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fxus66 ksew 201538 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
830 am PDT Sat Mar 20 2010 


Synopsis...an upper level ridge will give one more day of sunny 
and dry weather. A weak cold front will approach the coast tonight 
and move inland Sunday. Light rain tonight will change to scattered 
showers Sunday afternoon...which will then taper Monday as an upper 
trough moves east. High pressure will build on Tuesday bringing dry 
weather. The next front will bring unsettled weather Wednesday night 
through Friday. 


&& 


Short term...no changes to the forecast. Mostly sunny skies today 
with an upper ridge just to the east and surface offshore flow. High 
temperatures will be mostly in the 60s. Models are consistent in 
bringing a weak front toward Washington...developing a little rain 
along the coast early tonight and spreading a chance of rain inland 
late tonight. A broad upper trough will make showers likely Sunday. 
Showers will taper Sunday night into Monday. High temperatures on 
Sunday and Monday are likely to be at least 10 degrees lower than 
today...in the low to middle 50s. Burke 


Long term...previous discussion...models have been consistent in 
building an upper ridge Monday night which transverses the area on 
Tuesday. This will bring a period of dry and slightly warmer 
temperatures to the area through Wednesday. The GFS has slowed the 
timing of the next system...now shown to move into western Washington Wednesday 
night. The models have also trended deeper with the associated 
surface low...down to 990 mb tracking into southern Vancouver Island. 
This could result in breezy to locally windy conditions. Will need 
to keep a watch on model trends with this system although I suspect 
the development may be overdone. Additional weaker system will move 
through the area Thursday and Friday with a continued threat of 
showers and cool temperatures. Mercer 


&& 


Aviation...southwest flow aloft will continue through tonight...as 
an upper level ridge just east of the Cascade crest progresses to 
The Rockies and a frontal system over the offshore waters moves into 
western Washington. At the surface weak northerly offshore flow will 
shift to southerly today with the approach of the front. The air 
mass will remain stable and dry today...then become moist all levels 
tonight. Light precipitation will reach the coast around 6 
PM...develop over the Olympics and Cascades later this evening...and 
spread to the interior lowlands late tonight. Ceilings will likely 
deteriorate to something like scattered-bkn025 broken-ovc050 around 3 am 
Sunday. 


Ksea...northeast wind 6-10 knots...switching to southerly 8-14 knots middle 
afternoon. Occasional MVFR ceilings will probably develop around 3 
am Sunday. Mcdonnal 


&& 


Marine...northerly offshore flow will shift to southerly midday 
today as a frontal system offshore approaches western Washington. 
The easterly component of the pressure gradient has begun to 
ease...and I will allow the Gale Warning for the West Entrance 
Strait of Juan Delaware fuca to expire at 9 am. The incoming cold front 
will move across the forecast area tonight and early Sunday...with 
Small Craft Advisory winds spreading to the East Entrance Strait... 
Admiralty Inlet...and northern inland waters. Onshore flow will 
develop behind the front tonight through Monday as high pressure 
builds offshore. The high will shift inland on Tuesday...with the 
next frontal system approaching the region from the west on 
Wednesday. Mcdonnal 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory coastal waters...West Entrance Strait of 
Juan Delaware fuca. 


$$ 


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