Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201623
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT Fri Oct 20 2017

.SYNOPSIS...Showers will continue over the area through tonight with
some mountain snow. A strong storm system will affect the region
over the weekend. Rain will increase Saturday and become heavy at
times, especially over the coast and mountains. There may be snow
accumulation in the Cascades Saturday before snow levels rise. Drier
weather will develop Monday and Tuesday.

&&

.SHORT TERM...A shallow upper trough will approach Washington today,
then move through tonight. A cool and showery pattern will persist
through tonight. The snow level is around 4000 feet, with snow on
the ground at both Stevens and White Passes. A few inches of snow
is visible in the Paradise parking lot. Mesoscale models show a
convergence zone forming this afternoon through this evening,
mainly affecting King and Snohomish counties. This will produce
locally heavy precipitation from around north Seattle to Everett and
east over the Cascades. Locally heavy snow in the Cascades of King
and Snohomish counties could produce 4 to 10 inches of snow above
4000 feet by late tonight, including Stevens pass. The snow level
could fall to as low as 3000 feet this evening as the upper trough
passes. A snow advisory is in effect for the Cascades of King and
Snohomish counties to cover this.

The air mass is weakly unstable, with the most unstable air near the
coast. Isolated thunderstorms are possible on the coast today.

A warm front will bring rain to the coast late tonight; the rain
will spread into the interior Saturday morning. The heaviest rain
with the warm front will first reach Western Oregon, then move north
over Western Washington Saturday afternoon and evening. A cold front
will reach the area Sunday morning and rain will turn to showers.
High temperatures will be in the low 50s today, rise to the mid 50s
Saturday, and be near 60 Sunday. Burke

.LONG TERM...Previous discussion...Residual showers will decrease
and end Monday as high pressure aloft quickly builds north over the
region. Dry weather is expected across the area Monday night and
Tuesday with sunshine and a little warmer daytime temperatures.

Most models still bring a weak front through the area on Wednesday.
This will be a progressive front, so light rain will not stick
around for long. River flooding with this system is not a concern. A
return to dry weather is expected Thursday. Mercer

&&

.HYDROLOGY...Model runs this morning show overall QPF in most places
similar to previous runs. There has been some change, however. The
NAM and GFS both bring heavy rain a bit further north in the
Cascades and also now have heavy rain over the Olympics. This seems
to be a typical progression in our atmospheric river events--the
rain in early model runs starts out concentrated in certain areas
but in later runs spreads to most areas. Another change is that the
heavy rain appears concentrated in a 12 to 18 hour period, from 18Z
Saturday through 12Z Sunday.

Total rain amounts could exceed the last storm, especially over the
south/central Cascades where 4 to 7 inches could fall through
Sunday. Locally higher amounts are possible. Over the lowlands an
inch or so of rain is forecast, except over the south Puget Sound
region and the southwest interior where around two inches could
fall. Two inches of rain in 12 or 18 hours will probably be enough
to cause some urban and small stream flooding.

The river model shows minor to moderate flooding on most rivers
flowing off the Cascades from the Skykomish southward, and no
flooding in the northern Cascades or the Olympics. When the model is
run next, it may show flooding farther north and in the Olympics. In
any case, the flood watch remains in effect for all of Western
Washington. Burke

&&

.AVIATION...Southwest flow aloft continues over Western Washington
early this morning, ahead of an upper trough over the offshore
waters. The air mass is moist and unstable with low level onshore
flow. The upper flow will veer to westerly today as the trough
moves inland, then increase tonight as a vigorous Pacific frontal
system reaches the Pacific Northwest. The air mass will remain
moist tonight but stabilize.

There is a mix of VFR and MVFR conditions across the forecast
area, with the lower conditions mainly associated with showers.
Showers should increase somewhat today as the upper trough moves
through, then taper off this evening as it moves east. The
exception will be in a Puget Sound convergence zone, which will
be north of Seattle this evening.

KSEA...MVFR low clouds will linger this morning, with ceilings
increasing to the mid level deck at times. The lower clouds should
be gone by afternoon except perhaps near showers. Ceilings will
lower again late tonight and Saturday morning as a warm front
arrives. South wind 7-13 knots. McDonnal

&&

.MARINE...Onshore flow today will turn easterly tonight ahead of
an approaching warm front. Small craft advisory winds will develop
over most of the waters today; the strongest winds will likely be
the westerlies in the Central and Eastern Strait of Juan de Fuca.
Gales are not out of the question in the Strait but they would be
marginal and brief at best.

A vigorous Pacific frontal system will move through the area
Saturday and Sunday, with widespread small craft advisory winds.
Gales are possible over the Coastal Waters, entrances to the
Strait, and Northern Inland Waters late tonight and Saturday
morning. There could be another period of gales over the Coastal
Waters Saturday evening, especially south part.

Onshore flow behind the frontal system will ease and eventually
turn weakly offshore later Monday and continue Tuesday.

West swells of 16 to 19 feet over the Coastal Waters will
gradually subside today and tonight. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Saturday afternoon through Sunday afternoon for
     Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
     Vicinity-Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Central Coast-East Puget Sound Lowlands-Eastern
     Strait of Juan de Fuca-Everett and Vicinity-Hood Canal Area-
     Lower Chehalis Valley Area-North Coast-Olympics-Seattle and
     Vicinity-Southwest Interior-Tacoma Area-Western Skagit
     County-Western Strait of Juan De Fuca-Western Whatcom
     County.

     Winter Weather Advisory from 1 PM this afternoon to midnight PDT
     tonight for Cascades of Snohomish and King Counties.

PZ...Small Craft Advisory for rough bar until 6 PM PDT this evening
     for Grays Harbor Bar.

     Gale Watch from late tonight through Saturday 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.

     Small Craft Advisory until 1 AM PDT Saturday 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 Watch from late tonight through Saturday afternoon for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory from 1 PM this afternoon to 3 AM PDT
     Saturday for East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory from 1 PM this afternoon to 11 PM PDT this
     evening for Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory until 6 PM PDT Saturday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to 5 PM PDT
     Saturday for Admiralty Inlet-Central U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening for Puget
     Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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