Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 190418
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
817 PM PST Sun Feb 18 2018

.SYNOPSIS...Cold and dry conditions will persist as north and
northeast winds remain over Western Washington. A widespread hard
freeze is expected tonight through Tuesday night. A weak system
dropping down offshore late Tuesday could bring a little snow to the
coast and Southwest Washington. Other systems around Wednesday
night, Friday, and next Sunday could bring more precipitation, with
lowland snow possible with the Wednesday night and Friday systems.

&&

.SHORT TERM...Gradients continue to gradually ease this evening
which will resulting wind speeds slowly relaxing. Current obs around
the CWA continue to show gusts around the area...the strongest being
at any sites near water...but the vast majority fall below any
headline criteria. The exception to this...Western Whatcom county
and the San Juans...with their current High Wind Warning in
place...looks to have fallen below that criteria. As such...will
monitor next set of obs before making a decision whether or not to
downgrade to a wind advisory or just scuttle the headline altogether.

Regardless of speed...these winds are responsible for bringing a
cold and dry continental air mass into the region. In fact...likely
to be the coldest air mass of this winter and this will remain in
place at least through Monday night. A widespread hard freeze is
forecast tonight, Monday night, and possibly Tuesday night. Lows in
the low-mid 20s will be common over the next two nights. Sunny skies
forecast during the day on Monday.

Late Tuesday, the ECMWF is consistent with its previous solutions
in dropping a system southward off the coast and slopping some
precipitation onto the coast and into Southwest Washington. A
growing minority of GFS ensemble members bring a few hundredths of
precip to Hoquiam late Tuesday as well, and the 18z NAM brings
precip to within 25 miles of the coast. If precip moves onshore,
the air mass will still be cold enough for snow to sea level. Have
about 20-40 percent PoPs late Tuesday for the coast and SW Wa, but
have a dry forecast from Seattle metro on north. This system
should exit southward early Wed, with a mainly dry but cool day
expected. Temperatures on Wed will still be well below normal.

Models agree in dropping another shortwave down along the
southeast Alaska and B.C. coasts and into Western Washington Wed
night and early Thu. The NAM is perhaps most bullish with precip
amounts in the warm advection pattern preceding the shortwave. The
air mass will still be sufficiently cold for all snow, except
perhaps at the immediate coast of Grays Harbor County.
Possibilities over the lowlands range from non-impactful flurries
to a few inches of snow. The NAM favors neither the coast nor
inland, while the EC favors the coast. The latest GFS just has
flurries. Ensembles point to a range of possibilities, most on the
lighter side. Haner/SMR

.LONG TERM...From Previous Discussion...A highly amplified longwave
pattern will continue, with cold north-northwest flow aloft from
Thursday through next weekend. Looks like additional systems
dropping down in cold northwest flow aloft around Friday and next
Sunday. The Friday system could see a transition from some snow to
rain as warmer air spreads inland. Snow levels by next Sunday should
rise above the lowlands, perhaps to 1000 to 1500 feet. Still, below
normal temperatures.  Haner

&&

.AVIATION...Northerly flow aloft will continue over W WA tonight and
Monday. VFR conditions in place thanks to cold...dry air mass. Clear
skies tonight will persist into Monday with some occasional SCT high
clouds. Surface winds this evening will remain gusty as Fraser
outflow slowly starts to subside with calmer winds expected in most
locations by 12Z Monday morning.

KSEA...Dry cold air and northerly winds as Fraser outflow continues
tonight. Wind speeds gradually diminishing with gusts expected to
end by early Monday morning. Skies should be mostly clear from here
on out til weather systems arrive starting on Tuesday.

&&

.MARINE...Strong Fraser outflow winds still having
impacts...particularly over the Northern Inland Waters. While most
waters have fallen below gale...still isolated obs coming in from
within Gale Warning zones...so will leave inherited headlines alone
there.  Remainder of waters look to be behaving...so not much
expected in ways of updates for evening package.

A series of weather systems will drop down out of the northwest Tue-
Sat so there will probably be about three periods where fairly light
southerly winds develop, and then a pretty decent push of NW winds
behind each front comes through the region--especially later in the
week when the systems seem be be successively stronger.

&&

.HYDROLOGY...River flooding is not expected the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Wind Warning until midnight PST tonight for San Juan
     County-Western Whatcom County.

PZ...Small Craft Advisory until 2 AM PST Monday for Admiralty Inlet-
     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-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 4 AM PST Monday for
     Grays Harbor Bar.

     Gale Warning until 4 AM PST Monday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca-Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory until 2 AM PST Monday for Puget Sound and
     Hood Canal.

     Gale Warning until 9 PM PST this evening for Puget Sound and
     Hood Canal.

&&

$$

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