095 
fxus66 ksew 220436 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
936 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...moderate low level onshore flow in conjunction with a 
short wave upper level trough will maintain scattered showers 
across the region tonight. May see an uptick in shower activity 
around Admiralty Inlet area and Snohomish County where the Puget Sound 
sound convergence zone lies. The nam12 shows the pscz shifting south 
into King County early Monday a.M. The uw WRF GFS shows the convergence 
zone dissipating in the north Cascades with winds remaining 
southerly through the central sound. In either case...will see 
showers taper down by Monday afternoon as the upper level trough axis 
moves east of the Cascades. Will see sun breaks in the cumulus field with 
highs in the 50s. 


An upper level ridge will build inland Monday night into Tuesday providing 
another break in the weather. Some morning fog...otherwise mostly 
sunny skies with highs on Tuesday back into the 50s to lower 60s. The 
ridge will begin to shift east of the crest on Wednesday as the next 
system approaches. Should squeeze in one more dry day with 
increasing high clouds Wednesday afternoon and evening. Temperatures nudged a few 
degrees warmer on Wednesday with 60s plentiful. 33 


Long term...previous discussion...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 evening with a band 
of heavier showers along the South Coast moving through the 
southwest interior in the next couple of hours. Ceilings as low as 
2000 feet with visibilities down to 3 sm in moderate rain showers 
with this feature. Outside of this feature multiple cloud decks 
between 4000 and 10000 feet overnight into Monday morning. A weak 
Puget Sound sound convergence zone over the north Puget Sound sound area this 
evening will drift south and dissipate Monday morning. Ceilings down 
around 2000 feet possible central sound in the dissipating 
convergence zone 14z-18z Monday morning. Scattered to broken SC deck 
around 6000 feet Monday afternoon. 


Ksea...south or southwest wind 8-12 knots through Monday morning. 
Ceilings near or above 5000 feet overnight with ceilings lowering to 
near 2000 in the 15z-18z time frame. The 2000 foot deck will 
dissipate midday Monday. Felton 


&& 


Marine...weak onshore flow overnight into Monday with surface high 
pressure over the coastal and offshore waters. Small Craft Advisory 
westerlies in the central and eastern Strait will ease by midnight. 
High pressure will remain over the coastal waters Monday maintaining 
the light low level 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. Felton 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory until midnight central Strait...East 
Entrance. 
.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).