High Wind in Washington, December 17 to 22, 2018

Area Forecast Discussion
National Weather Service Seattle WA
844 AM PST Mon Dec 17 2018

.UPDATE...Forecasts are generally on track with a vigorous front
associated with a 160 kt jet stream at 300 mb aimed toward the
southern portion of the forecast area. Gridded forecasts were
nudged to a blend of incoming 12Z forecast models to increase QPF
and snow amounts slightly, and that results in the need to issue
a winter weather advisory this afternoon and evening for the
Olympics above 4000 feet. The remainder of the forecast is
essentially unchanged. The aviation and marine sections below have
been updated. Albrecht

&&

.SYNOPSIS...Lingering showers are still present over portions of
Western Washington in the wake of an exiting front this morning.
After a brief break this afternoon, a strong front will move slowly
southeast across the area tonight through Tuesday bringing locally
heavy rain and windy conditions, with snow in the mountains.
Additional strong fronts will move through the area on Thursday and
late next weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...From the 330 AM PST
discussion: Showers continue over portions of the area this early
morning thanks to lingering shortwaves behind the front that
passed through yesterday. This will keep some precip in the
forecast this morning. These showers are expected to dissipate
somewhat by this afternoon...just as the next frontal system makes
its way to the coast.

As this front makes its way into W WA a fair amount of headlines
come with it. Pressure gradients with this front will result in
strong and gusty winds along the coast by afternoon and then in the
north interior this evening. As such...a high wind warning will go
out with the morning forecast package to cover this. Precip with
this system is likely to exacerbate flooding along the
Skokomish...but may also begin to trigger other rivers in the area
to rise to near flood stages as well. This is covered further in the
Hydrology section below as well as in the ESF/Hydrologic Outlook.
Then...plodding slowly eastward...the front will likely result in
some heavy snowfall for the North Cascades of Whatcom and Skagit
Counties...so feel good about upgrading the inherited Winter Storm
Watch into a Warning. Model precip output is also putting 11 to 17
inches in the vicinity of Stevens Pass for the period from tonight
through the day Tuesday and into Tuesday evening...landing it right
in Snow Advisory criteria. Those headlines will also go out in the
morning package. One point of interest will be how snow levels will
rise throughout the progression of the system...starting out around
3000-3500 feet and then lifting to above 4000 Tuesday morning and
afternoon.

Hinted at earlier in this discussion...this system is a slow mover
with effects lingering through most of the day Tuesday before very
gradually tapering off Tuesday night into Wednesday. Models continue
to suggest some degree of upper level ridging over the Pac NW for
Wednesday...but for W WA...the feature looks pretty dirty...with the
GFS retaining its shortwaves keeping the prospect for some showers
in place while the ECMWF keeps the ridge progressive enough for the
next system to overrun the ridge and spill into the area.
Thus...pretty hesitant to advertise any sort of break for midweek.
The pattern remains active as yet another system is on tap for
Thursday. SMR

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...From the 330 AM PST
discussion: Some model disagreement on finer details of Thursday
system with the GFS a little bit faster with it. Both models do
agree that precip will be present most of Thursday...but the GFS
is again a little faster...pushing the system out by Friday
morning while the ECMWF lets it linger. Both models do agree in
dry conditions by midday Friday as an upper level ridge builds
into the area. The GFS keeps dry conditions into Saturday while
the Euro dirties the ridge up with some precip starting to creep
into the area through the day. Another system looks to sweep
through the area for Sunday. SMR

&&

.AVIATION...A weak upper trough over the area will move inland
today. A strong frontal system will move through the area
tonight. Increasing southwest flow aloft. The air mass is moist
and stable.

Mid and high clouds through early afternoon with some lower
ceilings near the coast. Ceilings will gradually lower late in
the afternoon and evening with MVFR low clouds becoming widespread
tonight as the front moves onshore.

KSEA...Mid clouds today. Ceilings will lower tonight as a front
moves onshore. Winds aloft will increase this afternoon and
evening leading to some possible low level wind shear. South wind
5-10 knots will increase to 10-15 knots tonight and become gusty.
Schneider

&&

.MARINE...A strong front will approach the coast today and move
inland tonight. High end southerly gales are likely over the
Coastal Waters with this system. There will also be gales in the
Strait of Juan de Fuca and over the Northern Inland Waters, with
at least small craft advisory strength winds elsewhere.

Moderate to strong onshore flow will follow on Tuesday and then ease
on Wednesday. Another strong frontal system will bring more gales
Wednesday night and Thursday. Onshore flow behind the Thursday
front will ease on Friday.

Long period west around 20 feet will affect the coast this
morning. A high surf advisory is in effect for the coast until 10
AM. Swells will fall to 17-19 feet late this morning and afternoon.
Schneider

&&

.HYDROLOGY...From the 330 AM PST Discussion: The Skokomish river
remains above flood stage. It will slowly fall today but rise
again tonight in response to more rain. The river is unlikely to
fall below flood stage for at least a few days, and could reach
the moderate flood stage late tonight.

Rainfall from the system tonight will be focused on the Olympics and
Southwest interior. There is a possibility of flooding on some
rivers in those areas on Tuesday. A hydrologic outlook covers that
possibility. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 6 PM this evening to 6 PM PST
     Tuesday for Cascades of Snohomish and King Counties.

     Winter Storm Warning from 6 PM this evening to 6 PM PST Tuesday
     for Cascades of Whatcom and Skagit Counties.

     High Wind Warning from 4 PM this afternoon to 1 AM PST Tuesday
     for Admiralty Inlet Area-San Juan County-Western Skagit
     County-Western Whatcom County.

     High Wind Warning from 1 PM this afternoon to 10 PM PST this
     evening for Central Coast-North Coast.

     High Surf Advisory until 10 AM PST this morning for Central
     Coast-North Coast.

     Winter Weather Advisory from 2 PM this afternoon to 4 AM PST
     Tuesday for Olympics.

PZ...Small Craft Advisory for rough bar until 4 PM PST Tuesday for
     Grays Harbor Bar.

     Small Craft Advisory until 10 AM PST this morning 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 Warning 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 until 1 PM PST this afternoon for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Gale Warning from 1 PM this afternoon to 4 AM PST Tuesday 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 4 AM PST Tuesday for Admiralty Inlet-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.

&&

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