Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PST Tuesday Nov 24 2015
Synopsis...light rain or showers will decrease from north to south
across western Washington today as cold dry air begins pushing in
from the north. Fraser outflow will produce windy conditions in the
north interior through tonight. Cool dry weather will prevail
through the weekend...with sunny days and freezing temperatures at
night. Wet weather will probably return early next week.
Short term...satellite imagery shows the upper level closed low
circulation center roughly 100 miles off the Washington coast dropping due
southward this morning. Wrap around moisture and deformation aloft is
generating lift across western Washington from about Everett southward.
Radar shows areas of rain but amounts are light with only an
additional tenth or so expected through midday. Snow levels are
generally above 1000 feet this morning where an inch or two of snow
could fall which includes lower passes. Interestingly...mesoscale models
show a pocket of 1000-850 mb thickness values down to 1300m with
temperatures zero to -1c at 925 mb over the SW interior. A spotter
at 700 feet NE of Eatonville reported wet snow with a skiff on the
grass this past hour. Would not be surprised to see a brief mixed
rain/snow above a few hundred feet in the SW interior or far south
Puget Sound sound until around 11 am this morning. Little or no
accumulation expected. Models are all in agreement including the
hrrr and wrfgfs that precipitation will end from north to south by
The kbli-ywl gradient is up to +15.5 mb and forecasted to peak
around +18 or 19 mb this afternoon/evening. The Wind Advisory for
northeast winds 20-35 miles per hour and gusts 50 to 55 miles per hour looks on track at
this point with winds peaking later today and this evening. The
advisory will go through early Wednesday morning. Northerlies will
also filter down through Puget Sound sound but mainly in the breezy to
locally windy category 15 to 25 miles per hour west/ gusts to 35 miles per hour.
Low dew point air will gradually filter across western tonight and
Wednesday. Drier air means clearing skies but strong radiational
cooling during the overnight hours. Expect low temperatures
Wednesday and Thursday to fall to below freezing most spots. Cold
pocket locations like Olympia and Shelton could get down the low or
middle 20s. The typically warmer Seattle metropolitan area may reach the upper
20s Thanksgiving morning which looks to be the coldest of the next
several days. Motorists should use caution as black ice is possible
during the latter part of the week. Days will be sunny with highs
reaching the upper 30s to low or middle 40s. Mercer
Long term...no update from previous discussion...dry northeast flow
aloft will continue Friday through Sunday...with northerly offshore
flow at the surface. The pattern will gradually break down over the
weekend though...with the upper ridge weakening as it shifts across
British Columbia and into Alberta. The air mass over western
Washington will remain in place but gradually moderate. On Monday
moist southwest flow will probably reach western Washington again.
Aviation...an upper level low will drop south into Oregon today.
The areas of precipitation and clouds will sag south with it and
there will be clearing from the north this afternoon and evening.
Although the Fraser outflow has developed temperatures across The
Lowlands are still warm enough for rain almost everywhere...only a
couple reports of wet snow were coming in this morning .
Ksea...rain and low ceilings will give way to clearing this afternoon
and north winds.
Marine...Fraser outflow is going and gale warnings are up for those
areas. The swell is 14 feet on the coast and the Grays Harbor bar
was closed to rec/upv on the 8am Coast Guard report. The NE flow
should pick up over the coastal waters today and then the NE flow
and Fraser outflow will ease on Wednesday. Light to moderate
offshore flow will then prevail for the rest of the week.
Hydrology...flooding is not expected for the next seven days.
Washington...Wind Advisory until 6 am PST Wednesday for San Juan County-
western Whatcom County.
Pz...Gale Warning central and eastern Strait of Juan Delaware fuca and for the
northern inland waters...Small Craft Advisory coastal waters and West
Entrance Strait of Juan Delaware fuca...Small Craft Advisory Admiralty Inlet
... Craft advisory and Puget Sound sound and good canal.
Small Craft Advisory for rough to severe bar conditons Grays Harbor bar.
You can see an illustrated version of this discussion at