Winter Storm Warning 2, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
948 AM PST Sun Jan 7 2024

.SYNOPSIS...A few lingering snow showers over the mountains this
morning, but today will be a break as weak high pressure brushes us
to the north. The active pattern returns Monday as the first frontal
system noses southward across the northeast Pacific towards the
coast with a leading warm front. A new and stronger surface low
quickly overtakes this system and moves across Western Washington
Monday night into Tuesday. A stout cold front digs southward beneath
strong high pressure across Alberta, keeping precipitation chances
alive along with the potential for much colder temperatures and
lowland snow Thursday into Friday.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...There are a few light
lingering showers this morning (interior and coast) but overall
it's a quiet day (compared to all that's coming the rest of the
week). Temps are in the 30s this morning with highs around 40-45
this afternoon. 33

Previous discussion...A few lingering snow showers in the
Cascades and light showers along the southern coast early this
morning, but this activity should gradually wane through the
morning hours as yesterday's system exits further to our east
today. Overall, a brief lull in activity today as weak high
pressure skirts across southern Canada. A few breaks of sun
possible this morning, but otherwise mostly cloudy skies as high
clouds spill onshore ahead of our next weather feature for Monday.
Highs today will be on the cool side in the low 40s.

As mentioned, a period of active weather arrives Monday as low
pressure dips southward from the Gulf of Alaska along a warm front
extending southeast into our coastal waters. Precipitation will move
onshore through Monday morning ahead of this front with snow levels
starting off around 1500 feet along the coast to below 500 feet into
the North Cascades, thus most precipitation should begin as rain in
the lowlands Monday morning. Some wet snowflakes could mix in from
Everett northward through western Snohomish, Skagit, and Whatcom
Counties, but surface temperatures in the low to mid 30s should
preclude any impactful accumulation. Any accumulation that does
occur would be confined to mostly grassy surfaces and will quickly
melt as temperatures climb into the low to mid 40s in the afternoon
as the aforementioned warm front lifts across Western Washington.
Warmest temperatures will be over the Southwest Interior and along
the coast in the upper 40s to near 50.

Snow continues through the mountains Monday night, though snow
levels begin climbing in the wake of the warm front closer to 5500
feet. Latest model guidance has also been hinting at a break in
precipitation or intensity, at least, during this time. A Winter
Storm Watch was issued for Monday morning through Wednesday night to
encompass most of this prolonged period of heavy snowfall.
Subsequent forecast shifts will convert this into warnings and
advisories appropriately once this evolution comes into better
light.

Much to discuss Tuesday as a new surface low develops ahead of a
vigorous shortwave trough over the northeast Pacific, eventually
moving onshore somewhere into southern British Columbia, likely sub-
980 mb as it does so. Snow levels jump above pass-level to 5000-6000
feet Monday night into early Tuesday morning, but quickly plummet
back to around 2000 feet by late Tuesday afternoon with heavy snow
returning in earnest to the passes. Moderate to heavy rain in the
lowlands will persist through much of the day. Weak instability may
create a few thunderstorms or rumbles of thunder at the least along
the coast in the post-frontal airmass Tuesday afternoon. Strong wind
gusts will also be a concern with this system due to the tight
pressure gradient as the surface low moves across British Columbia.
Wind Advisories probable along the coast and across Whidbey Island
and the San Juan Islands into the North Interior (60-70% chance)
with some potential for High Wind Warnings even (40% chance).
Coastal flooding also becomes a concern with the Tuesday morning
high tides due to a combination of the low atmospheric pressure,
strong onshore/SE winds, and westerly swell surpassing 20 feet along
the coast.

Little overall change to the total snowfall Monday night through
Wednesday night over the Cascades and Olympics with several feet of
snow still on the table. Probabilities for 3-day combined snowfall
to exceed 48 inches through Wednesday night is around 40% at
Snoqualmie Pass and 60% at Stevens Pass. Heaviest snowfall will be
Monday night through Tuesday evening with snowfall rates of half to
even 1 inch per hour possible at times. This, combined with strong
wind gusts up to 50 mph are likely to create difficult and even
dangerous travel conditions with localized white-out conditions with
blowing and drifting snow.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Continued active
inclement weather Wednesday as deep troughing persists over the
northeast Pacific and Pacific northwest with strong ridging shooting
northward into Alaska and over the North Atlantic as a pattern shift
begins. Closer to home, a strong Arctic high digs southward into
Alberta, pushing a cold front southward into Western Washington
Wednesday. Considerable disagreement continues with the trajectory
of the core of the upper-level trough and cold air associated with
it from Thursday night onward. Western Washington will be on the
edge of this high pressure center, thus minor differences in its
western extent and how far south the cold front pushes will have
major implications into precip type and lowland snow potential for
Thursday, Friday, and Saturday. A period of below normal
temperatures is increasingly likely but it is hard to say just how
cold at this point. Best chances for lows well below freezing will
be north of Seattle into western Snohomish, Skagit, and Whatcom
Counties.

Here are probabilities for 3-day snowfall totals through Saturday
night:
* 1 inch or more: 40-60%
* 4 inches or more: 10-15%

Davis

&&

.AVIATION...Northwest flow aloft over the region with weak high
upper high pressure moving through today. Lingering convergence
precipitation band remains from KCLM to K0S9 and then to southeast
laying in between KPAE and KBFI. This will likely continue to remain
for the next few hours before weakening. Otherwise, mix of lower IFR
stratus and fog near the coast and far north and south, and broken
to overcast VFR to MVFR stratus elsewhere. Overall improvement to
VFR is expected across the region this afternoon as remaining shower
activity dissipates. Ceilings lower again tonight with MVFR in rain
developing at the coast with approaching warm front by 12Z Monday.
These conditions will spread inland Monday morning.

KSEA...Few residual showers near/around the terminal area through
21z with ceilings trending primarily to 3000-3500 ft or so today,
though can't rule out a brief lowering under a shower. Expect
ceilings to again lower late tonight with MVFR heights the most
likely setup by 09z, though still around a 20 percent chance of
lower stratus if mid clouds ahead of the next front don't fill in
quick enough. Rain returns after 14z with low MVFR conditions
favored. Surface winds generally 5 to 7 kt and from the
south/southeast. Cullen

&&

.MARINE...There will be a brief period of light winds across area
waters today as a weak surface ridge shifts inland. Small craft
advisories continue for the coastal waters where seas remain
hazardous. A warm front approaching late tonight will bring a return
of small craft advisory level winds to the coastal waters, but
attention remains fixated on the trailing cold front and associated
surface low that is expected to rapidly deepen to 975 millibars or
less as it curls toward Haida Gwaii Monday night. Models have
remained fairly consistent with this idea which would produce a
broad swath of full gales over the coastal waters both ahead of and
behind the cold front. The surface low is expected to gradually fill
as it shifts inland over southern British Columbia on Tuesday. This
sets up the potential for an extended period of gales across
interior waters on Tuesday as pressure rises to the south of the
surface low create some very tight gradients. A number of models
have been advertising an extended period of gradients exceeding 12
to 14 millibars PDX-BLI and 5 to 7 millibars UIL-BLI.

Unsurprisingly, probabilistic guidance gives a 90% to 100% chance of
gales for the coastal waters and most of the Strait of Juan de Fuca
by midday Tuesday and 70% to 80% probability of gales for the
Northern Inland Waters, Admiralty Inlet, and Puget Sound.
Topographic enhancement of the westerly surge through the strait
Tuesday afternoon is clearly illustrated with a 50-60% probability
of gusts reaching 55 knots near the east entrance to the strait.

Gale watches have been posted for nearly all waters (with the
exception of Puget Sound) for Monday afternoon/night through
Tuesday. Seas will gradually subside today and Monday before
building sharply behind the front on Tuesday. Seas of 23 to 27 ft
look probable across the coastal waters Tuesday and Wednesday.

27

&&

.HYDROLOGY...Heavy rainfall of 2-4 inches in the Olympics associated
with a strong frontal system will lead to sharp rises on streams and
rivers. The Skokomish River in Mason County is forecast to rise into
Minor Flood Stage late Monday night or Tuesday morning. No other
river flooding is forecast over the next 7 days.

Davis

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from late tonight through Wednesday evening
     for Olympics-West Slopes North Cascades and Passes-West
     Slopes North Central Cascades and Passes-West Slopes South
     Central Cascades and Passes.

PZ...Small Craft Advisory until midnight PST tonight for Grays Harbor
     Bar.

     Small Craft Advisory until 4 PM PST this afternoon for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Monday evening through late Tuesday night for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST Monday 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 Monday afternoon through Tuesday 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.

&&