Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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668
FXUS66 KSEW 021745
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Fri Dec 2 2016

.SYNOPSIS...Rain and mountain snow will prevail across Western
Washington today as frontal system moves through the region.
precipitation will taper to showers tonight and Saturday morning. A
vigorous front arrives Saturday night, followed by a cold upper
trough Sunday and Monday. The snow levels will fall to around 500
feet with snow mixed with rain at times near sea-level. Colder
fraser outflow Monday night may meet up with residual moisture to
bring a better chance of some snow to the greater Puget Sound metro
region. Cold and mainly dry weather develops Tuesday. A system
arriving Wednesday night and Thursday could bring some low elevation
snow before warm air brings rising snow levels.

&&

.SHORT TERM...Light to moderate warm advection rain has overspread
the area this morning with snow levels near 4000 feet except near
3000 feet this morning in the passes which will have some colder
air trapped for a few hours before rising. The cold front will
arrive from the northwest later this afternoon which will drop the
main snow levels slightly as precipitation tapers to showers by this
evening. A Puget Sound convergence zone is possible in post-frontal
flow, mainly over Snohomish county. Showers will continue in the
mountains with the flow giving some orographic enhancement to snow
amounts. About 3 to 6 inches are expected through tonight above
around 3500 to 4000 feet, so Stevens may see some a few inches but
Snoqualmie not so much.

Showers persist Saturday but much of Puget Sound will be rain
shadowed and sunbreaks are possible. A few more inches of snow could
fall in the mountains but the passes should not see more than a
couple inches. The confluent moist westerly flow aloft Saturday
afternoon brings increasing rain and mountain snow later in the day.
A strong wave develops along the front and moves through Saturday
night which will be the start of a major pattern shift for the
region. There is still some uncertainty as to how the strength of
any surface low development. Most models still take a weaker low
inland south of our area as much colder air aloft pours in behind the
front Saturday night and Sunday. Rain will change showers as a cold
upper low in the Gulf of Alaska drops south over or the Pacific
Northwest later Sunday and Monday. 500 mb temps are shown by models
as low as -40c which is unusually cold even for a low coming from
the north. Instability should increase with showers and perhaps even
a chance of isolated thunderstorms Sunday afternoon. Snow levels
will fall to 500 to 1000 feet but if showers are heavy enough, snow
could mix in with rain even down to sea-levels Sunday but
accumulations would be be unlikely with the warm ground and surface
temperatures still in the upper 30s to low 40s in the afternoon.

Things get much more interesting with the next low pressure system
arriving from the northwest Monday, with snow levels getting close
to sea-level. With Fraser outflow developing, there is some threat
of a little lowland snow, even the Puget Sound metro area by Monday
night. Lots of things need to come together to snow in the lowlands
and will be evaluating the new model runs today. Mercer



.LONG TERM...Previous discussion...Extended models still not in good
agreement for the system on Monday with the GFS being wetter and
faster then the ECMWF. The air mass in both models is definitely
cold enough for snow with 1000-850 mb thickness values between 1290-
1275 meters and 850 mb temperatures -7 to 8c. The ECMWF is also
slower in spilling the cool northeasterly Fraser river outflow into
the area versus the GFS keeping some low level onshore flow at the
surface. Right now the best chances for snow are in the 06z/10 pm
Sunday to 18z/10 am Monday window. The ECMWF snow accumulations are
up to an inch. The operational GFS has more than that but looking at
the GFS ensembles the ensemble mean is also right around an inch for
Sunday night into Monday morning. The ground will still be plenty
warm with the lack of any cold weather so far this season so the
snow will have a hard time sticking to the road surfaces in the
lower elevations. With both models indicating precipitation have
increased the pops in to the likely category for Monday.

Models in better agreement with an upper level trough moving
through Western Washington Monday night and drier northerly flow
behind the trough for Tuesday. Lows Tuesday morning will be below
freezing across the area.

Models solution become inconsistent again in the middle of the
week with the bringing a warm wet system into the area later
Wednesday into Thursday while the ECMWF holds this system off
until late Thursday. Current forecast follows the GFS solution and
will stay with that idea in the morning forecast package. Felton

&&

.AVIATION...Westerly flow aloft. A cold front over the offshore
waters will move rapidly eastward and inland late this afternoon.
The air mass will be moist and stable ahead of the front and will be
unstable and somewhat moist behind the front. Rain today will taper
to scattered showers behind the front late this afternoon with
precipitation increasingly tied to the terrain, the coastal zones,
and a Central Puget Sound Convergence Zone. Conditions will be
mainly MVFR across the area through Friday morning with some
improvement at times in rain shadowed areas to the east of the
Olympics and mountains of Vancouver Island. Albrecht

KSEA...South to southeast wind 7-11 kt, rising to 11 to 18 knots
with gusts to 26 knots a few hours this afternoon around FROPA about
00Z. With rain shadowing expect mainly VFR conditions at the
terminal with MVFR at times ahead of the front this afternoon in
-RA. Albrecht

&&

.MARINE...A cold front over the offshore waters will move rapidly
inland late this afternoon. Onshore flow will follow the front late
this afternoon and tonight. Small craft advisory southerly winds are
expected all waters except the central Strait of Juan de Fuca today,
with westerly small craft advisory conditions over the coastal
waters and through the central and eastern portions of the Strait of
Juan de Fuca this evening. Small craft advisory conditions will
continue over the coastal waters and at the west entrance to the
Strait of Juan de Fuca tonight into Saturday as a westerly swell
builds to 18 feet over the near coastal waters and 21 feet over to
outer coastal waters. The swell will peak late tonight and early
Saturday morning.

Another frontal system is expected to bring small craft advisory
conditions to the coastal waters and portions of the Strait of Juan
de Fuca late Saturday night into Sunday.

Northerly flow will develop later Monday or Monday night as strong
high pressure forms over interior British Columbia. Fraser River
outflow may develop Monday night or Tuesday. Albrecht

&&

.HYDROLOGY...No river flooding is expected over the next week.
Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PST this evening 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.

     Small Craft Advisory for hazardous seas from 10 PM this evening
     to 6 PM PST Saturday 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.

     Small Craft Advisory for rough bar until 6 PM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 3 AM PST Saturday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST this evening for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.

     Small Craft Advisory for hazardous seas from 6 PM this evening
     to 6 PM PST Saturday for West Entrance U.S. Waters Strait
     Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PST
     Saturday for Central U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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