Heavy Snow in Washington, February 22, 2021

Area Forecast Discussion
National Weather Service Seattle WA
342 AM PST Mon Feb 22 2021

.SYNOPSIS...An atmospheric river will continue to bring wet
weather to Western Washington through Tuesday. Snow levels will
fall today, with mountain rain changing to snow this afternoon and
persisting through Tuesday. Following a break on Wednesday,
additional systems are likely for Thursday and Friday before drier
weather possibly returns this weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...

***KEY POINTS***

* Moderate, persistent rain will continue across much of the area
  through early this afternoon, including in the Cascades and
  Olympics where rainfall totals between 3 and 6 inches will be
  common by this afternoon.

* Snow levels will fall below pass level this afternoon and remain
  below through the end of the period. Bands of heavy snow
  expected, with snowfall rates generally in the 1-2"/hr range,
  potentially as high as 2+"/hr at times this evening through
  Tuesday. Very Difficult travel conditions expected.

* From NWAC: High to extreme avalanche danger will persist across
  much of the Washington Cascades through at least this evening.
  Refer to WSDOT for the latest road status and conditions.

***DISCUSSION***

The AR event is underway across the region as heavy precipitation
has set in across the region as the plume of moisture starts to
migrate southward. While still breezy in some areas, hazardous winds
have ceased across the region as the boundary layer in the
previously shadowed region have moistened over the past few hours.
Snow levels will remain elevated across the region through early
this afternoon with rain continuing across the region, including
across the Cascades. While rainfall rates look to remain low enough
to not yield a high urban flooding threat, rises on area rivers are
expected through Tuesday. See the hydrology section below for
flooding concerns. Continued rainfall in the Cascades this morning
will continue to yield very dangerous to extremely dangerous
avalanche conditions in the Cascades through at least this
afternoon. A quick moving cold front will push across the region and
bring snow levels down below pass level by this evening. There is
great agreement from the latest HREF members in heavy snow bands
setting up across Stevens and Snoqualmie Passes this evening.
Snowfall rates of 1 to 2"/hr will be common, with the potential for
2"+/hr rates not out of the question at times.

Lowland rain and mountain snow showers will persist through much of
the day Tuesday. A shortwave trough will eject across Western
Washington during the day, bringing 500 mb temperatures down to -30
to -35C. This, along with the potential for a few sun breaks will
bring some instability into the mix, with MUCAPE values generally
ranging from 100 to ~300 J/KG. Convective showers expected, with the
potential for small hail and isolated thunder, especially east of
Puget Sound. These showers will be capable of yielding briefly
higher snowfall rates in the passes through the day Tuesday. In all,
snowfall totals in the passes will generally range from 18 to 24
inches, with the heaviest totals expected Monday night.
Precipitation will taper from north to south Tuesday night.

Ensemble guidance remains in solid agreement in the upper level
ridge off of the west coast amplifying on Tuesday as the
aforementioned system departs. Clearing skies overnight will result,
allowing temperatures to fall into the mid to lower 30s across the
lowlands. On Wednesday, an east/west oriented shortwave ridge will
traverse Western Washington, giving the region a break from wet and
active weather, as well as a chance for some sun.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Confidence is increasing in
northwesterly flow through the period as high pressure over the
northeast Pacific remains stout. Several progressive shortwave
troughs are expected to bring another round of lowland rain and
mountain snow to Western Washington Thursday and Friday. Some
indication in mid- range ensemble guidance in the ridge axis
extending eastward into the northwest. While there is still plenty
of time for different variables to come into play, such as
scenario would favor drier conditions this weekend. Stay tuned.

Pullin

&&

.AVIATION...Strong westerly flow aloft continues today as a frontal
boundary sags southward across the region with an associated
atmospheric river. Widespread MVFR ceilings with pockets of IFR will
persist through most of the day. Improvement expected later this
evening into Tuesday morning as heavy rain shifts south of the area.
Scattered showers will linger into Tuesday but steadily decrease in
coverage. Gusty south to southwesterly surface winds will continue
today.

KSEA...MVFR conditions with periods of IFR through this afternoon.
Ceilings could lift to VFR after 00Z Tuesday as frontal boundary
sags south of the area. Gusty S-SW surface winds to continue with
gusts of 25 to 30 knots possible...especially early this morning and
again tonight.

Carpenter

&&

.MARINE...Gusty winds will continue much of this week as a series of
fronts move through the waters. A brief lull in winds expected for
the inland waters later this morning but winds will increase again
later today. Small craft advisories in place for most waters.
Another period of gales is likely tonight through the Strait of Juan
de Fuca.  Another front late in the week may bring another round of
potential gales late Thursday into Friday.

Seas over the coastal waters build will remain above 10 ft through
at least the first half of the week, reaching 14-17 ft Tuesday.
Another larger swell likely arrives late in the week behind the next
stronger front, with combined seas possibly around 20 ft over the
coastal waters Friday.

Carpenter

&&

.HYDROLOGY...Persistent moderate rain will continue below 5000 to
6000 feet this morning before snow levels fall to 1500 to 2000
feet by this evening. In total, 3 to 6 inches of rain expected in
the Olympics and Cascades below 4000 feet. Rivers are expected to
respond to rainfall today, with rises expected through Tuesday. At
this time, the Skokomish is the only river forecast to reach
minor flood stage, with a few others showing some potential to
approach minor flood stage today (Bogachiel). While showers will
persist into Tuesday, lower snow levels means that showers will
not be efficient rainfall producers - thus additional rainfall
through this afternoon will be the main factor in how much rivers
rise and which ones, if any, outside of the Skokomish reach flood
stage.

Pullin

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 4 PM this afternoon to 10 PM PST
     Tuesday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Tuesday for Olympics.

PZ...Small Craft Advisory until 10 AM PST Tuesday 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-Grays Harbor Bar-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PST early this morning for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 1 PM this afternoon to 1 AM PST Tuesday 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 AM PST early this morning for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 1 PM this afternoon to 1 AM PST
     Tuesday for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands-Puget Sound and Hood Canal.

     Gale Warning until 4 AM PST early this morning for Puget Sound
     and Hood Canal.

&&

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