Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 101154 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 354 AM PST Sun Dec 10 2017 .SYNOPSIS...
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A strong upper ridge will maintain dry and stagnant conditions across Western Washington through the middle of the week. The ridge looks like it will break down mid week with the first chance of rain Thursday or Thursday night. Another system will follow the first one keeping at least a chance of rain in the forecast into next weekend.
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&& .SHORT TERM...
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Satellite imagery shows plenty of high clouds over Western Washington early this morning. The high clouds inhibited most of the fog formation overnight with just a couple of observations reporting fog. There was a wide variety of temperatures at 3 am/11z ranging from the mid 20s to near 40. Weak system moving by to the north is already east of the Cascades. The high clouds associated with the tail end of the system will move out of the area later today leaving mostly sunny skies across the region. Temperature inversion still place with the temperatures at Mount Baker ( 4200 feet ), Hurricane Ridge ( 5250 feet ) and Paradise on Mount Rainier ( 5400 feet ) in the mid 40s to lower 50s. Sandpoint profiler shows light winds in the lower levels up to 1500 feet and model soundings show this scenario continuing today. Some of the air quality sensors are now in the unhealthful for sensitive groups category, a downgrade from 24 hours ago. With no improvement in the conditions air quality wise the air stagnation advisory will remain in effect. Highs this afternoon with some sunshine will mostly be in the 40s in the lowlands. Downslope easterly winds off the Olympics will make the North Coast the warm spot with temperatures in the mid 50s. Little change in the pattern tonight into Monday with the upper level ridge over the area and low level offshore flow. With the lack of high cloud cover tonight into Monday morning fog coverage will increase versus this morning by early Monday morning and low temperatures will be colder with the clear skies. Most locations will again be below freezing Monday morning. This sets up the possibility of some black ice on the roadways during the Monday morning commute. What fog that does form will be shallow and burn off by early afternoon. Highs Monday will once again be mostly in the 40s with mid 50s along the North Coast. Model soundings show the inversion still in place. There is a good possibility the air stagnation advisory will get extended into the middle of the week. Another weak system riding moving through the ridge to the north of the area Monday night. The system weakens rapidly Monday night so just expect a few high clouds overnight. Lows will be in the mid 20s to mid 30s. Upper level ridge rebuilding a little on Tuesday in the wake of the system. Low level offshore gradients weakening with surface high pressure building offshore behind the front. The weakening surface gradients early Tuesday will allow more fog to form Tuesday morning. Most places will again be in the sunshine by afternoon with highs in the 40s and lower 50s. .LONG TERM...Extended models consistent in the idea that the dry spell comes to an end after Wednesday. The 00z run is a little slower with the first system after the ridge versus the 12z run but at this point will let the chance pops on Thursday ride. All of the models have some precipitation Thursday night. Beyond Thursday night it is back to our regularly scheduled December weather with zonal flow aloft and a series of weather systems taking aim at the area into next weekend. Felton
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&& .AVIATION...
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A strong upper ridge axis over Central Washington will remain in place through Sunday night. Flow aloft will be light to moderate from the southwest, with low-level offshore gradients. The air mass will remain dry and very stable. Forecasting shallow dense fog over the Puget Sound lowlands will continue to be the main challenge. So far this morning, fog has not materialized. This is likely due to thicker high cirrus and enough wind to entrain drier air from above into the boundary layer. Even if we escape this morning with little or no significant fog, lack of cirrus clouds tonight should make fog formation more likely then. KSEA...Just a little light fog down around Federal Way early this morning, but the terminal continues to experience good visibility. The next TAF issuance will likely remove fog from the SEA forecast through today. Conditions for fog formation should be a bit more favorable tonight, so will bring some fog in after the Sunday evening arrival rush. Haner
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&& .MARINE...
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Easterly gradients between a 1041 mb high over Eastern Washington and lower pres offshore will keep offshore winds going through Monday morning. The offshore flow will weaken on Monday afternoon. Light northerly offshore flow is expected Tuesday and Wednesday.
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&& .HYDROLOGY...River Flooding is not expected for the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Air Stagnation Advisory until noon PST Monday for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity- East Puget Sound Lowlands-Eastern Strait of Juan de Fuca- Everett and Vicinity-Hood Canal Area-Seattle and Vicinity- Southwest Interior-Tacoma Area-Western Skagit County- Western Whatcom County. PZ...Small Craft Advisory until 9 AM PST Monday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle

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