Washington State Forecast Discussion

Western Washington


990
FXUS66 KSEW 210440
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
840 PM PST Wed Nov 20 2024

.SYNOPSIS...A large upper level low will continue to churn off the
coast of Washington through the weekend. Showers and isolated
thunderstorms will continue this afternoon along the coast ahead
of a stalled out occluded front, with snow showers continuing in
the Cascades and Olympics. A second surface low will track towards
the region offshore on Friday, bringing more widespread
precipitation and the potential for gusty winds near coastal
areas. The unsettled weather pattern continues this weekend into
early next week.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...An upper trough offshore
continues to spin showers and isolated thunderstorms into Western
Washington this evening. Overall activity will begin to diminish
overnight as the upper low continues to gradually weaken and is
expected to retrograde further offshore toward the early morning
hours of Thursday. Models rotate another upper level disturbance
toward the coast during the day on Thursday for an expected uptick
in shower activity...especially west of Puget Sound. Another brief
downward trend in precipitation is expected Thursday evening before
our attention turns toward the next impactful area of low pressure
that will deepen as it curls northeastward offshore. Again, this
next system is not expected to be as impactful as that of Tuesday
night. By early Friday, models are advertising a cross-Cascade
easterly gradient of roughly 60% of what we saw Tuesday
night...which bodes well as we continue to clean up from the recent
storm. Windiest areas will likely be along the central coast where
ensembles are showing some gusts in the 40 to 45 mph range. We'll
continue to monitor this as new data comes in. Current forecasts are
on track this evening. Previous discussion follows with updates to
marine and aviation portions. 27

Upper level analysis and satellite shows a large upper level low
still offshore this afternoon. A weak occluded front still remains
offshore (as it stalls approaching the Washington Coast). With cool
air aloft,this frontal boundary has been the primary focus area for
morning and afternoon convection, in the form of showers and
thunderstorms. A couple strong storms may be capable of producing
small hail and gusty winds.

Elsewhere, a few showers continue inland across Puget Sound.
These will mostly continue along and west of the I-5 corridor
through the afternoon. Snow showers also continue in the Cascades
and Olympics this afternoon. Additionally, coastal flooding up in
San Juan County has begun to recede (with the inundation
beginning to drop below 2 feet/minor flooding). High surf
conditions will remain a concern for anyone going to beaches
through early Thursday morning as waves of 20 to 24 feet continue
with this low. Winds remain a little breezy this afternoon (with
peak gusts around 20 to 25 mph along coastal areas), but otherwise
winds will continue to decrease later tonight into Thursday
morning over most areas.

The Thursday pattern will be similar to Wednesday with off an on
showers across the entire coverage area. The most wet time appears
to be early Thursday afternoon with a band of showers moving up
from south to north on high resolution models. The coastline will
continue to see a chance of isolated thunderstorms Thursday. High
temperatures will range in the upper 50s to low 50s Thursday, with
winds expected to be significantly lighter out of the southeast at
around 5 to 10 mph.

Friday/Saturday: Another deep surface low is expected to track
towards our region offshore early in the morning. WPC guidance and
models show the low deepening to around 980 mb (which is not as
strong as the low that passed through the region Tuesday
night/Wednesday morning. Want to iterate to the public that the
impacts are not expected to be as widespread as with the low
earlier in the week (winds will be lighter out of the south, and
precipitation amounts will be lighter). Most likely scenario will
be a period of breezy southerly winds approaching 30 to 35 mph
Friday morning along the coastline, and North Coast. There is only
a 10 to 15 percent chance that wind gusts would exceed 45 mph
(for a wind advisory). Remaining areas will most likely only see
wind gusts peak at 20 to 25 mph.

Precipitation amounts will also be significantly less,
with 24 hour QPF totals approaching half an inch in the lowlands,
and 1-2 inches in the Cascades, Olympics and coast. Snow levels on
Friday will increase with the warm front to around 5,000 feet
during the day, then fall to around 3,000 feet Saturday. The
heaviest of the snow will be at higher elevations, but still could
be an inch or two in some of the passes through the weekend. The
showers will continue into the weekend, with winds beginning to
diminish more Saturday.

This system will continue to be monitored, and as higher
resolution guidance comes in closer to the event, details will be
shared.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...The consensus between
longer range and ensemble models is for the upper level low to
move inland early next week. There is some uncertainty with how
the low will move inland (some solutions show the low tracking
into northern California, with some keeping it only down into
Oregon). With the low expected to depart, it appears the wet
weather will be replaced with drier weather come midweek as
ridging builds back into the region.

HPR

&&

.AVIATION...A deep upper level low continues to spin offshore with
strong south/southwesterly flow aloft. VFR conditions prevail across
western Washington this afternoon. Scattered showers and a few
isolated thunderstorms along the coast continue this afternoon.
Shower activity should decrease in coverage tonight before another
round of light rain is possible tomorrow. Ceilings should remain
mostly VFR though the TAF period, though may lower to MVFR at times
in any showers. Ceilings may lower to low-end VFR to high-end MVFR
with the rain coming through tomorrow.

Winds generally east to southeast 5 to 15 kt decreasing to 5 kt or
less tonight. Winds again will be east to southeast tomorrow but
lighter (around 5 to 8 kt).

KSEA...VFR conditions prevail this afternoon and should remain like
so through most of the TAF period. Scattered showers remain across
the area which could bring brief periods of MVFR this evening and
early tonight. Shower coverage will decrease before another round of
rain arrives tomorrow, which will bring ceilings down close to the
MVFR threshold. Winds east to southeast 8 to 12 kt (with a few gusts
here and there) but decrease tonight. Winds may turn north (up to 5
kt) for a few hours early Thursday morning before returning back to
southeast late Thursday morning.

62

&&

.MARINE...An area of low pressure remains well offshore, west of
Vancouver Island. South-southeasterly continue to diminish this
evening, with Small Craft Advisories remaining for the coastal
waters and the East Strait of Juan de Fuca. Winds will continue to
ease into early tomorrow morning. Winds will turn southeast to east
on Thursday as another low pressure system develops and strengthens
well offshore of Washington. Peak winds in the coastal waters will
depend on the exact strength and track of this low, but winds are
expected to reach high-end gale to potentially storm force (with a
50-70% probability of wind gusts exceeding storm force). A Storm
Watch has been issued for Friday. Winds will also increase through
the inner coastal waters on Friday, with the potential for gale
force winds through Admiralty Inlet and the East Strait of Juan de
Fuca. The low pressure system will gradually move northward towards
Vancouver Island, allowing winds to decrease across the area Friday
night into Saturday.

Seas are beginning to subside below 20 ft, with recent observation
showing around 16 to 18 ft. Seas will subside tonight and tomorrow,
subsiding below 10 ft Thursday night. Seas will once again quickly
rise Friday morning, potentially reaching up to 20-25 ft through the
outer coastal waters. With the uncertainty in the track of the low,
it is more uncertain what the waves will be like in the inner
coastal waters, through seas 15 to 18 ft are certainly not out of
the question. Seas will subside Friday night into Saturday as the
system departs.

With the east winds on Friday and building seas, conditions at Grays
Harbor Bar will likely be hazardous, especially at the time of ebb.

62

&&

.HYDROLOGY...The Skokomish river will stay between action
and minor flood stage through Saturday. As rain falls and with some
snowmelt contribution beginning on Friday morning, the river may
briefly rise into minor flood stage by late Friday into early
Saturday morning.

For the remainder of the rivers no flooding is expected. Lower snow
levels in the Cascades will limit runoff and reduce any flooding
chances for rivers flowing off the Cascades. Precipitation for the
remainder of the period will come in bursts. This combined with snow
levels remaining relatively low will keep rivers in their banks.
Felton/Kristell

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Coastal Flood Advisory until 4 PM PST this afternoon for San
Juan County-Western Whatcom County.

Flood Watch through late Friday night for Hood Canal Area-Lower
Chehalis Valley Area-Olympics.

High Surf Advisory until 4 AM PST Thursday for Central Coast-
North Coast.

Winter Weather Advisory until 6 PM PST this evening for West
Slopes North Cascades and Passes.

PZ...Small Craft Advisory until 10 PM PST this evening for Grays
Harbor Bar.

Gale Warning until 1 AM PST Thursday for West Entrance U.S.
Waters Strait Of Juan De Fuca.

Gale Warning until 11 PM PST this evening for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.

Gale Warning until 4 PM PST this afternoon for Northern Inland
Waters Including The San Juan Islands.

Gale Warning until 2 AM PST Thursday 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.

&&