Winter Weather Advisory 7, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
351 PM PST Sun Jan 31 2021

.SYNOPSIS...Upper level trough offshore with moist southwesterly
flow aloft over Western Washington through Monday. The trough will
move through the area Tuesday. Upper level ridge building well
offshore with weather systems trying to moving over the top of
the ridge into Western Washington Wednesday through the end of the
week.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Satellite imagery shows
deep upper level trough offshore with the trough axis between 135
and 140W. Southwesterly flow aloft over Western Washington with a
continuous stream of moisture extending back to southwest of
40N/130W. Doppler radar has rain west of a line from about Astoria
northeast to east of Mount Baker. Temperatures at 3 pm/23z were
in the mid and upper 40s.

Little change in the pattern for the next 36 hours with the upper
level trough remaining offshore and southwesterly flow aloft
sending bands of moisture over Western Washington. Snow levels
slowly rising peaking out around 5000 feet over the South Cascades
and 4000 to 4500 feet over the North Cascades and the Olympics.
Temperatures will remain in a narrow range, in the mid 40s in the
evening and early morning hours creeping up to near 50 Monday
afternoon. Will continue the winter storm warning for the North
Cascades through Monday afternoon and add a Winter Weather
Advisory for the Olympics overnight.

Upper level trough moving inland Tuesday and Tuesday night with
the steady rain giving way to scattered showers during the day Tuesday.
Snow levels will drop to below pass levels during the day as well.
Temperatures will remain fairly steady Tuesday, in the mid 40s.
Lows Tuesday night in the mid 30s to lower 40s.

Wednesday looks to be a transition day with an upper level ridge
building offshore and drying northwesterly flow aloft increasing
during the day. A few lingering showers in the morning will dry
up in the afternoon. Highs will be in the mid 40s.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Extended models having
trouble with the placement and strength of the upper level ridge
offshore. There is some agreement with a system moving over the
top of the ridge but the models are not in sync timing wise.
Ensembles are not much help at least half of the solutions showing
light precipitation Thursday into early Friday morning. Will go
with chance pops during this period. Ensemble solutions don't
completely dry out for the Friday and Saturday time frame but
only a few of the solutions have light precipitation so have gone
mostly dry in this window. Some better consistency Sunday with
the ridge breaking down and a weak system moving over the top of
what is left of the ridge into Western Washington. Temperatures
will be near or a little below normal through the period. Felton

&&

.AVIATION...Southwesterly flow will persist aloft as an upper
level trough remains the dominant influence across the Pacific
Northwest through Monday. The trough will continue to send
disturbances across Western Washington, bringing additional
rounds of rain to the region. Radar shows that the heaviest
precip associated with the occluded front has made its way
further inland into the Sound this afternoon. Current conditions
across area terminals are predominantly MVFR to IFR as a result.
Expect a rather wet afternoon and evening to continue for most
terminals as this system continues to very slowly move across the
region. Cigs and visibilities look to remain lower MVFR to IFR
through much of Monday morning. The mountains will remain mostly
obscured in clouds and precip. Surface winds are primarily
southerly at 3-8 kts, will become more easterly tonight. 14

KSEA...Light rain at the terminals this afternoon, with conditions
primarily MVFR. Could see cigs and visibilities lower to IFR at
times in steadier, heavier precip. S/SE surface winds 3-8 kts
look to become more easterly tonight, before becoming
southeasterly again by Monday morning. 14

&&

.MARINE...An upper level trough will remain the dominant influence
across the Pacific Northwest through Monday. Meanwhile, at the
surface, an occluded frontal system continues to make its way
across the inland waters this afternoon. A surface low will then
track northward across the coastal waters Monday morning, allowing
for southerly winds to turn more easterly for a brief period of
time. Expect another round of SCA winds for the Coastal Waters,
the Northern Inland Waters, Eastern Strait, and Admiralty Inlet
Monday. Winds will then transition to more westerly midweek as low
pressure slides down from British Columbia and interacts with
high pressure attempting to nudge into the region from the west.

Seas along the coastal waters and the West Entrance to the Strait
continue to hover between at 10-14 feet and will remain elevated
into Monday. 14

&&

.HYDROLOGY...Steady rain continuing over the southern slopes of
the Olympics for the next 36 hours with another 1.5 to 2.5 inches
of precipitation in the forecast. Snow levels slowly rising to
near 4500 feet. This combination will push the Skokomish river to
near flood stage Monday evening/early Tuesday morning. A flood
watch will be issued with this forecast package.

For the remainder of the rivers in Western Washington no flooding
in the next 7 days. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 4 PM PST Monday for Cascades of
     Whatcom and Skagit Counties.

     Winter Weather Advisory until 6 AM PST Monday for Olympics.

PZ...Small Craft Advisory until 4 PM PST Monday 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 10 AM to 4 PM PST Monday for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 10 AM PST Monday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

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