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fxus66 ksew 190948 cca 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
300 am PDT Friday Mar 19 2010 


Synopsis...an upper level ridge will dominate the Pacific northwest 
through tonight. The ridge will shift inland Saturday as cold front 
and upper trough approach the region. Cool moist and unstable air 
following the cold front will maintain a showery pattern Sunday and 
Monday. High pressure will build on Tuesday bringing a period of dry 
weather before the next front Ushers in unsettled weather Wednesday. 


&& 


Short term...a strong ridge of high pressure will dominate the Pacific 
northwest through tonight...then shift eastward on Saturday. Low level offshore 
flow will maintain dry air at the lower levels...inhibiting stratus 
and fog for the most part. The exception will be prone valleys of 
the SW interior where radiational cooling will result in patchy fog 
this morning. Otherwise...mostly clear skies through tonight. High 
temperatures will warm to above average today and Saturday ahead of a front 
which will approach the area Saturday afternoon. The warmest day 
along the coast will probably be today as the flow becomes onshore 
there by midday Saturday cutting off warming there. However...the 
warmest day across the interior will probably occur Saturday 
afternoon before the switch to onshore flow occurs. High temperatures could 
reach into the middle and upper 60s...a good 10+ degrees above the 
average. 


A front moves into western Washington Saturday night but appears to lack 
moisture initially. The 00z models have backed off slightly with quantitative precipitation forecast 
and suggest the best chance for light rain across the interior will 
occur after midnight Saturday night as more energy rounds the trough 
and energizes the front. Made some slight adjustments to push back 
timing of precipitation across the interior until after midnight. The upper 
trough will follow on Sunday with light rain becoming showers as 
cooler air begins to filter into the area. 


Long term...the upper trough remains parked over the Pacific northwest Sunday 
night and Monday with a cool...unstable...showery pattern in place. 
Snow levels will fall below 3k feet but accumulations appear light 
with not much flow over the area. Could see a strike or two but will 
hold off on putting any thunder in the forecast. Drier northwest flow 
develops Monday night as the trough begins to pull east allowing 
showers to decrease. A temporary upper ridge transverses the area on 
Tuesday with a period of dry weather expected. The progressive 
pattern continues into middle week with the next trough and associated 
front quickly moving into western Washington on Wednesday. While not 
strong...this system will usher in moisture...cool temperatures...and a 
chance of showers. Mercer 


&& 


Hydrology...the flood risk across western Washington and for the 
Green River is very low for the next 10 days. 


High pressure will maintain dry weather across the area through 
Saturday. A frontal system will move across western Washington Saturday night 
followed by a cool trough Sunday and Monday. This system will lack 
substantial moisture with weak flow not aiding orographics. The snow 
level will also fall below 3 knots feet by Sunday night. Bottom 
line...light rainfall/snowfall amounts are expected with no flooding 
concerns. Another system on Wednesday will also be fast moving and 
rather weak. Rainfall amounts will also be light with this system. 
Mercer 


&& 


Aviation...an upper level ridge continues to build offshore with 
northwest flow aloft over the region. Near the surface...low level 
offshore flow will increase through early Saturday. VFR prevails 
through the forecast period except for localized valley stratus 
early this am. 


Ksea...VFR. North to northeast wind through the forecast period. 


&& 


Marine...offshore flow will increase today in response to a surface 
ridge building over southern British Columbia. The offshore flow 
will strengthen further tonight into Saturday as pressures fall over 
the northeast Pacific. Winds might approach gale force near the West 
Entrance to the Strait early Saturday morning. A cold front will 
sweep across the area Saturday night with probable Small Craft 
Advisory wind for much of the waters. Onshore flow will persist 
behind the front Sunday into Monday as a surface ridge builds over 
the waters. 27 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory in effect for West Entrance to the Strait. 


$$ 


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