Heavy Snow in Washington, February 19 to 21, 2023

Area Forecast Discussion
National Weather Service Seattle WA
305 AM PST Sun Feb 19 2023

.SYNOPSIS...A strong frontal system will enter our region tonight
through Tuesday bringing lowland rain and heavy snow to the
mountains. Strong onshore winds will develop Monday night. Expect
gusty Fraser River outflow winds Tuesday night. 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 /TODAY THROUGH TUESDAY/...Western WA will be under
strong, cold, moist W to NW flow over the 48 hours for an active
period of weather. The main impacts through Tuesday night will be
heavy mountain snow (especially for the Cascades) and strong wind
(primarily Monday evening through Tuesday).

For the Cascades, heavy snow is forecast across the west slopes
and a long-fused Winter Storm Warning is in effect. Snow will
start to increase tonight and will be persistent through Tuesday
afternoon with a range of 1-3 feet expected. Snow levels will
initially hover around 4000 feet but will drop to around 2000 ft
Monday night. So Snoqualmie Pass may start out as rain but
eventually all highway passes will see accumulating snow, thus
making travel over the Cascades difficult.

As for the wind, western WA will be under very strong onshore flow with
gusty S/SW winds beginning late Monday afternoon or evening. The
strongest winds will be through the Strait of Juan de Fuca with
Storm Force winds possible. We'll likely see High Wind Warnings or
Wind Advisories for most lowland areas through this period.

Heavy snow tapers off Tuesday night in the Cascades as the low
level flow starts to turn offshore. However, the air mass will be
cold with low snow levels and a threat of lowland snow with
lingering moisture. For now, significant/widespread accumulating
snowfall is not anticipated. The probability of snow greater than
1" is low and less than 10 percent for most lowland areas.
Offshore winds will strengthen Tuesday night (and Wednesday) with
strong Fraser River outflow winds expected. This could bring high
wind to areas like western Whatcom, San Juan islands and the
western strait. 33

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The air mass will remain
cold Wednesday through Friday with offshore flow. Gusty Fraser
River outflow winds will continue Wednesday and Thursday.
Temperatures will be well below normal with morning lows in the
20s and afternoon highs in the 30s each day. There's still some
lingering moisture around with troughing over the Pac NW and
there's still a threat of lowland snow through the period. There's
around a 20-30 percent chance of seeing greater than 1" of snow
over the 72-hour period ending Friday afternoon, with higher
probabilities closer to the mountains. Heavier amounts (like
greater than 4") is much lower and generally less than 10 percent.

Moving forward, the air mass does start to moderate as we move
into Friday and Saturday. By then the low level flow will be
flipping back to onshore to bring in warmer temperatures and
higher snow levels. However, there is another frontal system
slated to move inland over this time frame and we may see snow
changing over to rain. Exact timing on this is still unclear. 33

&&

.AVIATION...Increasingly moist northwest flow aloft today and
tonight. At the surface, onshore flow will increase as well. The
air mass will be stable with abundant low level and mid level
moisture.

KSEA...Low clouds should develop early this morning and persist
today and tonight. Light south wind will increase to 6-12 knots
this afternoon. 16

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.MARINE...A vigorous frontal system will move into the area from
the northwest on Monday with onshore flow gradually ramping up. By
Monday night, gale force westerly winds will be likely most
waters with storm force winds possible in the Strait. Strong
onshore flow will rapidly ease on Tuesday, turning offshore
Tuesday night and Wednesday with Fraser outflow developing. Fraser
outflow will continue into Thursday, easing late in the day.

Combined seas over the Coastal Waters will hover around 10 feet
today and Monday then build to 18-23 feet Monday night and
Tuesday. Seas will gradually subside Wednesday and Wednesday
night, eventually falling below 10 feet on Thursday. 16

&&

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

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 4 PM this afternoon to 4 PM PST
     Tuesday 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 2 PM this afternoon to 6 PM PST this
     evening for Grays Harbor Bar.

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

     Gale Warning from 4 PM this afternoon to 4 AM PST Monday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Storm Watch from Monday evening through Tuesday morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to 4 PM PST Monday for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 4 PM this afternoon to 7 AM PST Monday
     for Admiralty Inlet.

     Small Craft Advisory from 4 PM this afternoon to 4 PM PST Monday
     for Puget Sound and Hood Canal.

     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.

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