Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201721
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
920 AM PST Mon Feb 20 2017

.SYNOPSIS...A frontal system will move northeast across the area
today, affecting mainly southern areas. Expect rain at times with
snow in the mountains. Another system will brush the area Tuesday as
it moves by even further to the south. An upper trough over the
region Wednesday through Friday will bring cool and showery weather.
Sunny and cool weather is likely Saturday.

&&

.SHORT TERM...Southwest flow aloft will continue over the region
today. A frontal system will move northeast across the southern
half of the area. Radar shows moderate rain in the far south with
light rain as far north as Seattle. Models show everywhere getting
at least a little rain today, though there is a chance the far north
will remain dry.

Snow will fall in the mountains, heavy enough in the Cascades of
Pierce and Lewis counties to warrant a snow advisory in effect
through this evening. New snow amounts there will be 5 to 10 inches.
Snoqualmie and Stevens passes are forecast to get just below
advisory amounts--3 to 6 inches. Highs today will be in the 40s. The
snow level will be 2500 to 3500 feet.

Showers will decrease tonight as the system moves northeast. Another
frontal system will develop off northern California late today, then
move inland across northern California and Oregon tonight and
Tuesday. The north side of this system will bring some precipitation
to mainly the southeast corner of our forecast area. Precipitation
amounts will be relatively light, and much of the area north and
west of Seattle could have dry weather all day.

An upper trough will move over the region from the west Tuesday
night, and then deepen over the area as it slowly moves east through
Wednesday night. It will bring generally light, scattered showers
and probably a weak Puget Sound convergence zone. The snow level
will fall to 1000 to 2000 ft, with several more inches of snow in
the mountains.

Highs across the Western Washington lowlands on Tuesday will be in
the mid to upper 40s, then in the lower to mid 40s Wednesday. Lows
should be in the mid 30s to lower 40s tonight, cooling into the
lower to mid 30s Wednesday night. Burke/McDonnal

.LONG TERM...Previous discussion...Northwest flow aloft will develop
over Western Washington Thursday, as the trough discussed above
moves east. Another trough will be carved out in the northerly flow
aloft on Friday. Both days should be partly sunny to mostly cloudy,
with spotty light showers, and highs in the lower to mid 40s. The
snow level will be only 500 to 1000 ft. The trough will dig into
California by Saturday, leaving dry northwest flow aloft over our
forecast area; it should be sunny and cool. Models agree fairly
well on all of this, and that another upper trough will dig over
the region from the northwest Saturday night or Sunday. That could
renew showers across the area, with the snow level only around
500 ft. McDonnal

&&

.HYDROLOGY...No flooding is expected during the next week. USGS
landslide guidance remains above the threshold at which landslides
typically occur. This means there will continue to be an elevated
threat of landslides for at least the next few days.

&&

.AVIATION...Active southwest flow aloft will carry a surface low
from northwest Oregon into eastern Washington this afternoon, with
a solid area of rain to the left of the low track over Southwest
Washington and the South Sound Region. Cigs and vsbys will lower
through early afternoon as rain and large-scale lift pick up.
Northerly surface winds will trend more westerly late this
afternoon as the surface low moves into Eastern Washington. Some
improvement in cigs and vsbys expected overnight, but any breaks
in the clouds could allow patchy fog to develop.

KSEA...Gradients supporting northerly surface winds will peak
around noon (20z), then quickly become southwesterly around mid-
afternoon and continue SW through the evening rush. A return of
northerly surface winds is expected Tue morning in response to yet
another low moving by to our south over Oregon. Otherwise, rain
will intensity and peak in the 21z-24z time frame this afternoon,
with cigs and vsbys bottoming out in that time frame as well.
Vsbys will improve early this evening as the rain eases, but cigs
of 015-030 will be common through tonight.  Haner

&&

.MARINE...A 994 mb low was located over the southern Willamette
Valley this morning. It will move northeast to Eastern Washington
this afternoon, causing a shift from northerly to westerly gradients
later today. Have added small craft advisory level westerlies to the
Strait for early this evening as westerly gradients peak.

A second low, this one with a central pressure of about 1000 mb,
will move inland on the southern Oregon coast Tuesday morning. In
response, winds will turn back to northerly again late tonight into
Tuesday as the second low moves by well to the south of the area.
Winds are expected to remain below 20 knots through Tuesday over the
coastal and inland waters.

A surface ridge will build offshore from Western Washington later
Tuesday into early Wednesday. This ridge will produce moderate
northwesterly flow over the coastal waters. Small craft advisory
conditions are likely over the coastal waters and through the Strait
of Juan de Fuca Tuesday afternoon and evening. Winds will ease
Wednesday into Thursday as a weak low forms around Cape Flattery
then drifts southward through the coastal waters.  Haner/Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 PM PST this evening for
     Cascades of Pierce and Lewis Counties.

PZ...Small Craft Advisory for hazardous seas until noon PST today 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 for rough bar until 4 PM PST this afternoon
     for Grays Harbor Bar.

     Small Craft Advisory from 1 PM this afternoon to 9 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PST
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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