Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171054

Area Forecast Discussion
National Weather Service Seattle WA
254 AM PST Wed Jan 17 2018

.SYNOPSIS...A warm front will clip the area today followed by a
cold front tonight. Large waves will impact the coast on
Thursday. A broad upper trough will traverse the area Thursday and
Friday, keeping a cool, showery pattern in place. Unsettled
weather will continue into this weekend, with a front enhancing
rainfall and mountain snow Saturday night and Sunday. An upper
ridge may build around next Tuesday for a drying trend.


.SHORT TERM...A strong Pacific system, low near 965 mb, will track
NE toward southern B.C. today, then gradually stall/weaken off
Haida Gwaii on Friday. The main impacts from this system will be
strong winds at the coast and north interior today then large
swells reaching the coast on Thursday. For the winds, generally 20
to 35 MPH with gusts to 45 MPH for the zones mentioned above.
Winds will ease tonight. A Coastal Flood Watch is in effect for
the north and central coast with the large swells arriving on
Thursday. Models show 25-30 foot swells over the inner coastal
waters during this time with flooding and beach erosion possible.

Precip-wise, we have a warm front clipping the area this morning
then a brief break in the afternoon as the front lifts north into
B.C. Rain will increase in coverage tonight as the cold front
moves inland. Snow levels are high but easterly winds may keep
temperatures near freezing in the passes. The passes may see a
rain/snow/freezing rain mix with up to 5 inches of snow on the
volcanoes. Moist, southwest flow will continue on Thursday with
showers likely. There is a chance of showers on Friday as the cool
upper level trough moves in. Snow levels in the mountains will
lower to around 2,000 feet.

The flow aloft is nearly zonal on Saturday with showers mainly on
the coast and in the mountains. Temperatures are trending close to
normal. 33

.LONG TERM...A wetter frontal system is slated for late Saturday
night into Sunday. Snow levels will remain low, near 2,000 feet,
for more snow in the mountains. A weak ridge may bring drier
weather Monday night and Tuesday. A series of fronts will then
keep the weather unsettled around midweek. 33


.AVIATION...Moderate southwesterly flow aloft today will become
stronger tonight as a warm front and then a cold front move through
the region. At the surface, southeasterly winds will increase today,
especially on the coast. The air mass is moist and stable.

Rain will spread inland this morning, then the warm front will exit
to the north this afternoon. Precipitation will become more showery
this afternoon and evening, then increase again as a cold front
moves through the area tonight. Ceilings will be mostly VFR today,
then MVFR tonight.

KSEA...Discussion above applies. Southeast wind 10-15 kt today
becoming more southerly and increasing to 10-20 kt tonight. CHB


.MARINE...A deep low far to the northwest will create strong
southeasterly pressure gradients over area waters for the next
several days. Today a warm front will move south to north through
Western Washington. Strong gale force winds are forecast on the
coast, with low end gales at the east entrance and the northern
inland waters. Small craft advisories are in effect elsewhere. The
offshore coastal waters will have the strongest winds; they may
approach storm force.

A cold front will move through the area from west to east tonight
and it will probably generate gales again. Gale force winds are also
possible Friday and Saturday, especially on the coast.

Strong southwest winds well off the coast will generate large swells
that will reach the Washington coast Thursday. Models have been
consistent in bringing westerly swells of 25-30 ft to the coast,
with even larger swells offshore. Seas have not reached 30 ft
at Buoy 41 since December 2015. A coastal flood watch is in effect
for the coast. This sort of coastal flooding mainly affects the
beaches as waves run far up beyond where they usually do. Exposed
towns along the beaches, such as Ocean Shores and Westport, could
have waves crash over their levees, and sea water could collect in
low areas. CHB


.HYDROLOGY...A brief period of heavy rain is possible in the
Skokomish Basin tonight. Current rainfall forecasts support
bringing the Skokomish to within a foot of flood stage, but it is
still not expected to flood.

The next round of heavier rain will come around Saturday night or
Sunday. This rainfall should cause most rivers to rise, but only
the Skokomish appears to be at risk of flooding at this time. An
inhibiting factor will be that snow levels could remain at or
below 3000 feet, which would reduce runoff down the river.


WA...Wind Advisory from 10 AM this morning to 6 PM PST this evening
     for Admiralty Inlet Area-Central Coast-North Coast-San Juan
     County-Western Skagit County-Western Whatcom County.

     Coastal Flood Watch from late tonight through Thursday evening
     for Central Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 4 AM PST Thursday for
     Grays Harbor Bar.

     Gale Warning until 4 AM PST Thursday 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 this afternoon for 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 Thursday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-Puget Sound and
     Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.


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