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fxus66 ksew 212205 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
300 PM PDT sun Mar 21 2010 


Synopsis...a cool upper level trough will move inland tonight. 
Showers will taper as the shifts east on Monday with an upper level 
ridge building off the coast. The ridge will slowly move over the 
area Tuesday and Wednesday...bringing dry weather. The next front 
will bring unsettled weather Wednesday night through Friday. 


&& 


Short term...satellite imagery shows clouds and sun breaks and 
radar shows scattered showers. There will be a minor surge of 
onshore flow down the Strait this evening...which should get a Puget Sound 
sound convergence zone going. This will likely remain in southern 
Snohomish and northern King counties. Have put this pscz signature 
in the grids and forecasts...with higher probability of precipitation in that area amid a 
sea of likely probability of precipitation. 


The convergence zone could keep going in the morning...but elsewhere 
showers will taper Monday as the upper trough moves east. Both an 
upper and a surface ridge begin nosing into western Washington in 
the afternoon and there should be at least partly sunny conditions 
under a field of cumulus clouds. Highs Monday will be in the 50s. 


Real drying gets underway Tuesday and Wednesday. There will also be 
a warming trend as upper heights build and offshore surface flow 
develops. Have highs as mostly 55-60 on Tuesday and well into the 
60s on Wednesday. Latest GFS and Euro runs keep the next system well 
to the southwest through Wednesday so mostly sunny be the rule. Burke 


Long term...both the GFS and Euro spread a little rain into the 
area Wednesday night...the Euro is slower. Trend in the models 
lately has been to slow this system and the Euro also takes the bulk 
of it into Oregon. Have kept climatological probability of precipitation for Wednesday night 
and boosted probability of precipitation into the likely category for Thursday. Showers then 
taper Thursday night...and have low chance probability of precipitation for Friday and 
Saturday as the GFS builds a strong ridge and the Euro dries out 
somewhat. Back to climatological chance probability of precipitation for Sunday as both 
models drag a front through the area. Burke 


&& 


Aviation...southwest flow aloft over western Washington will veer 
to northwest late tonight as the upper trough offshore moves inland. 
An upper ridge will progress over the area on Monday. Scattered 
showers continue across the forecast area this afternoon. A weak 
Puget Sound sound convergence zone will develop over the north Puget Sound sound 
area this evening...then gradually shift southward and dissipate 
Monday morning. 


Ksea...south or southwest wind 8-14 knots. The convergence zone will 
shift southward from the north Puget Sound sound tonight...but it will 
likely dissipate Monday morning before reaching ksea. Mcdonnal 


&& 


Marine...surface flow has turned onshore more weakly than expected 
but marginal Small Craft Advisory winds are still likely for a 
period in the central Strait and East Entrance this evening. 
Otherwise pressure gradients are generally decreasing...and small 
craft advisories for the remaining waters will be allowed to expire 
at 6 PM this evening. A Puget Sound sound convergence zone will form over 
the north Puget Sound sound area late today or this evening...then shift 
slowly southward later tonight and dissipate. 


Small craft advisories will be in effect for the coastal waters for 
hazardous seas...as well as for Grays Harbor for rough bar 
conditions...tonight and Monday. 


Stronger high pressure will build over the offshore waters tonight 
and Monday...maintaining weak onshore flow. The high will shift 
inland on Tuesday...and offshore flow will strengthen on Wednesday 
ahead of the next approaching frontal system. That feature will move 
through the forecast area Wednesday night and Thursday. Mcdonnal 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory until 6 PM coastal waters...West Entrance 
Strait...northern inland waters...Puget Sound sound and Hood Canal. 
.Small Craft Advisory until midnight central Strait...East 
Entrance. 
.Small Craft Advisory tonight and Monday coastal waters for 
hazardous seas. 
.Small Craft Advisory tonight and Monday Grays Harbor for rough 
bar conditions. 


$$ 


Www.Weather.Gov/Seattle 


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