Heavy Snow in Washington, December 9 to 10, 2023

Area Forecast Discussion
National Weather Service Seattle WA
826 AM PST Sat Dec 9 2023

.SYNOPSIS...Strong frontal system moving into Western Washington
this weekend bringing moderate to heavy rain, mountain snow, and
gusty winds to the area. Upper level ridge building in behind the
front Monday will continue to build Tuesday before moving east
Wednesday. A weakening system will move through later Wednesday
into Thursday. Another upper level ridge will try to build Friday.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...No significant changes to the
forecast this morning. Rain has started to move in along the coast
and will spread inland late this morning into the afternoon. The
rest of the discussion can be found below:

Next system, a warm front, reaching Western Washington today.
Rain developing along the coast this morning with rain spreading
inland this afternoon. There could be a little cool air trapped
along the east slopes of the Olympics, around the Hood Canal and
up in Whatcom county for the precipitation to start as rain but
the increasing southerly winds will warm the air mass up enough to
turn the precipitation to all rain by early afternoon. Snow levels
will be low to start out with in the mountains, 1000 to 2000
feet, with the snow levels rising to around 3500 feet late in the
day in the Olympics and South Cascades. New snow amounts in the
mountains ranging from 2 to 4 inches in the Olympics to 3 to 11
inches in the Cascades with the higher amounts near Mount
Rainier. Winds increasing in the usual locations with a warm
front, coast and Northwest Interior including Whidbey Island and
Admiralty Inlet. Winds getting strong enough for a wind advisory
beginning in the morning along the coast and early afternoon hours
for the locations in the interior. Southeast winds with gusts as
high as 45 mph in the forecast. Highs today in the mid 40s over
the interior. A little warmer on the coast with the warm air
arriving earlier with highs in the lower 50s.

Warm front remaining over the area tonight and weakening. Snow
levels rising to 5500 to 6500 feet in the Cascades and 7000 feet
in the Olympics. Some additional snow in the evening for the pass
elevations in the evening before the precipitation turns over to
rain. Tough call in the passes on precipitation type. Surface
gradients are easterly which will pull in a thin layer of cold air
right near the surface of the passes. 925 and 850 mb winds are
southwesterly which opens up the possibility of the cold air
remaining trapped near the surface ( would like to see more
westerly winds aloft to scour or eliminate the cold air in the
passes. Have put a chance of freezing rain in for Stevens and
Snoqualmie pass overnight. Winds easing along the coast early in
the evening and over the interior around midnight. Lows in the
upper 30s to mid 40s.

Trailing cold front moving northwest to southeast across Western
Washington Sunday ending up in Northern Oregon late in the day.
Precipitation rates easing in the afternoon. Snow levels dropping
slightly in the afternoon but still remaining above 5000 feet
keeping the threat of minor river flooding going in a couple of
locations. See hydrology section for move details. With the front
moving into Northern Oregon cross Cascade gradients remain
negative so the possibly of the shallow layer of cold air near
the surface in the passes remains so will have a chance of
freezing rain in the passes again Sunday. Highs in the lower to
mid 50s.

Front continues to weaken and move southeast Sunday night. Weak
upper level trough moving into Western Washington keeping at least
a chance of showers in the forecast. Snow levels remaining above
the passes and the cross Cascade gradient goes flat keeping the
precipitation type just rain in the passes. Lows in the upper 30s
to mid 40s.

Upper level trough moving southeast out of the area Monday
afternoon with a weak upper level ridge building offshore. Shower
chances decreasing as the day wears on. Highs a little cooler, in
the mid 40s to lower 50s.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Upper level ridge offshore
continuing to build and moves into Western Washington Tuesday for
a dry day. The ridge shifts east Wednesday with a weakening front
approaching the area. Front moving through Wednesday night into
Thursday. Another upper level ridge will try to build over the
area Friday. Models are all over the place for the weekend after
this one. Temperatures near normal with highs mostly in the mid to
upper 40s and lows in the 30s. Felton

&&

.AVIATION...A frontal system will move into the region today,
bringing another round of widespread rainfall to the area. Flow
aloft is westerly. Mostly VFR early this morning across the region,
except for terminals in fog (KPWT, KOLM) which have dropped down
to IFR/LIFR overnight. MVFR to IFR conditions becoming more
widespread as rain spreads across the area today - most likely
between 14-16Z for terminals along the coast and between 16-19Z
for terminals across the interior. Breezy S/SE winds will pick up
through the morning hours, with gusts up to 30-35 kt for airfields
such as KHQM and KBLI. Winds will gradually ease through the
evening into Sunday.

KSEA...VFR conditions this morning, with high clouds streaming
overhead. Widespread rain looks to move into the terminal by
17-19Z, likely lowering conditions to MVFR and even IFR at times
throughout the day. Winds will be from the S/SE and will increase
to 7-12 kt through the morning hours. 14

&&

.MARINE...A sub 990 mb low pressure system will track just north
of Haida Gwaii today and will bring a front across the area waters.
Winds will increase through the morning hours, with gale force
winds expected for the Coastal Waters, East Entrance to the
Strait of Juan de Fuca, Admiralty Inlet, and the Northern Inland
Waters. Meanwhile, southerly to southeasterly winds along the Puget
Sound waters and West Entrance of the Strait will become breezy
with Small Craft Advisories in effect. Winds will gradually ease

Seas 7 to 9 ft across the coastal waters will build to around 13
to 18 ft tonight before gradually subsiding back to 10 ft
throughout the day Sunday. Seas will then continue to subside to 3
to 5 ft by Monday-Tuesday before building back up to 8-11 ft as
the next system approaches close to midweek. 14

&&

.HYDROLOGY...The Chehalis river at Porter has dropped below flood
stage and the warning has come to an end.

A brief lull in activity continues early this morning before the
next frontal system brings another round of rain and mountain
snow later today. Snow levels start off between 1000 and 2000 feet
then rise to 5500 to near 7000 feet tonight. Forecast rainfall
totals are similar to the last run with mountain locations ranging
from 2 to 3 inches with local higher amounts.

Antecedent conditions of high soil moisture and high river levels
are favorable for the potential of some new flooding. The
forecast amount of rain with higher snow level will drive
significant river rises in a few locations but for most areas it
will not result in flooding. The most likely river to flood is the
Skokomish where a flood watch is out for Mason County. There is a
low chance of flooding in additional areas, mainly for the
Snoqualmie River and the Chehalis River Basins. A Flood Potential
Outlook remains out for King, Thurston, Lewis, and Grays Harbor
counties. JBB/Felton

&&

.CLIMATE...Seattle rainfall for the month of December through the
8th is 5.78 inches. The normal fort the entire month is 5.72
inches. If Seattle gets the normal amount of rain for the rest of
the month, 4.20 inches, this will end up being the oth wettest
December on record in 79 years at Seattle-Tacoma airport. Only 3
years have surpassed 10 inches in December, 1979 11.85 inches,
2015 11.21 inches and 1996 with 10.18 inches. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 1 PM this afternoon to 1 AM PST Sunday for
     Admiralty Inlet Area-San Juan County-Western Skagit County-
     Western Whatcom County.

     Flood Watch through Monday morning for Hood Canal Area-Lower
     Chehalis Valley Area-Olympics.

     Wind Advisory until 7 PM PST this evening for Central Coast-
     North Coast.

     Winter Weather Advisory until 4 AM PST Sunday for West Slopes
     North Cascades and Passes-West Slopes South Central
     Cascades and Passes.

     Winter Storm Warning until 10 AM PST Sunday for West Slopes
     North Central Cascades and Passes.

PZ...Small Craft Advisory until 4 PM PST Sunday for Grays Harbor Bar.

     Small Craft Advisory until 7 PM PST this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 11 PM PST this evening for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 10 PM PST this evening for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.

     Gale Warning until 7 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-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.

&&