Heavy Snow in Washington, December 22 to 26, 2021

Area Forecast Discussion
National Weather Service Seattle WA
340 PM PST Wed Dec 22 2021

.SYNOPSIS...A Pacific frontal system will bring periods of rain to
the area tonight with the potential for isolated thunderstorms near
the Coast this evening. Passing weather systems coupled with slowly
penetrating Fraser outflow will set the stage for the coldest
temperatures of the season so far late in the week and a chance for
light lowland snow this weekend. Dry but cold conditions look to be
on tap for the start of next week.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Showers that were present
over the Olympic Peninsula this morning now finding themselves east
of Puget Sound. The Peninsula is not completely quiet with some
lingering activity there...and just off the coast starting to see
echoes associated with the incoming cold front. Only a couple of
isolated thunderstorms popped up during the span of the afternoon
hours with a very limited inland run before falling apart. Some
clearing ahead of the front might allow for a quick shot of
instability, but questionable. That said, isolated thunderstorm
threat might persist into the early evening, but likely game over
for that by 7 PM PST.

Forecast models remain consistent with the incoming cold front
making rainfall more stratiform tonight. The impact of the front on
overnight lows will not be very much, with lows practically
identical to what was observed last night/early this morning. The
upper low will slowly move through the area Thursday, keeping precip
in place, albeit becoming a bit more showery.

Up in the mountains, current Winter Storm Warning looks good
as snow continues to fall there. Although there looks to be very
little in the way of breaks, snow amounts drop off significantly
Thursday evening and as such, current expiration time looks good and
see no reason to extend at this time.

Friday sees another upper low dip into the area from off the BC
coast, mainly sitting and churning off the NW WA coast for much of
the day and allowing moisture to continue to cycle through the area.
An accompanying surface low will help to set up a scenario that
looks to get some Fraser outflow started. By this point in the
forecast, this cold air will not make much headway into much of W
WA, but will see winds turn more north to northeasterly over
portions of Whatcom County, heralding the cold air to come.

The upper low will move eastward over the area Saturday but not
fully exit W WA. This will keep moisture and thus the chance for
precip in the forecast. The catch at this point will be how
successful the previously mentioned Fraser outflow winds penetrate
into the CWA. Current forecast suggests that temps in the lowlands
will be warm enough for either rain or a wintery mix for much of the
day, conditions look to cool enough for a transition to snow
Saturday night. At this time, forecast accumulations in the lowlands
look to be pretty light with generally a half inch or less for
locations from Burlington south...while Western Whatcom county could
see up to around an inch. This will really depend on how successful
that cold air is in making its way southward, so monitor forecasts
for the latest.

Cooler afternoon highs tomorrow with temps over W WA slipping
slightly down into the lower to mid 40s and remain that way for
Friday. Saturday will see cooling continue with highs in the upper
30s to around 40.  18

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Best chunk of moisture
associated with the low looks to move through the area overnight
Saturday night and into Sunday. As such, this will likely be the
period for the best accumulation of snow in the lowlands. The trough
finally ejects out of the state by late Sunday afternoon allowing
for conditions to start to dry from the north as per deterministic
models. PoPs linger in the forecast for this period due to ensemble
members, while generally agreeing on a drying trend, do not remove
PoPs completely. As such, the forecast at this time probably looks
worse than what it is actually to be expected.

The placement of the trough over the Central US is going to play a
big role in the remainder of the extended. The ECMWF allows the
trough to retrograde enough in order to keep W WA dry even into
Wednesday. The GFS shifts the trough east enough to allow a
secondary trough to dip down into the area Tuesday to bring another
round of precip...keeping the area cold enough for any precip to
fall mainly as snow...and then dries things out for Wednesday, much
in the same way the ECMWF did after the initial trough. Ensemble
members prove to be no help in providing any insight on which
direction to lean. As such, have opted to lean the forecast in the
wetter direction...mainly because so far, it has been that kind of
season.

As alluded to, temperatures during the long term get cold. Sunday
sees highs in the mid 30s, then it all goes downhill from there as
the remainder of the period sees highs in the upper 20s to lower
30s.  18

&&

.AVIATION...Southwest flow aloft this evening and tonight with an
upper level low pressure dropping south along the coast and pushing
a cold front towards the area. Scattered rain showers in the early
evening becoming more stratiform as cold front moves through the
area. Cigs a mixed bag with locations reporting IFR and MVFR for
most locations, however, some spots along the Strait and in the San
Juans seeing VFR cigs. Should see cigs become more widespread MVFR
to IFR with fropa. Mountains and foothills will remain obscured.
Blustery southerly winds mainly north and coast. 18/Johnson/19

KSEA...Rain showers this evening becoming more stratiform tonight as
the front moves into the area. Southerly winds 10-15 kts for the TAF
period. Ceilings to remain generally MVFR through the period.

&&

.MARINE...Southerly winds early this evening will turn westerly over
the coastal waters and Strait tonight and into Thursday. As this
transition occurs, this should shut off the SCA level winds for the
interior waters, while speeds look to remain with SCA thresholds
over the coast. As cold air drops south through British Columbia
late in the week northeast Fraser outflow winds will develop. 18/19

&&

.HYDROLOGY...One half to one inch of rain over the south slopes of
the Olympics since late this morning has already pushed the
Skokomish River within a half a foot of flood stage this
afternoon. Steady rain will come to an end over the Olympics this
evening. The Skokomish will rise above flood stage this evening
cresting in the early morning hours Thursday. The river will begin
receding late Thursday and fall below flood stage Thursday
afternoon.

Outside of the Skokomish River event in the next 24 hours, no
addition river flooding expected for the next 7 days. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 4 PM PST Thursday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-Olympics.

     Flood Watch from 4 PM PST this afternoon through Thursday
     afternoon for Hood Canal Area-Lower Chehalis Valley Area-
     Olympics.

     Coastal Flood Advisory until 7 AM PST Friday for Admiralty Inlet
     Area-Bremerton and Vicinity-Everett and Vicinity-Hood Canal
     Area-San Juan County-Seattle and Vicinity-Western Skagit
     County-Western Whatcom County.

PZ...Small Craft Advisory until 11 PM PST this 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.

     Small Craft Advisory until midnight PST tonight for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory until 5 PM PST this afternoon for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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