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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Thursday
• 
• High: 61 °
• Low: 48 °
• T-Storms

• Friday
• 
• High: 66 °
• Low: 50 °
• Chance of Rain

• Saturday
• 
• High: 64 °
• Low: 50 °
• Chance of Rain

• Sunday
• 
• High: 64 °
• Low: 54 °
• Partly Cloudy

• Monday
• 
• High: 68 °
• Low: 55 °
• Chance of Rain

Area forecast discussion 
National Weather Service Seattle Washington 
915 am PDT Thursday may 23 2013 


Synopsis...a slow moving cold upper-level low will continue to 
produce showers with isolated afternoon and evening thunderstorms 
mainly over the mountains today. The upper low will wobble around 
the Pacific northwest into the Memorial Day weekend as it slowly 
fills and weakens. The cool air mass will gradually moderate into 
the weekend and showers will gradually diminish starting tonight. 


&& 


Short term...upper level low is spawning widespread showers over 
the coast and southern areas with scattered showers elsewhere. There 
are notably more sun breaks today and high temperatures should be 
higher than yesterday...mostly 55-60. As the low pivots east then 
south western Washington will be under an area of divergence aloft. 
Models show negative lifted indices this afternoon and evening 
mainly over the Cascades but also to some extent over the interior 
lowlands. Isolated thunderstorms with small hail are possible. 


Both the NAM and the GFS lift the upper low northeast and fill it on 
Friday. This would keep scattered showers in the forecast especially 
over the mountains. Then these models dampen the upper low further 
into a broad upper trough on Saturday and just zonal flow Sunday. 
The Euro is not in complete agreement with these ideas and keeps 
some semblance of the upper low over the area through Sunday and 
beyond. However the latest run of the Euro has gone some way toward 
the other models. So the forecast of slow warming and lowering of 
probability of precipitation as time progresses has some support. Highs over the interior 
will be in the low 60s Friday and the middle 60s Saturday and Sunday. 
Some sort of weak system is likely Monday. Burke 


Long term...previous discussion...another upper level low will 
bring more rain to western Washington on Memorial Day. But there are some 
timing differences in the models. The GFS is the wetter model. The 
European model (ecmwf) is slower with precipitation holding off until the afternoon...and 
mainly affecting areas south. So exact details are still unclear at 
the moment. Models keep a longwave trough over the region through 
midweek for additional showers with temperatures near or slightly below 
normal. 33 


&& 


Aviation...a deep upper low centered over western Washington this 
morning will remain over the region through tonight. Flow at the 
surface and aloft will be generally light and variable. The air mass 
will be moist and weakly unstable. Showery precipitation is mainly 
along the coast and in the southwest interior this morning. It will 
continue through tonight especially over the southern two-thirds of 
the forecast area. There is roughly a 15 percent chance of 
thunderstorms this afternoon and evening. 


There was a wide mix of conditions across the forecast area at 8 am 
-- IFR due to fog at kbli where skies were clear part of the 
night...low-end VFR and MVFR over most of the Puget Sound sound region... 
and mainly lower conditions in showers along the coast and in the 
southwest interior. This kind of variability will be the rule 
through the forecast period...making confidence in forecast details 
rather low. 


Ksea...south to southwest wind 4-8 knots. Roughly a 10 percent chance 
of a thunderstorm this afternoon and evening. Mcdonnal 


&& 


Marine...a 1014 mb surface low will move slowly southward across 
the Washington offshore waters today...then weaken further as it 
moves southeast into Oregon tonight. This feature is producing south 
to southwest winds up to 20 knots over the coastal waters this 
morning...and the plan to keep winds 20 knots or less -- ie below Small 
Craft Advisory threshold -- looks correct. 


Onshore flow will develop Friday and continue Saturday and Sunday. 
Small Craft Advisory west winds are possible in the central Strait 
of Juan Delaware fuca and East Entrance Strait each day during the late 
afternoon and nighttime hours. A front will probably approach the 
Pacific northwest from the offshore waters Monday. Mcdonnal 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...none. 


&& 


$$ 


You can see an illustrated version of the forecast discussion at 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html