Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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809
FXUS66 KSEW 101154
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
354 AM PST Sun Dec 10 2017

.SYNOPSIS...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.

&&

.SHORT TERM...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

&&

.AVIATION...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

&&

.MARINE...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.

&&

.HYDROLOGY...River Flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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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