Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 181235
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
435 AM PST Mon Dec 18 2017

.SYNOPSIS...A cold front will turn up stationary over Southwest
Washington this afternoon. A strengthening low center will track
across Northwest Washington on Tuesday, with a sharp cold front
trailing the low on Tuesday afternoon. This will bring heavy
mountain snow and generally windy weather on Tuesday. A few
lingering showers will continue Tuesday night and Wednesday
morning. A cold and dry air mass will end the showers on
Wednesday. Around Thursday night, a wave diving southeast through
B.C. will increase clouds with a low chance of some light lowland
snow. Cool and dry weather expected under high pressure from later
Friday through next weekend.

&&

.SHORT TERM...Slow-moving cold front sinking southward through
Western Washington this morning. The large area of precip is
slowly sinking south with it. For northern locations from the
North Coast over to the San Juans and Bellingham, looking for a
break in precip for some of the day. Further south toward Seattle
and Olympia, will be hard to discern the difference in today`s
front and tomorrow`s storm just by looking out the window, since
the two systems will only be separated by a period of lighter
precip this afternoon.

Later today, cyclogenesis will occur along the offshore portion of
the front between 130W and 140W. The deepening low center will
then track east-northeast, reaching the North Washington coast
around sunrise on Tuesday as about a 997-999 mb low center.
Central pressure in the low will continue to fall until it reaches
the Cascades, a sign of the large-scale lift and forcing in
advance of the low. Heavy precip will develop early this evening
and peak in intensity late Tuesday morning. With snow levels
ranging from 2000-4500 feet depending on time and location, this
will spell heavy snow in the mountains. Snow will be measured in
feet over the course of tonight and Tuesday. During the most
intense precip, the snow level for Stevens and Snoqualmie Passes
will be around 3500 feet, so Stevens will get heavy snow, while
Snoqualmie Pass should pick up mere inches, mainly after a sharp
cold front later Tuesday lowers snow levels.

The other story on Tuesday will be wind. Having the low center
track directly over the area actually helps some. The strongest
winds will come in the vicinity of the cold front as it swings
through around mid-day Wed. In the Puget Sound region, wind gusts
around 35-40 mph are expected. Based on the current forecast, the
Central Coast around Grays Harbor is the only zone that would
eventually necessitate a Wind Advisory.

By Tuesday night, looking for convective showers in the cold,
unstable post-frontal air mass. With onshore flow, showers will
focus on the terrain and in a Puget Sound Convergence Zone
roughly near Everett. The air mass will continue to cool on
Wednesday morning as northeast wind near Bellingham brings colder
and drier continental air out of the interior of B.C.. This could
cause a few lingering showers over the lowlands to mix with snow
Wed morning. Showers will be in the process of dissipating as it
gets cold enough for some snow, so only minor accumulations are
expected at worst in a Convergence Zone or near the Cascade
Foothills. Most lowland locales will get no snow.

Despite some sunshine on Wed afternoon, the cold air mass will
mean high temps only in the upper 30s and lower 40s.

.LONG TERM...The latest wrinkle in the forecast is a shortwave
that will slide southeast through interior B.C. late Thu and early
Fri. The 00z ECMWF is sharper with this feature and blossoms some
light precip over the interior Western Washington on Thursday
night and Friday morning. Meanwhile, the 06z GFS is weaker and
brings no precip. Only a few of the GEFS 20 ensemble members shows
measurable precip for this time period. Still, with a cold air
mass and a weak precip signal, have gone with low chance PoPs in a
rain/snow mix over the lowlands Thu night and Fri morning.

Models then agree in showing an upper ridge axis with cold and dry
weather from late Friday through the end of Day 7.  Haner

&&

.AVIATION...Westerly flow aloft will continue. A front over the
area will sag south today and then lift back north as a warm front
tonight. A trailing cold front will move onshore Tuesday. An
associated deepening surface low will make landfall over the north
coast Tuesday morning. The air mass is moist and stable.

KSEA...MVFR low clouds are expected to be continue today and
tonight although ceilings might lift to VFR at times this
afternoon. Gusty south wind 10-20 knots will ease this morning and
probably turn more easterly this evening before becoming southerly
and increasing again Tuesday morning. Schneider

&&

.MARINE...A front over the area will sag south today with winds
generally easing. A brief westerly gale is still possible early
this morning in the Strait of Juan de Fuca. Winds in the Strait
will ease to small craft later this morning and then below small
craft by afternoon. Small craft advisory strength south winds in
Puget Sound should ease this morning as well.

A second vigorous frontal system will move through the area
tonight and Tuesday with a warm front lifting north through the
area tonight and a cold front shifting inland Tuesday. An
associated deepening surface low will move across the area
Tuesday. The low is expected to make landfall just south of UIL
Tuesday morning. South of the low, gale force wind are expected
Tuesday over the Coastal Waters. Small craft advisory strength
winds are expected all other waters. Portions of the inland
waters could also hit gale Tuesday if the low winds up slightly
farther north so this bears watching.

Offshore flow will develop Wednesday and continue Thursday and
Friday. Schneider

&&

.HYDROLOGY...Rises on area rivers are expected on Tuesday due to
heavy precipitation, but snow levels around 2500 to 3500 feet will
limit runoff. Therefore, river flooding is not expected the next
7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST this morning for
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties.

     Winter Storm Warning from 6 PM this evening to 10 PM PST Tuesday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Storm Warning from 4 PM this afternoon to 3 PM PST
     Tuesday for Olympics.

PZ...Gale Warning from 4 AM to 4 PM PST Tuesday for 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 from 10 PM this evening to 4 PM PST Tuesday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until noon PST today for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Warning until 6 AM PST early this morning for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PST early this morning for Puget
     Sound and Hood Canal.

&&

$$

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