815
FXUS66 KSEW 220347
AFDSEW

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

.UPDATE...Overall a quiet evening across western Washington this
evening. Partly cloudy skies and a break in rain this evening.
Satellite imagery is beginning to show the next low pressure
system developing offshore west of Oregon, which will be the
primary feature responsible for the next round of active weather
tomorrow. The forecast remains on track, see the previous
discussion below, as well an update to the aviation section. 62

&&

.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 /TODAY 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 forecast) 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...South to southwest flow aloft will increase
overnight as a deepening upper trough and associated frontal system
approach the region. Patchy IFR/LIFR stratus will form mainly south
and west of Puget Sound this evening before increasing low level
easterly flow gradually erodes it. Otherwise, ceilings will
deteriorate to MVFR at times most areas by 12Z as precipitation
increases ahead of a warm front. The trailing cold front will catch
up and occlude it as it lifts northward across the region Friday
afternoon. Gusty east winds near gaps in terrain will increase 09Z-
12Z before winds shift southerly mid to late morning and increase
along the coast and through Puget Sound as low pressure offshore
shifts northward. Some modest improvement to low end VFR in
scattered showers is expected by late afternoon in the unstable post-
frontal air mass.

KSEA...Ceilings lowering to MVFR 10Z-12Z as main precipitation band
lifts northward across the terminal. These conditions will persist
at times through around 20Z before precipitation turns more showery.
Surface winds become easterly overnight by 11z, with the potential
for east gusts continuing through 15z. Southerly winds, also gusty,
will follow during the day on Friday. 27

&&

.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. 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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 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 AM PST Saturday for Grays Harbor
     Bar.

     Gale Warning until 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 until 7 AM PST Saturday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory from 1 AM Friday to 2 AM PST Saturday 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.

&&

$$