Scientific Forecaster Discussion
fxus66 ksew 121159
Area forecast discussion
National Weather Service Seattle Washington
359 am PST Tue Dec 12 2017
Synopsis...a weak front will dissipate along the coast this
morning. 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...satellite imagery shows cloudy skies over western
Washington this morning. A weak front along the coast brought 0.05
inches to kuil overnight. The remainder of the area was dry. With
the cloud cover 3 am/11z temperatures were warmer than last night,
in the mid 20s to near 40.
Weak front dissipating along the coast this morning with just a
chance of some sprinkles along the coast. While not producing any
precipitation in most locations the cloud cover associated with
the front will hang over western Washington for most of the day.
The cloud cover overnight held fog formation to a minimum so far
this morning and this trend will continue with just patchy fog in
the forecast for the south sound and the southwest interior. Low
level offshore gradients weakening today. This combined with the
cloud cover will keep temperatures over the area mostly in the 40s
and lower 50s. The north coast will be 5 to 10 degrees cooler
than the last two days with the decreasing offshore flow.
Cloud cover dissipating tonight with the upper level building
offshore. Offshore surface gradients will continue to weaken
overnight. With the lighter gradients and decreasing clouds expect
areas of fog and freezing fog to form in the early morning hours
from about Seattle southward. Many places will be below freezing
again Wednesday morning with lows ranging from the mid 20s to the
Upper level ridge axis remaining offshore Wednesday and Wednesday
night for another sunny day and cool night after the morning fog
Burns off. Highs on Wednesday will be in the mid 40s to lower 50s
with lows Wednesday night in the 20s to mid 30s.
Upper level ridge moving east on Thursday with the upper level
ridge axis east of the area by Thursday night. Approaching front a
little slower on the 00z runs so have cut back the pops Thursday
evening with the best chance for rain after midnight.
With the upper level ridge in place and light flow at the lower
levels through the short term the air stagnation advisory will
continue through Thursday afternoon.
Long term...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...the weak dissipating front brought clouds to the
coast overnight but elsewhere just high clouds so far. A few fog
patches may yet set up in the Foggy Bottoms and river valleys and
near Puget Sound--otherwise the high clouds will clear and the
stratus on the coast will break up today as high pres rebuilds.
Ksea...the weak front that is breaking up near the coast probably
wont have much in the way of cloudiness today.
Marine...a weak system is dissipating near the coast this
morning. High pressure over the area through midweek will give way
to a cold front Thursday night into Friday. A warm front will
approach the coastal waters on Saturday.
Hydrology...river flooding is not expected for the next 7 days.
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.