Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 152219
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
219 PM PST Sat Dec 15 2018

.SYNOPSIS...A front will move through the area late tonight and
Sunday bringing rain, locally windy conditions, and high mountain
snowfall. A stronger front will move across the area Monday night
through Tuesday giving locally windy conditions with and heavy
rainfall. Another strong front will move across the region
Thursday or Thursday night.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...Conditions are mainly dry
across the region this afternoon as we sit between weather
systems. A strong frontal system that is now seen over the
Washington and Oregon offshore waters is moving slowly eastward
and will cross the area late Sunday afternoon.

While the front will be stretching and weakening somewhat as it
moves into the area, pressure gradients will be strong enough to
produce southeast winds 20-35 mph with gusts to 45 to 50 mph over
areas near the water in Western Whatcom County, the San Juan
Islands, and the area around Admiralty Inlet and the eastern
Strait of Juan de Fuca. A wind advisory has been issued for these
zones tonight through Sunday morning.

Precipitation with the front is not expected to be exceptionally
heavy, but it will be heavy enough over the southern slopes of the
Olympics to potentially cause the Skokomish River in Mason County
to go back above flood stage late Sunday. This river is running
just below flood stage at this time and it would not take too much
rain to cause it to flood again. A flood watch is out for Mason
County.

Snowfall will be heavy, especially Sunday morning through early
afternoon, in the higher Cascades of Skagit and Whatcom counties.
The Mount Baker Ski Area will likely see 12-18 inches of new snow
before precipitation turns to rain and becomes lighter Sunday
afternoon. A Winter Storm Warning has been issued for the Cascades
of Whatcom and Skagit county for tonight into Sunday afternoon as
a result.

Sunday night through early Monday will see post frontal showers
gradually diminish across the area. There may be a brief break in
the action between systems Monday afternoon.

From Monday night through Tuesday, a strong frontal system
associated with a deep low moving into the SE Alaska Panhandle
will move SE across the area. This front will bring more wind and
rain to the area. The air mass ahead of this front will be quite
moist, and SSW winds 60-75 mph at 5000 ft will produce good
orographic uplift over the southwestward facing Olympics and
Cascades. Models have been advertising heavy precipitation
amounts with this front for several days now, but the more recent
model solutions have picked up on a more progressive frontal
system for less of a flood threat over the northern half of
Western Washington. Snow levels ahead of the front will only be
modestly high -around 4500-6000 ft. Flooding may be a threat to
rivers south of the Seattle area, but it is too early to tell at
this time. Albrecht

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...Extended models
have moved toward a solution where there is a break in the heavier
rainfall between Tuesday`s front and the next progressive frontal
system Thursday night. Some models show the potential for
Thursday night`s front to be quite windy with the deterministic
ECMWF and some GFS ensemble members showing a 990ish mb low moving
eastward along the Canadian border. Another front may move through
the area next weekend.

A blended model solution was used as a first guess forecast for
the Tuesday night through Saturday period. Albrecht

&&

.AVIATION...A strong frontal system will reach the area tonight
and move inland on Sunday. Southerly flow aloft. At the surface,
easterly flow tonight will become southerly late Sunday as the
front moves through. The air mass is moist and stable.

Mostly mid and high clouds currently although there are some
5000-7000 foot ceilings floating around. The trend will be for
gradual lowering ceilings this evening as the front nears and
rain develops. Low clouds should become widespread later tonight
but easterly winds ahead of the front will likely keep things VFR
for the interior. There should be some MVFR ceilings Sunday,
mainly west of Puget Sound.

KSEA...Mid and high clouds will lower to 5000-6000 feet this
evening as rain develops. Easterly wind 5-10 knots will increase a
little tonight and continue into Sunday, then turn southerly late
in the day as the front passes. Schneider

&&

.MARINE...A strong frontal system will bring another round of
gale force southerly winds to most waters tonight and Sunday
morning. There will be a relative lull in between fronts later
Sunday through Monday morning. A second strong frontal system will
bring more gales later Monday and Monday night. Moderate onshore
flow behind the second front will ease later Tuesday and Tuesday
night. A third frontal system will approach from the west on
Wednesday and likely bring another period of gales on Thursday.

The latest wave model shows 20-22 foot west swell reaching the
coast late Sunday afternoon and Sunday night. Schneider

&&

.HYDROLOGY...The flood prone Skokomish river is below flood stage
for now but the rain on Sunday will probably drive the river back
above flood stage: A flood watch is in effect for Mason county.

The weather pattern will remain active through next week and we will
have to keep an eye on rainfall amounts, snow levels, and river
forecasts. Overall, there seems to be enough of a gap between the
heavier rain in each system, and looking at the plumes of moisture--
the so called atmospheric river of moisture does not seem to stall
over Western Washington long enough to put us in an obvious flood
pattern. We will need to watch rivers over the southern portion of
the forecast area closely if the front slows in its southeastward
movement.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 6 PM this evening to 4 PM PST Sunday
     for Cascades of Whatcom and Skagit Counties.

     Flood Watch through Wednesday morning for Mason County.

     Wind Advisory from 6 PM this evening to noon PST Sunday for
     Admiralty Inlet Area-San Juan County-Western Whatcom County.

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

     Gale Warning until 10 AM PST Sunday 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 4 PM PST Sunday for East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Gale Warning until 4 AM PST Sunday for West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Warning until 4 PM PST Sunday for Central U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 10 AM PST Sunday for Admiralty Inlet.

     Small Craft Advisory until 4 PM PST Sunday for Puget Sound and
     Hood Canal.

&&

$$

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