Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181240 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
440 AM PST Sat Nov 18 2017

Corrected a couple of minor typos.

.SYNOPSIS...Weak high pressure aloft will prevail through
tonight. Expect a Pacific storm to approach the area during the
day Sunday. The cold front will sweep across the region during
the late Sunday afternoon through Sunday evening time period.
Expect locally strong winds and heavy precipitation ahead of and
with the front. Weak high pressure aloft will return on Monday.

&&

.SHORT TERM...
There were scattered showers over the CWA at this time. Expect the
showers to decrease in coverage this morning, becoming confined to
the far northwest part of the CWA by this afternoon.

Meanwhile, weak high pressure aloft will prevail over the region
through tonight. The ridge aloft will weaken further early Sunday,
allowing an upper level trof and associated cold front to approach
from the northwest. The models were a little slower with this
system than the previous runs; therefore, it appeared that the
southeast part of the CWA will probably not see any precipitation
until Sunday afternoon. The front will sweep across the area
during the late Sunday afternoon through Sunday evening time
frame. Expect locally strong southerly winds over parts of the
coast and northwest interior ahead of and with the front. A Wind
Advisory will probably be needed for some of these zones for
Sunday.

Concerning snowfall: The north Cascades will remain in the colder
air throughout the day Sunday; therefore, the precipitation will
fall as snow above the 3000 foot level. The Mount Baker ski area
could pick up about a foot of snow during the day Sunday;
therefore, a Winter Storm Watch was issued for Whatcom and Skagit
Counties. The snow will taper off over this area behind the cold
front Sunday evening.

Locally heavy precipitation will continue on the Cascades of
Pierce and Lewis Counties as the snow levels fall Sunday night;
therefore, the Crystal Mountain ski area and the southwest slopes
of Mount Rainier could receive up to about a foot of snow Sunday
night before the precip tapers off. Therefore, a Winter Storm
Watch was issued for this area.

Expect advisory snowfall amounts (5-10 inches) in the Cascades of
Snohomish and King Counties Sunday night. Stevens Pass may pick
up about 10 inches of new snow.

The GFS and ECMWF solutions were in better agreement as far as
Monday was concerned. The Canadian model was the outlier. It looks
as though a warm front will approach the region from the south
during the day. Much of the CWA will be dry on Monday.

.LONG TERM...
Much of this period appeared mild, wet, and unsettled. Temps are
anticipated to be above normal during this period. Expect locally
windy periods, with the possibility of flooding on some area
rivers. Confidence in the details was not high for this period.

&&

.AVIATION...A flat upper ridge centered over Western Washington
early this morning will shift east into the Idaho Panhandle late
tonight. A strong and wet cold front will sag southeast through
British Columbia today then will shift southward through Western
Washington on Sunday. Moderate westerly flow aloft. The air mass
is moist and somewhat stable and will dry somewhat at lower levels
today. Deep moisture will shift southward into the area late
tonight into Sunday morning.

Isolated convective showers over the coastal waters and extending
into the interior from about KSHN southward continue to weaken and
diminish in coverage early this morning. Conditions are variable
with pockets of MVFR or IFR ceilings and other areas seeing
ceilings 035-045. Expect the lower level moisture to decrease this
morning leaving just mid and high level clouds above about FL100.
Cigs will fall to 008-015 after 10Z tonight through Sunday
morning as rain ahead of the incoming front shifts southeast into
the area. Albrecht

KSEA...South wind 8-11 knots. CIGS 030-035 this morning then above
FL100 this afternoon and evening. CIGS falling to 010-015 after
10Z Sunday. Albrecht

&&

.MARINE...A strong cold front extending WSW from the Queen
Charlotte Sound into the Pacific will slowly drag southeast down
the British Columbia coastline today then will push southeastward
through the coastal and inland waters on Sunday. Winds will
increase gradually from north to south through tonight with gales
expected over all of the waters except for Puget Sound and Hood
Canal and the Central Strait of Juan de Fuca late tonight into
Sunday ahead of the front. Winds will rapidly ease behind the
front later Sunday into Sunday night.

High pressure will bring light winds to the waters on Monday. A
strong warm front will push northward through the waters on
Tuesday. A strong low pressure center developing well offshore
will push northeast into Haida Gwaii on Wednesday. Albrecht

&&

.HYDROLOGY...
The models were not as wet with the Sunday system as the previous
runs; however, the forecast rainfall amount for the Skokomish
River basin was still enough to drive the uniquely flood-prone
Skokomish River near or slightly above flood stage Sunday night.

Beyond Sunday, the models did not exactly agree on the details.
However, it still appears that much of this period will be wet.
The snow levels will be atypically high (generally at or above 8K
feet) for this time of the year Tuesday through early Thursday.
This means that the precip will fall primarily as rain in the
mountains. It looks like this firehose will be a bit different in
that instead of staying on for X amount of time, it will be more
of an on and off type of deal. Thus, expect occasional heavy rain
in the mountains and on the coast. Although rainfall amounts may
not be significant for any given time period, the cumulative
rainfall for the Tuesday through Thursday period will probably be
enough to drive some rivers above flood stage, not just the
Skokomish River.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch in effect for Mason County from Sunday morning
 through Sunday evening.

     Winter Storm Watch from Sunday afternoon through late Sunday
  night for Cascades of Pierce and Lewis Counties above 4500
 feet.

     Winter Storm Watch from late tonight through Sunday afternoon
     for Cascades of Whatcom and Skagit Counties above 3500 feet.

PZ...Small Craft Advisory until 10 PM PST this evening for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Northern Inland Waters Including The San Juan Islands.

     Gale Warning from 10 PM this evening to 3 PM PST Sunday for
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory until 6 PM PST this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm.

     Gale Warning from 6 PM this evening to noon PST Sunday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm.

     Gale Warning from 10 PM this evening to noon PST Sunday for
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PST Sunday for Coastal Waters
     From James Island To Point Grenville Out 10 Nm-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 3 AM to noon PST Sunday for Coastal Waters
     From James Island To Point Grenville Out 10 Nm.

     Small Craft Advisory from 9 AM this morning to 3 AM PST Sunday
     for Coastal Waters From Point Grenville To Cape Shoalwater
     Out 10 Nm.

     Gale Warning from 3 AM to 3 PM PST Sunday for Admiralty Inlet-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PST Sunday for Admiralty Inlet.

     Small Craft Advisory from 10 PM this evening to 9 PM PST Sunday
     for Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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