Heavy Snow in Washington, February 4 to 8, 2020

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PST Mon Feb 3 2020

.UPDATE...No notable changes were needed to the forecast this
evening. The region is under the influence of a weak, positively
tilted ridge centered over the eastern Pacific. High pressure is
also noted in the latest surface analysis from WPC. In general,
tranquil weather is expected for much of the overnight period
under these conditions.

A quick glance at MRMS and local radar imagery suggest much of
the isolated, pin-prick shower activity has diminished across the
area. About an hour or so ago, reports of an isolated lightning
strike near Des Moines filtered in. This was confirmed despite the
absence of instability and poor lapse rates. This cell has since
diminished during its slow migration southward near Auburn and
Federal Way. For the remainder of the night, a few flurries may
persist but nothing of impactful nature.

In addition, some patchy fog could develop late this evening,
however, already seeing some high level cloud cover advect into
the area ahead of tomorrow's system. This may prevent decent
radiational cooling despite high pressure influence.

Speaking of tomorrow's system, it still remain on track with hi-
res guidance suggesting warm frontal boundary precip pushing into
the coastal areas just ahead of the morning commute. Despite
continued ridge influence, WAA should be strong enough to support
enough lift for precip to sustain itself as it pushes east
through the day. This will mark the beginning of yet another
atmospheric river event...

Kovacik

&&

.PREV DISCUSSION... /issued 232 PM PST Mon Feb 3 2020/

.SYNOPSIS...Scattered showers will end by evening as weak high
pressure builds into the region. A warm front will arrive Tuesday
afternoon and the snow level will rise through Wednesday morning.
Wet weather with a high snow level will continue Wednesday
afternoon and night and into Thursday as a frontal system remains
stalled over the area. A lull in the heavy precip is possible
during the day on Friday. A cooler frontal system will arrive
Friday night and Saturday.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Scattered showers will
give out late this afternoon and there will be some partial
clearing into the evening. High clouds will increase overnight
ahead of the warm front. As precip arrives on Tuesday it will
probably start out as wet snow and a few areas in the lowlands
ought to see up to an inch of accumulation on the grass--but a
southerly breeze will be increasing and the snow will not last. By
mid to late afternoon the lowlands will see all rain and the snow
level rill rise through Tuesday night and Wednesday morning. Til
the snow level rises above the passes there will be a period of
decent snow accumulation and a winter weather advisory starts at
4pm Tuesday and goes til 10am Wed for the Cascades. After that,
the snow level will be fairly high--probably ranging from around
4500 feet in the north Cascades to 5500-6500 feet in the south
Cascades and Olympics. 19

.LONG TERM /FRIDAY THROUGH MONDAY/...There is likely to be a
break in the heavy precip during the day Friday before another
frontal system arrives Friday night and Saturday. That system will
see the snow level drop back below the passes with possibly heavy
snow in the mountains during the day Saturday. The associated
upper trough then digs south through Oregon and northern
California Saturday night with cool drier air for Western
Washington Sunday and Monday as a transitory upper ridge builds in
to the region. 19

&&

.AVIATION...A brief break in weather systems is expected overnight
(with the exception of a few flurries) as high pressure dominates
at both the sfc and aloft. Seeing generally VFR conditions this
evening, tho could be some brief MVFR into the overnight given
some lingering lower-level cu seen on satellite. Some brief
patchy fog cannot be ruled out, but given that higher level cloud
cover is beginning to advect in ahead of tomorrow's weather
system, do not think conditions will be optimal for it to persist.
Clouds will thicken and ceilings will drop thru the day Tuesday
as a warm front and associated rain shield move in. Some areas
near the terminals may experience wet snow at the onset of the
precip but confidence is low in this potential. Winds will also
pick up from the south on Tuesday, generally sustained 8-12kts
with gusts near 20kts possible, especially near BLI and HQM. Rain
will persist then thru the TAF cycle.

KSEA...Lightning in the vicinity was reported around 6PM and the
associated cell has since diminished. Aside from perhaps a flurry
or two, tranquil conditions with dominant VFR cigs expected
overnight. Cigs will drop to MVFR as rain approaches Tuesday
afternoon. Confidence is low in wet snow start to the precip at
this time, so will keep -RA the dominant precip type. Winds will
pick up from the south 8-12kts Tuesday afternoon.

Kovacik

&&

.MARINE...No changes were needed to the marine package this
evening. Recent observations and overall synoptic analysis
indicate a period of light winds for most of the overnight
hours tonight. A small craft advisory for rough seas will continue
thru 10pm this evening across the coastal waters and grays harbor
bar given swell lingering around 10ft. Seas will subside then for
much of the overnight. Still looking for the next frontal system
to move into the area early Tuesday morning, which will bring
winds and seas back up to small craft levels for the coastal
waters and western Strait. an eventual increase in winds to small
craft criteria will then follow for all interior waters except the
central Strait. Based on some of the hi-res guidance, in particular
the WRF, the potential for isolated gale force winds are worth
the following shifts to monitor Tuesday evening/overnight. Active
weather then expected to persist for much of the week. See
previous discussion below:

Weak onshore flow today with some small craft advisory
strength northwest winds over the Coastal Waters. A warm front
will move over the area on Tuesday, stall over the area Wednesday
and Thursday, then weaken on Friday. Small craft advisory strength
winds southerly winds are expected most waters off and on through
the period. Additional headlines may be warranted later in the
period.

Kovacik/Kristell

&&

.HYDROLOGY...Another period of heavy rain in the mountains will
occur after a warm front moves through the area Tuesday night and
Wednesday morning. For Wed afternoon through Thursday the snow
level will be above the passes and heavy precip in strong westerly
flow aloft will occur. The temp at 850mb will not be as warm this
time, and heavy snow will fall at the highest elevations, but
river flooding and elevated landslide risk is a good bet for Thu
and Fri. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

PZ...Small Craft Advisory until 10 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-Grays Harbor Bar.

     Small Craft Advisory from 7 AM Tuesday to 7 AM PST Wednesday for
     Admiralty Inlet-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-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory from 4 PM Tuesday to 7 AM PST Wednesday for
     Grays Harbor Bar.

&&

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