Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 062259
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Tue Dec 6 2016
.SYNOPSIS...Cooler and drier air will be over Western Washington
through Wednesday. There will continue to be Fraser River outflow in
the north. Easterly gap winds will increase Wednesday night. A front
will reach the area Thursday afternoon. Cool wet weather is likely
Friday through the weekend.
.SHORT TERM...A cooler drier air mass has arrived and that will give
Western Washington mostly clear weather through Wednesday. Fraser
outflow will keep breezy cold weather over Western Whatcom county
and into the San Juans. Easterly winds will increase Wednesday night
and Thursday--especially in the mountain foothills and passes. A
front will bring precip to the area Thursday afternoon and night and
many areas should get some snow. If the 12z gfs 1000-850mb thickness
is perfect the precip will be rain after daybreak Friday except for
a wintry mix up in Whatcom county.
.LONG TERM...There is a good chance that the modified arctic front
will be just north of the WA/BC border Saturday and for most of
Sunday. After that, the 12z GFS might have a wintry solution around
Sunday night but that`s super speculative. The ECMWF pulls the
gradient around to northerly next Monday night and Tuesday--in fact
dry cool weather is possible in strong offshore flow by the middle
of next week. But for now the forecast is closer to climatology than
to any of these interesting individual forecast model solutions.
.AVIATION...Dry northerly flow aloft will continue over W WA
tonight through Wednesday as upper level ridge offshore along 135W
offshore the region. The ridge will weaken on Wednesday with the
axis moving over W WA Wednesday evening. A surface high over B.C.
will maintain cold dry northerly low level flow. The air mass is
generally dry with clear skies or scattered clouds over the lowlands
and BKN-OVC layers over the mountains. The air mass will remain dry
tonight with good radiational cooling conditions. Low level moisture
is limited so patchy fog is only expected late tonight and Wednesday
morning over the south Puget Sound and the SW interior.
KSEA...Clear skies through Wednesday. Surface winds will be N-NE 4-
10 KT. Snow is possible Thursday afternoon and night. Kam
.MARINE...A surface ridge over B.C. will gradually weaken through
tonight...allowing the northeasterly offshore flow over W WA to
gradually diminish. Marginal SCA winds are expected to continue over
the north inland waters mainly north of the San Juan islands through
Wednesday morning. Borderline SCA N winds in Admiralty inlet and N
Puget sound should weaken this evening.
The big issue is the evolution of the low currently offshore W of
the N CA coast near 40N/143W. Models agree that the low will drift
slowly NE toward the Pacific Northwest over the next few days.
What seems reasonably certain is that easterly offshore flow will
increase to gale levels over the coastal waters and through the
Strait of Juan de Fuca Wednesday evening and continue through
Thursday night as the associated front moves NE across the area. A
gale watch is in effect for Wednesday night through Thursday night
for the coast and parts of the strait.
What is much less certain is the track and strength of the surface
low which will impact winds Thursday night and Friday as the low
nears the coastal waters. Confidence in the wind forecast for that
period is low. Kam
.HYDROLOGY...No river flooding is expected over the next week.
PZ...Small Craft advisory Puget Sound, Admiralty Inlet, and
North Inland waters.
Gale Watch coast, west entrance, central strait from
Wednesday evening through late Thursday night.
You can see an illustrated version of this discussion at