Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 090521
Area Forecast Discussion
National Weather Service Seattle WA
921 PM PST Thu Dec 8 2016
.SYNOPSIS...A warm front will lift northward across the area late
tonight into early Friday morning. Snow will turn to rain along
the front. Some light freezing rain can be expected in the
transition over the southern half of the area and in the Chehalis
Gap. Offshore winds will gradually relax overnight as the front
approaches. Rainy and milder weather is expected Friday through
the weekend. Cold air over British Columbia will likely drop
south into the area late Sunday night or Monday.
.SHORT TERM...With strong offshore flow in place, precipitation
has been taking its time spreading northward. Radar and
observations show that now taking place and at 9 PM snow is
falling from southern Snohomish county southward. Precipitation
appears to have changed to light freezing rain at Chehalis and at
Hoquiam. Elsewhere in the interior, it will have a hard time
changing over until the warm front passes. For anywhere that
changes to rain ahead of the change in wind direction from
northeast to south, there could be a period of light freezing
rain or sleet until the temperature comes up above freezing. As
far as timing goes, models show good variability with the 00z
NAM12 hinting at 09Z-12Z and the GFS 12Z-15Z. The south interior
and a wedge along the west slopes of the Cascades will change over
a bit faster as warm air aloft surges northward along the terrain.
Areas along Hood Canal will likely wait until about 15Z to change
to rain. Accumulations: the Southwest interior, Central coast, and
Chehalis Gap zones 2-5 inches (already occurred for the most
part), Hood Canal area 3-6 inches, Greater Seattle- Tacoma-Everett
areas around 2 inches, and north interior zones closer to 1-2
Winds are decreasing this evening, so we let the wind advisories
for the gaps and the west slopes of the Cascades expire.
The area northeast of Bellingham will possibly have problems
completely transitioning over to rain Friday with northeasterly
flow out the Fraser. Precipitation should be in the form of rain
after Friday afternoon and until the next batch of arctic air
starts moving southward later Sunday or Sunday night.
Once precipitation changes over to rain, Friday through Sunday
will see rain showers with snow levels rising to 500-1500 feet in
the Cascades and 2000 feet in the Olympics. Periods of snow will
give 1-2 feet of snowfall to the passes by Saturday morning. A
winter weather advisory is in effect for the Cascades through
early Saturday morning. Albrecht
.LONG TERM...Forecast models are showing arctic air lurking over
western Canada. This air mass will start making its move southward
on Sunday and may reach the Fraser outflow area around Bellingham
sometime late Sunday night before moving southward into western
Washington on Monday. Model timing is inconsistent with some
solutions bringing the cold air southward late Sunday night and
other model solutions during the day Monday. Many model solutions
show a few inches of snow accompanying the boundary with
temperatures plunging down into the 20s behind the boundary. Keep
an eye out on latest forecasts for the possibility of impacts on
Monday night through midweek looks quite cold and dry with
temperatures below freezing. Albrecht
.AVIATION...Weak high pres aloft will move slowly east through
Friday, allowing an upper level Low over the northeast Pacific to
approach the British Columbia coast. Meanwhile, an occluded front
over northwest OR will move northeast across southwest WA
overnight, and then slowly northeast across the rest of western WA
during the day Friday. Wly flow aloft will grdlly strengthen.
Strong low level offshore flow will weaken and become southerly on
Expect widespread MVFR conditions to dvlp overnight. There will
be areas of -FZRA and localized LIFR conditions (with +SN).
KSEA...It looks like the precip will remain as snow overnight
before transitioning to rain Friday morning (most likely after
1500 UTC or 7 AM PST). There is a possibility of brief -FZRA or
sleet before the switch to just plain rain. The total snowfall
amount is anticipated to be about 2-3 inches. Meanwhile,
anticipate MVFR conditions overnight, ocnl IFR with the heavier
An occluded front extended from a 992 MB Low well off the British
Columbia coast southeast to the central Oregon coast. This front
will move north across the Washington coastal waters overnight and
the interior waters on Friday. The flow will become southerly
behind the front.
Expect a trough of low pressure to move across the area late
Friday night through early Saturday, resulting in increased
onshore flow. A cold front will follow later in the day Saturday
for moderately strong onshore flow.
.HYDROLOGY...No flooding is expected over the next week.
WA...Winter Weather Advisory until 8 AM PST Friday for Admiralty
Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity-
East Puget Sound Lowlands-Eastern Strait of Juan de Fuca-
Everett and Vicinity-Lower Chehalis Valley Area-San Juan
County-Seattle and Vicinity-Southwest Interior-Tacoma Area-
Western Skagit County-Western Strait of Juan De Fuca.
Winter Weather Advisory until 3 AM PST Saturday for Cascades of
Pierce and Lewis Counties-Cascades of Snohomish and King
Counties-Cascades of Whatcom and Skagit Counties-Olympics.
Winter Storm Warning until 8 AM PST Friday for Hood Canal Area.
Winter Weather Advisory until noon PST Friday for Western
Winter Weather Advisory until midnight PST tonight for Central
PZ...Gale Warning until 3 AM PST Friday for Central U.S. Waters
Strait Of Juan De Fuca-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 until 9 PM PST Friday for Admiralty Inlet-
East Entrance U.S. Waters Strait Of Juan De Fuca.
Gale Warning until 2 AM PST Friday for Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory from noon to 9 PM PST Friday for Puget
Sound and Hood Canal.
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