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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Tuesday
• 
• High: 55 °
• Low: 46 °
• Rain

• Wednesday
• 
• High: 61 °
• Low: 45 °
• Rain Showers

• Thursday
• 
• High: 64 °
• Low: 46 °
• Chance of Rain

• Friday
• 
• High: 63 °
• Low: 48 °
• Mostly Cloudy

• Saturday
• 
• High: 61 °
• Low: 48 °
• Chance of Rain

Area forecast discussion 
National Weather Service Seattle Washington 
315 PM PDT Tuesday may 21 2013 


Synopsis...a cold upper level low will move into Oregon this 
evening then spin around over the Pacific northwest through Friday 
at least. The upper low will give cool and showery weather to the 
area through Thursday. After that temperatures will slowly rise and 
showers will decrease. 


&& 


Short term...upper level low is off the north Oregon coast and most 
of western Washington is in a showery pattern. There have been sun 
breaks this afternoon and this pattern will continue into the 
evening. No thunderstorms have broken out so removed the minor 
mention over southern areas this evening. Models spin the upper low 
around over Oregon and Washington over the next three days so cool 
and showery conditions will remain in the forecast. 


On Wednesday and Thursday skies will still be mostly cloudy and 
showers are likely but sun breaks will probably be more numerous. 
This could allow a few thunderstorms to develop in the afternoons 
and evenings so have added isolated thunderstorms to each day. As is 
so often the case in Spring...some thunderstorms may produce small 
hail. In fact there could be some hail without the thunderstorms. 
High temperatures will remain well below normal...rising into only 
the low 60s on Thursday. 


On Friday the upper low is right over the area but it is filling and 
broadening. This should diminish the showers somewhat so probability of precipitation are 
mostly in the chance category. A slow warming trend will continue 
with high temperatures climbing a few degrees from Thursday. Burke 


Long term...models diverge after Friday but the height rises 
continue. Would expect slow warming to continue Saturday through 
Monday with some upper 60s in the forecast. The GFS is pretty dry by 
Monday but the Euro swings an upper trough through the area Sunday 
night. Have kept the broad brush chance probability of precipitation. On Tuesday a weak 
system arrives in both models...keeping the chance probability of precipitation going but 
changing the weather from chance of showers to chance of rain. Burke 


&& 


Aviation...a deep upper low over the offshore waters this afternoon 
will continue to affect the forecast area as it shifts slowly inland 
tonight and Wednesday. The air mass will remain moist and somewhat 
unstable and showery precipitation will continue. Isolated 
thunderstorms could develop over the area Wednesday afternoon. The 
Cascades will remain generally obscured. 


Low-end VFR to MVFR conditions prevail over the forecast area this 
afternoon with scattered showers. Cloud cover is roughly sct010 
scattered-bkn030 bkn-ovc050. These conditions should continue generally 
speaking through this evening...then deteriorate somewhat tonight as 
more showers move into the region. 


Ksea...southwest wind 8-14 knots this afternoon...becoming south 
6-10 knots later tonight and veering to southwest again Wednesday 
afternoon. Mcdonnal 


&& 


Marine...Small Craft Advisory west to northwest winds over the 
coastal waters will ease tonight as a 1007 mb low forms near the 
northwest end of Vancouver Island. 


There is still lots of uncertainty...and differences in model 
solutions...with regard to the movement of the low and the winds it 
will produce. Most likely it will weaken as it moves slowly south 
across the Washington offshore waters Wednesday and Wednesday night 
and into the Oregon offshore waters Thursday and Thursday night. The 
low could produce Small Craft Advisory winds over the coastal waters 
and in some of the inland waters Wednesday and Wednesday night...an 
idea that is supported some of the uw WRF-GFS runs. The forecast 
that we are going with for now is a blend of that model and the 
weaker NAM solution...and the result keeps winds 20 knots or less over 
the marine zones. 


Weak onshore flow will develop Friday and continue Saturday and 
Sunday. 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory coastal waters through midnight...except 
for the northern inner coastal waters zone. 


&& 


$$ 


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