Flood and Heat in Washington, December 4 to 6, 2023

Area Forecast Discussion National Weather Service Seattle WA 838 PM
PST Sun Dec 3 2023

.SYNOPSIS...An atmospheric river will impact the region Monday
through Tuesday, bringing periods of heavy rain to western
Washington and an increased risk of river flooding. An upper level
trough will cross the region late in the week for a shift toward
more showery precipitation, cooler temperatures, and lowering snow
levels. A brief break in the active pattern is possible on Friday,
before additional systems move through the region over the weekend.

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.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Current radar not terribly
active, although there is an area of attention over central
Snohomish and Skagit counties looking like a convergence zone. The
trends do show this feature fizzling out and precip associated with
it has been pretty light. Otherwise, W WA finds itself in a bit of a
lull this evening and tonight before the expected atmospheric river
scenario sets up Monday. That said, inherited forecast remains on
track and as such no evening updates are expected. For additional
forecast details, please refer to the Previous Discussion section
below.  18

From Previous Discussion...Latest radar imagery this afternoon shows
some light shower activity persisting along the coast and a
convergence zone lingering along the King -Snohomish county border.
Overall, hi-res guidance is indicative of the convergence zone
lingering into the early evening hours before dissipating.
Elsewhere, expect shower activity to dissipate into the evening for
a brief lull in the weather in between systems.

An atmospheric river is slated to impact the region Monday through
Tuesday. Moderate to heavy precipitation is expected to spread
northward across western Washington starting Monday morning as a
warm front lifts across the region. Snow levels will rise rapidly to
7500-9000 feet through the morning hours as a result. The trailing
cold front then looks to stall offshore before making its way across
the region on Tuesday. The latest forecast calls for 48 hour
rainfall totals between 12Z Monday and 12Z Wednesday generally
ranging from 1.5 to 3 inches for the interior lowlands, 3 to 5
inches along the coast, 5 to 9 inches along the Cascades, and 7 to
10 inches across the Olympics. The combination of QPF and runoff
from rain falling on top of snow in higher elevations will lead to
sharp rises on area rivers and an increased risk of river flooding
across the region. See the hydro section below for more details. In
addition to rain, breezy to locally windy conditions will develop
across the region during the afternoon hours and persist through
Tuesday morning. Large waves along the coast will build towards 20
feet on Tuesday and may run up further on area beaches at times.

Heavier rain associated with the cold front will then gradually sag
southward into Oregon on Wednesday, though conditions look to remain
showery across western Washington. At this time, additional rainfall
amounts look to remain light, from a couple hundredths of an inch to
a few tenths. 14

.LONG TERM /THURSDAY THROUGH SUNDAY/...From Previous
Discussion...Deterministic models remain in good agreement with an
upper level trough shifting eastward through W WA through the day
Thursday, keeping precipitation in the forecast into the long term.
This system will eject Friday in favor of some long-overdue upper
level ridging. Enough lingering moisture may present for some
morning and early afternoon showers, but the majority of the
afternoon and evening will be dry. The side effect of this ridging
is that it will allow for continued cooling as daytime highs slide
solidly into the 40s and snow levels continue to lower below pass
levels. Deterministic models disagree by 6 or so hours as to when on
Saturday to bring the next frontal system into the area, and while
ensembles at this point in the forecast offer an array of solutions,
the ensemble mean suggests that while some rain can be expected,
amounts will be generally light. There are some hints at some
atmospheric river support to the back end of this system during the
day Sunday, but deterministic models disagree on both placement /GFS
favors W WA while ECMWF leans more toward W OR/ and strength /ECMWF
shows the AR connections being tenuous at best/. Ensembles however
are not biting into this solution at all, suggesting a general down
turn in activity at the close of the forecast period. Current
forecast blend of models is currently leaning slightly more
favorably in the direction of wetter solutions, but not convincingly
so, doing little to shore up any confidence for this time frame. 18

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.AVIATION...Westerly flow aloft becoming more northwesterly
overnight before shifting west/southwest Monday afternoon. Surface
winds remaining generally southerly over the next 24 hours, although
there may be some shifts to either the SW or SE depending on
location and local terrain effects. With FROPA Monday afternoon and
evening, winds will increase in strength resulting in widespread
gusts.

Generally VFR conditions this evening and overnight for most
terminals, however low level moisture from earlier rains is giving
rise to some scattered MVFR to IFR conditions at present. Given that
this is not simply confined to locations more prone to lower cigs
lowers confidence in actually forecasting when and where this will
occur. As such, sites that have already seen these lower cigs
materialize will be covered via TEMPO groups to address the
occurrence while there is hesitance to include such groups for
places where this has yet to materialize. This means updates may be
necessary between issuances. Widespread rainfall expected with the
next incoming system impacting most W WA terminals by mid to late
Monday morning. This will result in cigs gradually lowering
overnight and early Monday morning with widespread MVFR to IFR
conditions in place by Monday afternoon as rains move into the area.

KSEA...VFR conditions remain in place tonight and early Monday
morning. Latest guidance shows MVFR conditions in place over the
terminal by mid to late morning. Have opted to follow guidance that
brings these cigs in earlier given that FEW/SCT low clouds have
already been observed at the terminal this evening. By late
morning/early afternoon, while cigs remain MVFR, will see them dip a
bit further, leveling out between 1000-2000 ft. Cigs may dip even
further under moderate to heavy rainfall, but will leave that out of
TAFs for now. Southwest winds 10-15 knots this evening will ease to
4-8 kts and shift more southeasterly overnight and remain that way
into for much of Monday morning and afternoon. Wind speeds will
return to 10-15 kts with gusts up to 25 kts by mid to late afternoon
and into the evening as the front starts to pass through the area.

18

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.MARINE...A series of frontal systems will make their way across the
waters Monday through Thursday. Winds in the inland waters have come
down to below Small Craft Advisory, and this lull in winds is
expected to continue overnight. Winds will increase again Monday,
first along the coast and in the Central and East Strait of Juan de
Fuca in the early morning, then for the rest of the coastal waters
in the afternoon. Confidence is highest for gales along the coast
and for the East Strait, but is less so for the Northern Inland
Waters--thus keeping the watch until the next update. The Central
Strait looks to avoid headlines Monday night into Tuesday with the
southerly winds along the Puget Sound into the Strait of Georgia and
tapering down along the coast. While a break in wind headlines for
the long term is expected, SCA conditions for seas will likely
remain along the coast into the end of the week.

Seas at 10 to 12 ft will rise to 15 to 17 ft on Monday before
reaching 18 to 20 ft Tuesday. Seas then will drop to around 10 ft by
Thursday and look to remain around 8-10 ft Friday and into the
weekend.

LH/18

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.HYDROLOGY...From Previous Discussion...An atmospheric river will
impact the region Monday through Tuesday and looks to bring rainfall
amounts of 7 to 10 inches in the Olympics, 6 to 9 inches in the
Cascades, 3 to 5 inches along the coast and 1.5 to 3 inches to the
interior lowlands through early Wednesday. Snow levels will rise
rapidly across the region on Monday, topping out and generally
ranging between 8000 to 9000 feet by Monday afternoon. A Flood Watch
remains in effect for the area through late Wednesday night.

The combination of heavy precipitation and rising snow levels will
most likely result in river flooding on many rivers across western
Washington. Rivers currently look to reach flood stage Monday night
or Tuesday. Moderate to major flooding will be possible on some of
the bigger rivers like the Snoqualmie, Snohomish and Skagit as well
as the lower reaches of the Chehalis. While flooding looks likely
across the region, there remains a degree of uncertainty in its
scope and extent. This will depend largely on the reaction of the
area rivers from runoff produced from rain on snow for river basins
in higher elevations and the locations of the heaviest bands of
precipitation, so it will be important to stay up to date on the
latest forecast developments.

In addition to the river flooding, burn scars in the Cascades will
have to be monitored with the heavy precipitation forecast. Elevated
risks of landslide and debris flows will also be possible,
especially later Monday through much of Tuesday. 14/Felton

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through late Wednesday night for Admiralty Inlet
  Area-Bellevue and Vicinity-Bremerton and Vicinity-Central Coast-
East Puget Sound Lowlands-Eastern Strait of Juan de Fuca-Everett and
Vicinity-Hood Canal Area-Lower Chehalis Valley Area-North Coast-
Olympics-San Juan County-Seattle and Vicinity-Southwest Interior-
Tacoma Area-West Slopes North Cascades and Passes-West Slopes North
Central Cascades and Passes-West Slopes South Central Cascades and
Passes-Western Skagit County-Western Strait of Juan De Fuca- Western
Whatcom County.

PZ...Small Craft Advisory until 4 AM PST Tuesday for Grays Harbor
Bar-     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 AM to 1 PM PST Monday for Central
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 1 AM Monday to 1 AM PST Tuesday for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Monday morning through late Monday night for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 10 AM Monday to 4 AM PST Tuesday for
     Admiralty Inlet.

     Small Craft Advisory from 1 PM Monday to 4 AM PST Tuesday for
     Puget Sound and Hood Canal.

     Small Craft Advisory until 10 AM 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.

     Gale Warning from 10 AM Monday to 1 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.

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