation forecast will be light with higher snow 
levels...thus snow in the passes is not expected. 


High pressure builds stronger over the area Tuesday night and early 
Wednesday...then shifts eastward Wednesday afternoon as the next frontal 
band approaches. It looks like precipitation should hold off during the day 
The Heights and rising thickness values indicating warmer air aloft. 
Precipitation associated the the frontal band could start to affect the 
olympic peninsula and coast by late in the day. Mercer 


Long term...previous discussion follows...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 ... 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. 


Models differ on the Thursday system. The NAM is less progressive 
and hangs the frontal boundary up over the olympic peninsula. 
Rainfall rates do not look incredible but could be enough for minor 
flooding concerns on the Skokomish River. The other models are 
weaker and much more progressive with the front and would not imply 
any flood threat. 


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


&& 


Aviation...an upper level trough crossing southern British Columbia 
will push a weakening cold front to the Washington coast around 00z. Air 
mass is stable with gradually lowering ceilings in rain expected ahead 
of the weak front. The front will move inland after 00z then 
eventually dissipate over SW Washington 06z-12z. 


Latest observation show ceilings generally broken-0vc040-060 over the interior with 
some spotty MVFR still in the process of dissipating...with 
localized light rain interfering with this. Will show a minor break 
in the rain and keep ceilings up through about 22z based on an apparent 
break in the higher clouds on infrared imagery. Should see ceilings lower with 
more widespread rain toward 00z as the main front approaches. 


Ksea...will keep ceilings broken-ovc040-060 for most of the day then 
lowering to broken-ovc020-030 after 03z. Not completely confident that 
the ceilings will get down to bkn020 with the front so weak...but the 
winds will weaken tonight as well which would allow ceilings to lower a 
bit more. Expect surface winds to remain S through this evening. Kam 


&& 


Marine...the weakening cold front was up around 49n/128.5w around 
16z this morning. Models bring it to the north coast late this 
afternoon...12z NAM a little faster than the 12z GFS. Model 
solutions weakening this front as it moves inland seem on track 
since water vapor imagery shows what appears to be the last 
shortwave moving up the front almost to Queen Charlotte Sound. Main 
question at this point is how strong the winds will be. Should see a 
little strengthening in the winds through early afternoon...then 
weakening. Will leave the gales coast/West Entrance up and Small Craft Advisory 
elsewhere except central Strait. Forecast advertises Small Craft Advisory west winds in 
the Strait behind the front tonight...but neither the GFS or NAM 
support that. Will hold off on the advisories and see what the 12z 
WRF-GFS shows. Winds should be below advisory levels 
Tuesday...except for the coast. Kam 


&& 


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

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