Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201051
AFDSEW

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

.SYNOPSIS...Showers will continue over the area through tonight with
lowering snow levels and some mountain snow. The next 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. Snow accumulation in the Cascades may also be significant
on Saturday before snow levels rise. Some area river are at risk of
flooding and windy conditions are likely at times. Drier weather
develops Monday and Tuesday. Some light rain is possible Wednesday,
then drier weather returns Thursday.

&&

.SHORT TERM...A cool and showery pattern will persist through tonight
between weather systems. Snow level are falling with snow
accumulation likely in the mountains, mainly above 4000 feet. Meso
models show a strong convergence zone forming this afternoon through
this evening, 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.

It will be unstable today, with the best instability near the coast.
Kept a mention of isolated thunderstorms in the central/north coast
zones today. A strike or two could occur further inland but not
worth adding to the forecast given the low probability.

A warm front will bring rain to the coast late tonight and spread
into the rest of Western Washington Saturday morning. The heaviest
rain with the warm front will first reach Western Oregon, then lift
north over Western Washington late Saturday afternoon and evening.
The associated cold front will reach the area by Sunday morning with
rain eventually easing to showers late in the day. 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. Heaviest rain amounts,
up to 3 inches are possible from roughly Olympia southward with 1 to
2 inches north over the rest of the area. This will likely result in
minor flooding on some area rivers. See the hydrology section below
and flood watch statement for current details.

.LONG TERM...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 partial
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

&&

.AVIATION...An upper trough over the area today will shift inland
tonight. A warm front will move into the area from the west late
tonight. The flow aloft will be westerly. At the surface, onshore
flow today will turn easterly tonight. The air mass will be moist
and unstable today. It will remain moist tonight and become
stable. In general, ceilings will be generally VFR today. There
will be some lower ceilings, mostly in and around scattered
showers.

An upper short wave should cause an uptick in showers this
afternoon as it moves through. Showers should taper and most of
the lower clouds should be gone by this evening as the short wave
exits. 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 although ceilings
could pop up to a mid level deck at times. The low clouds are
expected to be mostly 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. Schneider

&&

.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 fairly strong 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. Schneider

&&

.HYDROLOGY...Models remaining consistent on the 00z run
indicating a heavy rain event Saturday afternoon through Sunday
morning. The heaviest rain will fall Saturday night. Focus of
heavier rainfall will be in Cascades from about Stevens Pass
southward, with less in the North Cascades and Olympics. The ECMWF
which was showing heavy rain over the North Cascades Saturday
night is more in line with the other models on the 00z run
focusing the heaviest precipitation in the central and southern
Cascades. Forecast precipitation amounts for Saturday through
Sunday morning range from 4 to 7 inches from Stevens Pass
southward in the Cascades with the higher amounts around Mount
Rainier to 2 to 4 inches in the Olympics and North Cascades.

The river model continues to show minor flooding on most rivers
flowing off the Cascades from the Skykomish River southward, with
a few rivers cresting in the moderate flood category. Flooding
will occur mainly late Saturday night into Sunday from the heavy
rain Saturday night.

A small shift in the pattern could still push some of the heavier
rain into the Olympics and North Cascades Saturday night into
Sunday. For this reason will stay with a flood watch for all of
western Washington except Island and San Juan counties from 5 pm
Saturday afternoon through 5 pm Sunday afternoon. Felton

&&

.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.

     High Surf Advisory until 6 AM PDT early this morning for Central
     Coast-North Coast.

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 for hazardous seas until midnight PDT
     tonight 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 from 11 AM this morning to 11 PM PDT this
     evening for Puget Sound and Hood Canal.



&&

$$

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You can see an illustrated version of this discussion at
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