Winter Storm Warning 3, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
1011 PM PST Sat Feb 18 2023

.SYNOPSIS... A stronger frontal system compared to the past
several systems will enter our region Monday bringing lowland
rain and heavy snow to the mountains. Colder temperatures will
start to affect the region into the middle of next week with light
lowland snow possible.

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.SHORT TERM /TONIGHT THROUGH TUESDAY/...The weak front that moved
southward across the region this afternoon has mostly fizzled and
taken most of the light rain and drizzle with it. Made a few
adjustments with the evening update to PoPs to account for some
lingering upslope showers and snow showers over the foothills.

A high pressure system orientated over the northeast Pacific will
continue to place Washington in a moist, northwest flow. There is
a slight break Sunday morning into the afternoon, but a wet, more
organized system will slide in from the northwest and start to
impact out region into late Sunday night. Temperatures over the
weekend will continue to remain in the mid to upper 40s.

Starting Sunday night, snow levels in the Cascades will be around
3500 feet, with the passes mainly starting off with a few inches
of snow. Snow levels will then begin to lower below 3500 feet
starting mid day Monday with snow becoming heavier at times.
Therefore, a Winter Storm Watch has been issued for all Cascade
zones above 2000 feet. Travel will be difficult through Snoqualmie
Pass and Stevens Pass during this time as snowfall will continue
at least through late Tuesday night.

A deep trough will start to slide southward from the north with
cool and unstable conditions on Tuesday night. Snow levels will
continue to lower throughout the region, with the mountains seeing
more snow.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...As this deep trough
slides through Tuesday night into Wednesday, the flow will turn
offshore and Fraser River outflow winds will start to ramp up.
Arctic air will start to be filtered from the north causing snow
levels to lower to sea level, with temperatures Wednesday morning
flirting around the freezing mark. Model consensus at this time
puts some lingering moisture in the lowlands through mid day
Wednesday, where very light lowland snow showers can be possible
during this time, likely less than half of an inch. High
temperatures will be in the upper 30s into Thursday with lows in
the 20s. Moisture will also pull away which will make Thursday a
generally dry day across the region.

Another weather system looks to ride a most north/northwest flow
regime Friday into Saturday which will bring another threat of
lowland snow to the area. With temperatures still remaining cold
(in the mid to upper 30s) combined with low snow levels, this
will also be a time period to monitor.

Davis/Mazurkiewicz

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.AVIATION...A weak front moved southeast across the region this
afternoon but has mostly fizzled along with the widespread light
rain and mist. Mostly MVFR ceilings are expected into Sunday
morning with IFR conditions at KOLM. The airmass will be stable.
Periods of light mist and drizzle will be possible through much of
tonight and Sunday, which may lead to MVFR VSBY restrictions as
well. Winds will favor a southerly direction through the early
afternoon then transition to more of southwesterly direction.
Winds will stay mostly westerly at KHQM and KCLM.

KSEA...VFR ceilings early tonight will gradually lower with MVFR
ceilings developing over the next few hours, which are expected
through most of Sunday. The airmass will be stable with periods
of drizzle and mist. This could lead to periods of MVFR VSBY
restrictions as well. South-southeasterly winds tonight will
transition to more of a southwesterly direction into the afternoon
from 5 to 10 knots.

Davis

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.MARINE...Modest onshore flow this evening will increase through
the Strait of Juan de Fuca early Sunday morning and continue
through Sunday night. A Small Craft Advisory remains in effect for
the coastal waters for wind gusts up to 25 knots and seas around
10 feet. A stronger westerly push is expected down the Strait of
Juan de Fuca with gale-force wind gusts Sunday afternoon into
Sunday night.

Strong surface low pressure will build along a frontal boundary as
it digs southward over the region early next week. There is
potential for storm-force onshore winds over the coastal waters
and Strait of Juan de Fuca Monday night into Tuesday as the
pressure gradient strengthens.

As the low pressure inflection passes southward with the front,
onshore flow will transition to an offshore direction Tuesday
night into Wednesday. Fraser Outflow will also developing over the
Northern Inland Waters and spread into the central/eastern Strait
of Juan de Fuca, bringing the chance for at least advisory-
strength winds.

Seas will remain elevated from 8 to 10 feet through Monday night
before building further in association with the low pressure
center moving southward over the region. Ensemble guidance
suggests a 60 to 80 percent chance for significant wave heights of
20 feet Tuesday night into early Wednesday over the coastal
waters. Seas look to finally drop below 10 feet later into
Thursday as the front and low sag south of our area.

Davis

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.HYDROLOGY...River flooding is not expected for the next 7 days.

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Sunday afternoon through late Tuesday
     night 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 until midnight PST tonight for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 AM to 4 PM PST Sunday for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Watch from Sunday afternoon through late Sunday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST Sunday 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.

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