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FXUS66 KSEW 010601
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
1001 PM PST Thu Nov 30 2023

.SYNOPSIS...Active and potentially significant weather is
expected to affect the area through the middle of next week.
Expect lowland rain and heavy mountain snow Friday and Saturday.
The pattern will begin to shift Saturday afternoon as the first of
two atmospheric rivers moves into the area. The second
atmospheric river Monday into Tuesday is expected to be an even stronger
event with much higher snow levels, heavier rainfall, potential
river flooding and breezy to windy conditions.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...The first round of rain and
mountain snow continues to move off to our east, lingering through
midnight over the North Cascades. The next round of precipitation
is moving closer to the coast this evening, which will follow a
similar trend as that of earlier today, though slightly stronger
in intensity. Snow levels will stay around 2800 feet tonight,
keeping precipitation rain over the lowlands.

Given the upward trend in forecast rainfall this weekend into next
week and additional rivers forecast to enter flood stage, opted to
convert the Hydrologic Outlook to a Flood Watch. Given the spatial
spread in forecast river flooding, kept the watch for all of
Western Washington from Saturday night through Wednesday night.

Generally light precipitation will begin to pick up after
midnight along the coast with the the second in a series of storm
systems to affect the region. Snow levels will rise 2500 later
this evening accumulations at Snoqualmie and Stevens Pass to build
with around 3 to 5 inches by late tonight and ultimately up to 2
to 3 feet in the Cascades at pass elevations through early Sunday
morning. Traveling through the passes will be difficult during
this time. Accumulations are expected to be lower over the
Olympics with around 12 inches possible through the same period.
Along with the rain, and higher elevation snow, gusty winds could
be possible along the coast and areas like Whidbey Island
northward. These are expected to fall in the 35 to 45 mph range
especially Friday evening. There is a 10 to 20% chance that gusts
to be 45 to 60 mph.

The pattern will shift late Saturday afternoon as a moderate
atmospheric river begins to move into the area. This will bring
much higher snow levels rising to well above 4000 feet by Sunday
morning, and a plume of moisture supporting widespread moderate to
heave precipitation - especially over the higher terrain. Rising
snow levels will begin to limit snow to higher elevations. Heavier
rain in the mountains and possibly some snowmelt will likely drive
increasing impact to area rivers. Snow levels will continue to
rise to well above 5500 feet by Sunday afternoon. The rainfall
will likely be he heaviest in the Olympics and Cascades south of
Skagit County by Sunday afternoon. This will drive rises on area
rivers - especially those flowing from the Olympics and Cascades.


.LONG TERM /MONDAY THROUGH THURSDAY/... Ensemble guidance,
including ensemble based members continue to highlight the second
atmospheric river pushing a swath of warm Pacific moisture in W
WA, entering Monday into Wednesday. Guidance today has increased
the potential for even heavier accumulations with this system -
coupled with very high snow levels and rivers already running at
elevated levels, confidence is increasing regarding the potential
for multiple rivers reaching flood stage with this event and
elevated hydrologic impacts.

&&

.AVIATION...A mixed bag of ceilings across Western Washington, but
most terminals are MVFR or lower with light rain as the first of a
series of frontal systems are making their way across the region.
Visibilities are reduced in areas of heavier showers or where
mist/fog has developed. Stratiform rain will become isolated showers
overnight before the next front approaches the area. This front will
bring another round of rain beginning early Friday morning which
will keep ceilings MVFR or lower through the rest of the TAF period.

Flow aloft remains west-southwesterly with shortwave impulses moving
across the region. At the surface, strong onshore flow will begin to
pick up later tonight into Friday, with south-southwesterly winds 10-
15 kt, gusting to 20-25 kt.

KSEA...MVFR with rain becoming scattered showers. Low level mist/fog
reducing visibility at the terminal. Ceilings remain MVFR dipping
down to IFR at times tonight. Ceilings remain mostly MVFR through
the period with another round of rain arriving around 12Z Friday.
Winds southerly picking up to 10-15 kt gusting to 20-25 kt.

LH

&&

.MARINE...A weakening cold front has moved inland into Western
Washington, with another front on its tracks nearing the coastal
waters. Winds gusting to 20-25 kt will allow for the continuation of
Small Craft Advisories for the coastal and inner waters. The second
front will be more robust, bring gales to the coastal waters and the
East Entrance to the Strait of Juan de Fuca on Friday. The remaining
regions will continue to be under Small Craft Advisories until
Friday night. A brief break precedes the next system which will
likely warrant additional headlines beginning early on Sunday and
into the beginning of next week.

Seas 5 to 7 ft will rise above 10 ft early Friday morning and remain
hazardous, at 12-18 ft, until further notice.

LH

&&

.HYDROLOGY...Increasingly wet storm systems will set the stage
for a series of two atmospheric rivers expected to affect the
region. The first Saturday evening into Sunday will bring moderate
to heave precipitation to the region and snowlevels rising to well
above 5500 feet Sunday. Precipitation amounts through the weekend
are high enough to drive the Skokomish River to minor flood stage,
as well as rises on all area rivers. The second, stronger
atmospheric river Monday into Tuesday even higher snow levels well
above 8000 feet by late Monday night. The second event following
on the heals of the first will surely elevate the potential for
more widespread and elevated hydrologic impacts, possibly
affecting the region through much of next week. A hydrologic
outlook remains in affect.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Saturday evening through late Wednesday night
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-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-
     San Juan County-Seattle and Vicinity-Southwest Interior-
     Tacoma Area-West Slopes North Cascades and Passes-West
     Slopes North Central Cascades and Passes-West Slopes South
     Central Cascades and Passes-Western Skagit County-Western
     Strait of Juan De Fuca-Western Whatcom County.

     Winter Weather Advisory from 4 AM Friday to 4 PM PST Saturday
     for Olympics.

     Winter Storm Warning from 4 AM Friday to 4 AM PST Sunday for
     West Slopes North Cascades and Passes-West Slopes North
     Central Cascades and Passes-West Slopes South Central
     Cascades and Passes.

PZ...Small Craft Advisory from 1 AM Friday to 4 AM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 4 PM PST Friday for Admiralty Inlet-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 AM to 4 PM PST Friday for Central
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Friday 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-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Warning from 4 AM to 4 PM PST Friday for East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST Friday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Saturday for Puget Sound and
     Hood Canal.

     Gale Warning from 4 AM Friday to 1 AM PST 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.

&&

$$

NWS SEW Office Area Forecast Discussion