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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: 55 °
• Low: 48 °
• Rain Showers

• Thursday
• 
• High: 61 °
• Low: 50 °
• Rain Showers

• Friday
• 
• High: 66 °
• Low: 54 °
• Mostly Cloudy

• Saturday
• 
• High: 66 °
• Low: 46 °
• Mostly Cloudy

Area forecast discussion 
National Weather Service Seattle Washington 
925 PM PDT Monday may 20 2013 


Synopsis...a cold front will move through western Washington 
tonight and Tuesday morning. A cold upper level low pressure system 
will linger over the Pacific northwest through the week. The upper 
low will give a cool showery weather pattern into the weekend. 


&& 


Short term...rain will continue to spread inland over western Washington 
tonight. This front will be over western Washington through Tuesday 
morning...then the precipitation turns more showery Tuesday afternoon. 
There could be some afternoon sun breaks after the front passes. 
The upper level low will wobble around over the pacnw all week. This 
is a showery pattern with high probability of precipitation each period. The air will become 
unstable...at first the cold air aloft is directed into Oregon...for 
instance the GFS shows the temperature over Oregon at 700mb Tuesday night to be 
-10 to -14c over a good portion of Oregon. By later Wednesday afternoon 
that puddle of cold air aloft has spread out and a good part of both 
Oregon and Washington are around -10c. At 500mb it cools off to around -30c 
over western Washington Wednesday afternoon...rather cold air aloft for this time 
of year. 


Long term...previous discussion...only minor differences between 
the Euro and GFS in that both keep an upper low in the area. The 
Euro keeps it more or less over western Washington through Monday 
while the GFS spins the original low away and develops a new one 
over the area. In both models heights slowly rise into the 560s. 
Have a broad brush forecast for the entire period Friday through 
Monday with chance probability of precipitation and highs in the upper 50s to middle 60s. Burke 


&& 


Aviation...a deep upper trough will settle over the Pacific northwest later 
tonight through middle week. The associated 1011 mb surface low will 
moving into Oregon late tonight with the northward extending front pushing 
through the Washington waters. By early Tuesday morning...the front will push 
through the interior waters. This will cause stronger onshore flow 
across western Washington. Light SW flow aloft will become light to moderate westerly 
flow. It will become moist at all levels across western Washington later tonight 
and Tuesday. Rain develops tonight...then showers Tuesday. Unstable 
conditions Tuesday...especially afternoon with isolated thunderstorms 
possible. Probability too low to put in forecast. 


Precipitation already reached the coast this evening and is 
spreading inland faster than expected. Adjusted tafs for quicker 
arrival of rainfall after midnight tonight with conditions 
deteriorating across the forecast area to MVFR and IFR. Marginal 
improvement is possible late Tuesday. 


Ksea...N/NW winds quickly surged southward reaching ksea/kbfi and the 
Tacoma area early this evening. North winds will switch around early Tuesday 
morning as rain increases. It appears the initial round of rainfall 
will reach the area faster than expected...by around 1-2 am 
Tuesday...and conditions will likely deteriorate to MVFR occasionally 
IFR. Dtm 


&& 


Marine...onshore flow will increase tonight and continue Tuesday as low 
pressure moves inland S of western Washington. Small craft winds will continue 
through the central/east Strait of Juan Delaware fuca and will affect the southern 
portion of the northern inland waters and north portion of Admiralty Inlet. 
By Tuesday...marginal small craft winds will be limited to the 
central/east Strait. Northwesterly onshore flow will increase over most the 
coastal waters to warrant a small craft Tuesday. 


Another low will probably move southeast across the Washington 
offshore waters Wednesday through Thursday. Dtm 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory central and east Strait of Juan Delaware fuca 
through Tuesday afternoon... 
Small Craft Advisory for Admiralty Inlet and the northern 
inland waters through 5 am PDT Tuesday. 
Small Craft Advisory coast...except the northern inner 
portion...from 5 am PDT Tuesday through the afternoon. 


&& 


$$ 


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