428
FXUS66 KSEW 212346
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
346 PM PST Thu Nov 21 2024

.SYNOPSIS...An upper level low continues to remain well offshore
Thursday into the weekend. A surface low will swing a couple of
fronts into Western Washington Friday, which will bring a couple
of rounds of easterly and southerly gusty winds. Most areas will
also see precipitation with this system, with a chance of thunder
along the Pacific Coast, and snow showers up at the passes and
Olympics and Cascades. The low appears to depart mid week next
week, which may set up the region for some quieter weather leading
up to Thanksgiving next week.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...An upper level 300 mb low
remains parked offshore in the Pacific this afternoon, with the
flow up there being more zonal today. As a result, weather this
afternoon is on the quieter side (compared to Wednesday). The
showers this afternoon have been concentrated along the coast and
over waters, ahead of a weak stationary/occluded front. High
resolution guidance shows the showers moving northward this
afternoon, primarily staying west of Puget Sound. Rain rates are
expected to be lighter, and thunder is not expected.

Friday morning into Saturday morning: A strong surface low
pressure system will track towards the Washington coast (but
remain offshore). The center of the low (per WPC surface analysis)
shows it deepening to 980 mb (not as strong as the
Tuesday/Wednesday storm). A triple point of fronts will swing up
from Oregon early Friday morning. The initial warm front along the
OR/WA border will produce the first round of precipitation and
gusty winds, beginning around 2 am to 4 am. No thunder is expected
with this initial round, but some heavier bands of precipitation
are possible within this line. The snow levels will rise up to
7,000-8,000 feet Friday with warm air bring brought north (highs
up to the mid 50s for the lowlands). Gusty east winds are expected
ahead of the warm front, with gusts reaching between 40 to 50 mph
in the wind advisory area (Bellevue, King/Snohomish/Pierce County
Cascade Foothills, and also northwestern Whatcom County).

A second round of precipitation/winds are expected late Friday
morning into the afternoon/evening ahead of the occluded front
associated with the triple point. This will produce more
widespread gusty winds out of the south late Friday morning into
the evening, with winds peaking Friday afternoon. The highest wind
gusts in this period will be along the Pacific Coast, with wind
gusts of 45 to 55 mph. For the Southwest Interior and Strait of
Juan de Fuca coast, wind gusts of 40 to 45 mph are expected. For
Seattle Metro and Cascades, wind gusts of 30 to 40 mph are
expected. Additional rounds of showers will spread inland with
this front into the afternoon. There will be a slight risk of
thunderstorms along the Pacific Coast and Southwest Interior this
in the afternoon. Severe storms are not expected, but a couple of
stronger storms producing small hail and gusty winds cannot be
ruled out. Additionally, a high surf advisory will be in effect
Friday for waves of 20 to 24 feet (primarily affecting ocean
beaches). Snow levels will drop back down to 3,500 feet Friday
night, with cooler air dropping temperatures down a few degrees
going into Saturday.

The low will begin to fill Saturday into Sunday, however showers
will continue into the weekend. Snow showers will also continue up
in the Cascades and Olympics. Snow accumulations will be light for
most areas, with Snoqualmie Pass only seeing 1 inch, and Stevens
Pass seeing 1-3 inches. QPF will be concentrated in the Olympics
and coast, with 2-3 inches, 0.5-1 inch in the Cascades, and around
half an inch in the lowlands. Most of the precipitation will fall
Friday, with rates tapering back Saturday into Sunday. Winds will
decrease to 5 to 10 mph southerly winds during the weekend.

.LONG TERM /MONDAY THROUGH THURSDAY/...As mentioned in the short
term section, the low is expected to fill in early next week, and
begin to move onshore Tuesday, beginning to break down over the
Rockies/Northern California as it positively tilts. The flow will
become northwesterly middle of the week, with potential for drier
conditions towards the Thanksgiving holiday (though long range
ensembles still hint at a slight chance/chance of precipitation
for the region during this time). The northwest flow will likely
cool high temperatures into the mid 40s, and lows into the mid
30s for most areas (expect cooler temperatures in the mountains).

HPR

&&

.AVIATION...Expect predominantly VFR ceilings and light wind
across the region with southerly flow aloft as a deep low remains
offshore over the eastern Pacific. The closer movement of this
deepening low will bring gusty east winds through the Cascades after
around 09z through 15z, strongest nearer the Cascades but still
gusting to 30 kt for favored locations downwind of the gaps. This
could pose a crosswind threat for terminals with only north/south
aligned runways. A warm front will bring some lower ceilings
(approaching or into MVFR range) along with some rain, followed by a
shift to southerly winds. These southerly winds will again be gusty,
strongest near the coast and the I-5 corridor/Puget Sound terminals.
Showers continue in the post-frontal air mass with a low (10-15%)
chance of a few isolated thunderstorms west of the Sound (highest
closer to the coast). Southerly winds eventually subside late Friday.

KSEA...Wind will become easterly today and increase tonight by 09z,
with the potential for east gusts continuing through 15z. Southerly
winds, also gusty, will follow during the day on Friday. Otherwise,
VFR conditions through the day, with increased likelihood of MVFR
ceilings with the warm front overnight along with the east wind
gusts.

&&

.MARINE...A brief lull in the marine conditions will come to an
end this evening as the next low approaches the waters as it
strengthens. This will again bring gusty east winds to the waters
tonight as it moves closer, followed by very strong southerly winds
(especially over the coastal waters, where storm force winds are
expected). Storm warnings cover the coastal waters with gales
expected through the Strait. Elsewhere, small craft advisories have
been issued for the remaining marine zones. A few notes about these
SCA: 1 -  the initial SCA for Puget Sound/Hood Canal and Admiralty
Inlet is covering the period of east winds, but it will either need
to be extended (or upgraded to a gale warning) for the southerlies
that will follow. 2 - There remains potential for east gusts to 35
kt north and east of the San Juans tonight, but confidence isn`t
high enough to warrant a Gale Warning at this time.  Meanwhile, seas
will again build significantly into the 20 to 25 ft range through
the day on Friday before gradually subsiding into Saturday. The low
will then weaken, bringing relatively less impactful conditions for
the start of next week.

&&

.HYDROLOGY...The Skokomish River will stay in action through early
Friday, before climbing up into minor flood stage Friday night.

For the remainder of the rivers no flooding is expected at this
time. Lower snow levels into the weekend 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.

HPR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 10 PM this evening to 10 AM PST Friday for
     Bellevue and Vicinity-East Puget Sound Lowlands-Western
     Whatcom County.

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

     High Surf Advisory from 9 AM to 10 PM PST Friday for Central
     Coast-North Coast.

     Wind Advisory from 10 AM Friday to 4 AM PST Saturday for Central
     Coast-North Coast.

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

     Small Craft Advisory from 10 PM this evening to 1 AM PST Friday
     for Grays Harbor Bar.

     Gale Warning from 10 PM this evening to 4 AM PST Saturday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 10 PM this evening to 4 AM PST
     Saturday for Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory from 1 AM to noon PST Friday for Admiralty
     Inlet-Puget Sound and Hood Canal.

     Storm Warning until 4 AM PST Saturday 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.

&&

$$