Scientific Forecaster Discussion
fxus66 ksew 121642
Area forecast discussion
National Weather Service Seattle Washington
845 am PST Tue Dec 12 2017
Synopsis...a strong upper level ridge will maintain dry and
stagnant conditions across most of western Washington through
Thursday afternoon. The ridge will break down on Thursday
allowing a cold front to bring rain late Thursday night and
Friday. Another frontal system will continue the wet weather
pattern into the weekend.
Short term...mid and high clouds cover western Washington this
morning. Winds are light but the cloud cover kept fog to a minimum.
The clouds also kept temperatures several degrees higher than
yesterday morning, with mostly 30s at 8 am along with a few low 40s
near the water. Satellite imagery shows a mass of clouds off the
coast, so today will be mostly cloudy with highs 45-50.
Guidance suggests clouds will not dissipate until after midnight,
when the upper ridge sharpens a bit. As the clouds fade, and
winds remain light, fog is likely to appear, especially around Puget
Sound. Overnight lows will be mostly in the 30s.
Wednesday and Thursday will be mostly sunny with areas of night and
morning fog. Highs will be in the mid 40s to lower 50s, but low
temperatures will fall into the mid 20s to mid 30s. An approaching
front will spread clouds over the area Thursday night and eventually
bring a little rain late Thursday night and Friday.
With the upper level ridge in place and light flow at the lower
levels through the short term, the air stagnation advisory will
remain in effect through Thursday afternoon. Burke
Long term...previous discussion...extended models in fairly good
agreement during the extended period with the trend on the 00z run a
little slower and weaker with the system over the weekend. Upper
level trough moving through western Washington on Friday behind the
front. Even through the models are not as strong with the warm front
as previous runs for Saturday will still keep at least chance pops in
the forecast. Trailing cold front arrives on Sunday. Another upper
level trough digs down off the coast on Monday keeping a chance of
precipitation in the forecast. Felton
Aviation...an upper ridge will build over the area through
tonight with light to moderate westerly flow aloft. The remnants
of a dissipating frontal system over western Washington is
bringing mid and high level moisture to the area today. Expect the
clouds to gradually decrease this afternoon and evening as surface
high pressure builds to the east and northeast of the area.
The combination of clearer skies and light offshore flow late
tonight into Wednesday morning will result in an increasing low-
level temperature inversion with moisture trapped near the
surface. Expect areas of fog to develop late tonight in valleys
with the fog becoming more widespread during the morning hours.
Ksea...the weak front that is breaking up near the coast probably
wont have much in the way of cloudiness today.
Marine...a ridge of high pressure extending from southern British
Columbia into eastern Washington will give light offshore flow to
the waters today through Thursday. A cold front will move east
through the waters Thursday night into Friday morning.
A warm front will approach the coastal waters on Saturday. Small
Craft Advisory conditions are likely across most of the waters
with these frontal systems. Albrecht
Washington...air stagnation advisory until 2 PM PST Thursday 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.