Heavy Snow in Washington, February 4 to 5, 2021

Area Forecast Discussion
National Weather Service Seattle WA
231 PM PST Thu Feb 4 2021

.SYNOPSIS...High pressure building offshore, but far enough west
for a series of disturbances to bring mountain snow and lowland
rain to the region tonight and at times into the weekend. A deep
upper trough over the central part of the country likely shifts
west next week, with notably colder air likely arriving across the
area.

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.SHORT TERM /TONIGHT THROUGH SUNDAY/...Moist NW flow will prevail
over the Pac NW tonight for more lowland rain and mountain snow.
Snow levels will hover between 3000-4000 ft with a mix of rain
and snow at Snoqualmie Pass. Precipitation rates will be higher
tonight and still expecting accumulating snow at higher passes and
elevations like Stevens Pass and Mt Baker. Down below, across the
lowlands, rain continues although the central sound (including
Seattle) is seeing some rain shadowing with westerly downslope
winds off the Olympics. Overall, we're looking at another
0.25-0.50" of rain along the coast tonight and 0.05-0.25" in the
interior (with lesser amounts in the rain shadow).

Friday will be drier under north flow as a ridge amplifies
offshore. Skies will remain mostly cloudy with low level onshore
flow. Mild temperatures expected with highs near 50.

Another wave of moisture will wrap around the ridge and clip
western WA Saturday and Sunday. The main impact over this period
will again be snow in the Cascades which will affect all passes
(snow levels around 3000 ft). There's no significant break in the
showers across the Cascades and total snowfall through Sunday
night is around 1-2 ft. Lowlands will see off-and-on rain with
overall totals 0.10-0.25" through the same period. 33

.LONG TERM /MONDAY THROUGH THURSDAY/...The focus as we move into
next week is a trend toward cooler and likely drier conditions
across western WA as we flip to offshore flow. However, the
ensemble temperature spreads are fairly large depending on how
strong of offshore flow we'll see (and thus how much cold air
will seep in). Most likely scenario is temperatures dipping into
the 20 to lower 30s Tuesday and Wednesday morning (with highs
near 40). Temperatures moderate a few degrees toward the end of
the week as offshore flow eases and a ridge moves in. 33

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.AVIATION...Northwest flow aloft through tonight with a ridge
offshore and an upper trough to the east of the area. Rain this
afternoon primarily focused north of King County and along areas of
the coast. Rain will likely become a bit more widespread tonight
into early Friday. Drier weather by Friday afternoon with just a few
spotty showers. MVFR cigs for most with IFR cigs possible at times
through most of Friday. Southwesterly winds at 8 to 12 knots through
tonight with a few gusts of 25 knots possible.

KSEA...MVFR cigs through tonight. IFR briefly possible at times
mainly late tonight. Rain possible this afternoon with rain chances
increasing after 04z into early Friday. Drier weather by Friday
morning. Southwest winds 8 to 14 knots with a few gusts to 25 knots
possible into Friday morning. JD

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.MARINE...An upper ridge will be in place off the coast for the next
week. A deepening upper trough west of the Cascades will create a
tight pressure gradient tonight into Friday. Gales are expected for
the central and eastern portions of the Strait of Juan de Fuca
tonight. Small Craft Advisory winds for the other area waters into
Friday morning. SCA winds for portions of the Strait of Juan de Fuca
and adjacent waters Friday afternoon into Friday night. Another
system moving through on Friday will bring another round of possible
gales or SCA winds for most waters. Northwest swells building to 12
to 15 feet for the outer coastal waters late tonight through Friday
and 10 to 13 feet for the inner coastal waters. Swells decreasing
into Saturday morning. JD

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.HYDROLOGY...No river flooding is expected through the next seven
days.

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 7 AM PST Friday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

PZ...Small Craft Advisory from 7 PM this evening to 10 PM 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-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 7 PM this evening to 10 AM PST Friday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 10 PM this evening to 10 AM PST Friday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal.

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