High Wind in Washington, December 21, 2020

Area Forecast Discussion
National Weather Service Seattle WA
1003 AM PST Mon Dec 21 2020

.UPDATE...Moderate to heavy rainfall continues to persist across much
of western Washington at this hour as the warm front remains
draped across the north/central Puget Sound. Though river flooding
will not be a factor outside of the Skokomish River in Mason
County, nuisance flooding in poor drainage areas, as well as
ponding of water on roadways will be a factor for the remainder of
the morning and into the afternoon. Use caution if traveling this
morning! Outside of rainfall, breezy to locally windy conditions
are expected along the central coast through this afternoon.
Localize power outages will be possible in this area.

The low pressure system will track eastward across the southcentral
Puget Sound later this afternoon, which will bring colder air into
the mix to the north of the low center. How cold? That's the key
question for the forecast for this afternoon.

Snow: The one thing everyone is curious about is the lowland snow
potential for this afternoon and evening. The forecast remains
complex, as several finer details will factor into what will occur.

-Temperatures: This appears to be the most important limiting factor
with respect to how much, if any accumulation will be observed in
the lowlands. Currently, temperatures are generally in the mid 40s
across Puget Sound, with mid to upper 30s along the Hwy 101 corridor
on northern coast of the Olympic Peninsula. Upstream in the Fraser
River Valley and into interior BC, temperatures are relatively mild
at this hour - with readings in the upper 30s to near 40 at this
hour. A strong push of cold air is not expected along the northern
Olympic coast, which will limit accumulation potential here. Cooling
in the Puget Sound Region will be limited as well for the same
reasons, with snow potential being contingent on how much the
atmospheric column can dynamically cool prior to precipitation moves
out.

-Timing: Another limiting factor in play today in two distinct
manners - time of day and precipitation duration time with cold air
in place. Lowland snow potential will exist in the northern Olympic
Peninsula lowlands during the daytime hours, extending into the
Puget Sound region 1 to 2 hours prior to sunset. The window for
wintry precipitation will also be limited everywhere on the back end
as the system pushes eastward this evening. In general, it looks
like a brief 3 to 4 hour window of wet snow/rain-snow mix at
individual points.

-Precipitation Rates: One factor that raises the potential of the
aforementioned areas seeing some flakes today are the moderate to
heavy (at times) precipitation rates. This will yield dynamic
cooling across the area, which could briefly lower snow levels below
500 feet this evening in parts of the lowlands.

-Expected Accumulations & Potential Impacts: In general, 1 to 2" is
possible along parts of Hwy 101 in the Olympic Peninsula, with some
of the higher parts of Hwy 101 potentially seeing locally higher
amounts. In the Puget Sound region, warm air temperatures currently
and warm road surface temperatures will keep accumulations limited
to grassy surfaces.

Outside of the lowland snow potential, impactful accumulations are
expected at pass level this evening. Snow totals this evening will
generally range from 6 to 10 inches, with the highest totals likely
at Stevens and Snoqualmie Passes. Precipitation will taper off
tonight with colder air filtering in behind the front. Overnight
lows in the low to mid 30s.

Pullin

&&

.PREV DISCUSSION... /issued 300 AM PST Mon Dec 21 2020/

SYNOPSIS...A frontal system will bring rain today. Snow levels
will lower as cold air filters in the area this afternoon and
evening with snow possibly mixing with rain. An upper ridge will
bring dry weather Tuesday through Thursday. Wet weather will
return Christmas Day into Saturday as a front moves through the
area.

SHORT TERM /TODAY THROUGH WEDNESDAY/...A messy and complex
frontal system will cross the area today. Rain is currently
lifting north into the area as a warm front. This will stall over
the area later this morning. An upper trough will move into the
area this afternoon and evening and push a cold front through the
area with precipitation tapering from the west.

Widespread river flooding does not look like a concern any more.
See the hydrology section below for more details.

A surface low will track west to east through the middle of
Western Washington today. South of the low, gusty south winds are
likely for the Central Coast. It will also be breezy to locally
windy elsewhere with the low tracking through the area but nothing
too impactful is expected.

Along and north of the low, northerly winds will develop today.
At the same time, cooler air aloft will work its way into the
region as the upper trough and cold front move through. This will
help drag snow levels down. Heavier rain rates could also drag
snow levels down this afternoon near the cold front.

All of this means there is a threat of lowland snow later today.
As usual, pinning down the details is very tricky and uncertainty
is high. Ensembles are sending a strong signal that there is at
least a chance of snow for the interior lowlands, mainly from
Tacoma north. However, there isn't any cold air in place and the
ground is not cold so it will be hard for accumulations to
develop on the roads. We probably won't really know how this will
all shake out until the event is underway. For now the main
messaging will be possible snow for the lowlands later today but
minimal accumulations and impacts. That could change of course. Stay
tuned.

There will also be a good shot of snow for the mountains. On the
north side of the stalled front, snow levels will be lower and
heavy snow is possible through early this evening. Further
south, snow levels will be high enough today for mostly rain below
6000 feet. Snow levels will fall below pass level this evening
and the passes could pick up advisory amounts tonight before
showers taper Tuesday morning.

Tuesday and Wednesday look dry with high pressure aloft in
control. A few showers will probably linger into Tuesday morning
for the Cascades. Besides that there will be some lingering clouds
on Tuesday then the lower levels dry for a mostly sunny day on
Wednesday. Schneider

LONG TERM /THURSDAY THROUGH SUNDAY/...High pressure will bring a
dry day on Thursday. A front will bring a little rain on Friday
(Christmas Day). A trailing upper trough will keep things
relatively wet into Saturday. A bit of a split flow pattern looks
to develop Sunday and Monday resulting in relatively quiet
weather for Western Washington. Schneider

AVIATION...Western WA will continue to see rain showers through
the rest of this morning, creating widespread MVFR/IFR ceilings.
However, with some of the heavier rain showers as well as some fog
forming due to the amount of moisture in the atmosphere it is not
out of the question for some short lived LIFR conditions this
morning and into the early afternoon. The trickier portion of
todays aviation forecast is the winds, particularly the wind
directions. As of 1600 UTC winds at most terminals remain variable
as the wind directions begin to switch around with the warm front
moving northward. Within the next few hours all W WA terminals
should have seen the switch to south/southwesterly winds ahead of
the cold front. As the cold front approaches the area winds will
be increasing. Expect sustained winds 15-20 knots with gust 25-30
knots possible. The cold front is expected to have moved through
by late tonight and winds will be west/northwesterly at 5-7
knots for terminals away from the coast.

KSEA...Current LIFR conditions will start the gradual improvement
to MVFR later this morning as the warm front moves north. The
variable wind direction will likely continue for the next few
hours until KSEA is completely in the warm sector. Winds will then
be south/southwesterly for this afternoon. Sustained speeds are
expected to be 10-15 knots with gusts up to 25 possible. Winds
will turn to westerly after the cold front passage later tonight.

Butwin

MARINE...Winds will increase over the waters today with a frontal
system over the region. Strong west to northwest flow will follow
tonight with a cold frontal passage - Gale Warnings or Small
Craft Advisories are in effect for all waters. Meanwhile, seas
will remain high at 10-16 feet. Winds will ease on Tuesday and
seas will subside under 10 ft Tuesday night. High pressure and
offshore flow will prevail Wednesday and Thursday. The next
frontal system will arrive Thursday night or Friday.

33/Butwin

HYDROLOGY...A frontal system will bring rain, heavy at times
today. Precipitation will taper from west to east late this
afternoon and evening.

The expected QPF with the frontal system today no longer looks
like enough for river flooding. The flood watch has been canceled.
The exception will be the Skokomish river in Mason county. It
fell below flood stage yesterday evening and is lower than
previously forecast but the rains today should push it back to
flood stage by this afternoon. The river should crest early this
evening then fall below flood stage once and for all tomorrow
morning.

An elevated threat of shallow landslides will also continue today
into Tuesday due to the recent heavy rains.

River flooding is not expected for the rest of the week. Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Tuesday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties.

     Winter Storm Warning until 7 PM PST this evening for Cascades of
     Whatcom and Skagit Counties.

     Winter Storm Warning until 4 PM PST this afternoon for Olympics.

     Wind Advisory until 4 PM PST this afternoon for Central Coast.

PZ...Small Craft Advisory until 4 PM PST Tuesday for Grays Harbor Bar.

     Gale Warning until 4 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.

     Small Craft Advisory until 6 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Gale Warning from 6 PM this evening to 4 AM PST Tuesday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

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

     Gale Warning from 4 PM this afternoon to 4 AM PST Tuesday for
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Tuesday for Admiralty Inlet-
     Puget Sound and Hood Canal.

&&

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