Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151625
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
825 AM PST Wed Feb 15 2017

.SYNOPSIS...A frontal system will bring moderate to heavy rain and
locally windy weather through tonight. A broad upper trough will
follow the front for cooler and somewhat showery weather Thursday
into early next week.

&&

.SHORT TERM...A frontal system in the form of two waves - one
earlier this morning and one late this afternoon and tonight -
will push through the area today and tonight. Showers will linger
on Thursday in the wake of the front as an upper trough settles
over the area. The area will mostly dry out on Friday as a
weather system moves inland well to the south and a weak upper
ridge builds over the area. Some moisture from the system to the
south will creep up into the area on Saturday. The best chance for
precipitation will be over the eastern half of the forecast area
as this will be glancing blow with most of the precipitation
staying south and east of the area.

There was a burst of winds and rain early today as the first part
of the frontal system moved onshore. Windy conditions coast and
north interior have been easing the last few hours behind the
first wave and the current wind advisory will be allowed to expire
at 10 am. Winds should rise again in these same areas late this
afternoon and tonight as the main front pushes onshore. Another
wind advisory will likely be needed.

Besides the wind, there has been some light freezing rain in the
passes but it does not seem to be having much impact with road
just bare and wet. Pass temperatures are just below freezing and
they will probably nudge up slightly today. The freezing rain
advisory will probably be allowed to expire at noon today.

There are also some flooding concerns. See the hydrology section
below for details. Schneider

.LONG TERM...Previous discussion...A longwave trough over the
West will keep the weather pattern active across Western WA
through the holiday weekend. Models are showing a series of fronts
impacting the region each day - Saturday through Monday. The
Monday system looks the strongest and wettest of the three. The
cool upper trough may linger over the region through the middle of
next week. Models seem on track so far. Temperatures will remain
below normal through the period. 33

&&

.AVIATION...There is a little break in the precip this morning
but the areas of precip on the radar will increase again by
afternoon as another round of heavier rain moves through the area.
There is ample moisture for increasing areas of low cigs and poor
vsby and the mountains and foothills that are not obscured this
morning will become obscured again this afternoon. Strong moist
southwesterly flow aloft will continue through tonight as a
frontal system moves through the region. The air mass is stable. A
deep low pressure center offshore will track to Queen Charlotte
sound Thursday morning.

KSEA...Rainy weather increasing again this afternoon. A southeast
breeze will increase tonight.

&&

.MARINE....Winds and pressure gradients are easing but a deep
low off the coast tonight will another round of gales. Southeast
gales are likely over most waters as the deep low pressure center
tracks to Queen Charlotte sound Thursday morning. Onshore flow
will follow on Friday. Current models bring westerly swell of
15-20 ft to the coast on Thursday.

&&

.HYDROLOGY...A frontal system will slowly push through the area
today with heavy rains and high snow levels. In general, 3.5-7
inches of rain is likely over the Olympics and slightly lesser
amounts for the Cascades for the 48 hour period ending early
Thursday. This will bring a renewed threat of flooding to some
area rivers beginning Wednesday.

The runoff in the river models are quite sensitive to the
temperatures for this storm so there is a fair amount of
uncertainty for the crests. The one exception is for the Skokomish
River where there is high confidence in river levels exceeding
flood stage.

The Skokomish river in Mason county will likely reach flood stage
Wednesday. A Flood Warning is in effect. Other rivers flowing off
the Olympics and possibly the Cascades, especially the north half,
could also flood Wednesday or Thursday. A Flood Watch is in
effect to address this threat.

In addition, the rainfall that occurs tonight through Thursday
should boost the USGS landslide guidance above the threshold at
which landslides typically occur. This means another cycle of
landslides is possible in the next few days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Freezing Rain Advisory until noon PST today for Cascades of
     Snohomish and King Counties.

     Flood Watch through late tonight for Cascades of Whatcom and
     Skagit Counties-Western Whatcom County.

     Flood Watch through late tonight for Admiralty Inlet Area-
     Central Coast-Eastern Strait of Juan de Fuca-Hood Canal
     Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Western Strait of Juan De Fuca.

     Wind Advisory until 10 AM PST this morning for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom
     County.

PZ...Small Craft Advisory for rough bar until 10 AM PST Thursday for
     Grays Harbor Bar.

     Gale Warning until 7 AM PST Thursday for Admiralty Inlet-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-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-West Entrance U.S. Waters Strait Of Juan
     De Fuca.

     Small Craft Advisory until 4 AM PST Thursday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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