Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 050502
Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sun Dec 4 2016
.SYNOPSIS...Scattered showers will become more organized later
tonight and bring up to two inches of snow to hilltops. Fraser River
outflow will bring colder air Monday night, and snow showers will
lower to sea level. A convergence zone may develop Monday night and
bring several inches of snow to north King and Snohomish counties.
Colder and drier weather will follow Tuesday and Wednesday. A system
Wednesday night and Thursday could also produce some lowland snow.
.SHORT TERM...Radar shows scattered showers, mainly over the Olympic
peninsula, though there is also a weak convergence zone in Snohomish
county. There could be some snow in the inland showers, but this
should not amount to much. A more organized shower band will reach
the coast after 09Z, then affect inland areas 15Z-20Z. Dew points
are mostly just below freezing over the interior, and temperatures
will be around freezing, so most of this precipitation will be in
the form of snow. Up to three inches is possible, with the WRF
showing the highest amounts along Hood Canal and possibly in
Snohomish county. Will refresh the winter weather advisory to
mention these trends. Hilltops will be most likely to get snow, and
sea level areas may not get any.
Colder air will move over the area from north to south Monday
afternoon through Monday night. There could be a convergence zone
where the colder, drier air meets the somewhat warmer, moist air. A
winter storm watch is in effect in north King and Snohomish counties
Monday night for the possibility that heavy snow could occur in a
convergence zone. The most recent 4km WRF has a bullseye over the
King-Snohomish county line with 8-12 inches or so in the center. It
should be noted the previous 1.3km WRF had nearly all the snow in
the east Puget Sound lowlands. No changes to the watch for now.
A period of dry and colder weather is on tap Tuesday and Wednesday
with highs mostly in the upper 30s and lows in the 20s to around 30.
Black ice will be a concern during the commutes Monday night through
Wednesday night. If any snow does fall Monday/Monday night, melting
and refreezing will increase the danger of slick roadways. Burke
.LONG TERM...Previous discussion...Global models seem to be trending
toward a more negative tilt front approaching the region Wednesday
night with dry/cold air firmly in place. Most models indicate
several inches of snow could fall across the interior lowlands
before a transition to warmer air bring plain old rainfall.
Forecasts should be monitored closely over the next few days as
significant travel impacts are possible Thursday morning.
Models vary considerably on the evolution of the system by Thursday
night and Friday. The GFS shows a deep low tracking near or north of
the area which could be a windy pattern for the region Friday. Heavy
mountain rain or snow is also possible. Long range models continue
to look quite active with a series of systems expected to affect the
region into the weekend. Mercer
.AVIATION...Westerly flow aloft over Western Washington this
evening will continue tonight and Monday. The air mass will be
moist, especially below 10000 ft. There are scattered showers over
the area this evening, mainly at the coast and in a weak Puget
Sound convergence zone extending from near Arlington to Stevens
Pass. A precipitation band associated with an upper shortwave
trough will move through the area late tonight and Monday morning,
bringing more widespread showers. Then another Puget Sound
convergence zone will develop midday Monday and continue into
Monday night. The air mass is cold enough that most precipitation
will fall as snow, especially at elevations 500 ft and higher.
Conditions are mainly VFR across the forecast area, but we could
see quick deterioration to low-end MVFR and local IFR as
Fraser outflow will develop during the late afternoon hours around
KBLI with N to NE winds increasing to 15 to 25 mph. McDonnal
KSEA..South to southeast winds 5 to 8 knots. VFR conditions this
evening. Snow showers with mainly MVFR conditions will begin
around the terminal roughly 12Z Monday. Accumulation of 1-2
inches is possible Monday morning by 18Z. McDonnal
.MARINE...Onshore flow will continue to ease tonight. Seas will
remain above 10 feet through Monday. A small craft advisory for
hazardous seas remains in effect through late tonight at the west
entrance to the strait while the small craft advisory will remain
in effect for the coastal waters for a combination of seas or
winds through Monday.
A weak low will move southward along the west coast of Vancouver
Island tonight. As the low continues south to the Washington coast
on Monday, a cold dome of high pressure will move south through
British Columbia. As a result, the flow across Western Washington
will turn northerly by late Monday. Cold Fraser River outflow
winds of 20 to 30 kt will likely develop across the northern
inland waters and Strait of Juan de Fuca Monday afternoon and
continue through Tuesday. A gale watch was issued for the Northern
Inland Waters for the potential of gales late Monday afternoon
Northeasterly flow will diminish Tuesday afternoon as it gradually
turns more easterly. Then on Wednesday night or Thursday, strong
easterly flow will develop with gales possible over the coastal
waters and through the strait. The GFS has been stronger and
faster with the incoming frontal system than the ECMWF model. The
forecast is a blend between the two at this point. McDonnal
.HYDROLOGY...No river flooding is expected over the next week.
WA...Winter Weather Advisory from 4 AM to noon PST Monday for
Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
Vicinity-East Puget Sound Lowlands-Everett and Vicinity-
Hood Canal Area-San Juan County-Seattle and Vicinity-
Southwest Interior-Tacoma Area-Western Skagit County-
Western Whatcom County.
Winter Storm Watch from Monday afternoon through late Monday
night for Bellevue and Vicinity-East Puget Sound Lowlands-
Eastern Strait of Juan de Fuca-Everett and Vicinity-Seattle
PZ...Small Craft Advisory for rough bar until 6 PM PST Monday for
Grays Harbor Bar.
Small Craft Advisory until 6 PM PST Monday for Coastal Waters
From Cape Flattery To James Island 10 To 60 Nm-Coastal
Waters From Cape Flattery To James Island Out 10 Nm-Coastal
Waters From James Island To Point Grenville 10 To 60 Nm-
Coastal Waters From James Island To Point Grenville Out 10
Nm-Coastal Waters From Point Grenville To Cape Shoalwater
10 To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater Out 10 Nm.
Gale Watch from Monday afternoon through Tuesday afternoon for
Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory for hazardous seas until 3 AM PST Monday
for West Entrance U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at