Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 281033

Area Forecast Discussion
National Weather Service Seattle WA
233 AM PST Tue Nov 28 2023

.SYNOPSIS...The upper ridge slides eastward later Tuesday with
more split flow on Wednesday. Dry weather, colder overnight
temperatures, and widespread morning fog through Wednesday. A weak
front moves across Western Washington Thursday with a more active
pattern Friday into next week.


.SHORT TERM /TODAY THROUGH THURSDAY/...Satellite imagery and obs
early this morning shows widespread fog across southern Puget
Sound, spreading towards Grays Harbor County, as well as across
western Whatcom County and towards the Strait of Juan de Fuca.
Temperatures in these areas generally range 25 to 35 degrees, for
this reason, a Freezing Fog Advisory is in place for the
aforementioned areas, with this forecast update including Seattle
metro. For those going out this morning, give yourself more time
with potential for areas of slick roads and sidewalks due to the
moisture and temperature at the surface. Some improvement to
visibilities late morning into the afternoon, however, given the
stagnant pattern and light flow, continued fog can be expected
through the afternoon towards south Puget Sound (including Olympia
and Shelton). Although the ridge axis begins to slide eastward
later today into tonight, a split flow regime for the Pacific
Northwest will result in another dry, cold morning on Wednesday.
Fog will once again become widespread across Puget Sound Wednesday
morning. Highs through Wednesday mainly in the low to mid 40s,
but highs may continue to remain in the 30s where fog does not
clear, particularly the southern Puget Sound area.

Troughing begins to develop across the NE Pacific Wednesday night
into Thursday morning highlighting a pattern change to Western
Washington. A front will slide across the area Thursday into
Thursday night for lowland rain and light mountain snow. Snow
levels for this initial system on Thursday will range 1500 to 2500
feet for accumulating snow at the Cascade Passes. Snow amounts
through Thursday night ranging 2 to 5 inches for both Snoqualmie
and Stevens Pass. A more active pattern begins on Friday for
additional mountain snow. Temperatures on Thursday similar with
highs in the mid to upper 40s.

.LONG TERM /FRIDAY THROUGH MONDAY/...The more active weather
pattern continues through this period as multiple systems move
across the Pacific Northwest for lowland rain, mountain snow, and
breezy winds. Troughing will provide a bit more of an influence
Friday into Saturday allowing for cooler temperatures, and thus
snow levels will be lower during this period. Guidance suggests
snow levels generally 2000-3000 feet for moderate to heavier snow
possible over both Snoqualmie and Stevens Pass. Ensemble guidance
suggests ridging then begins to build across the Western US Sunday
into early next week, for more of a southwest flow regime over
Western Washington. NBM surface temperatures depicts this warming
trend well, with highs into the low 50s early next week. However,
uncertainty does exist as far as the extent of ridging into the
Pacific Northwest with troughing well offshore. This pattern would
likely continue to support the active weather trend for
additional breezy winds and potentially heavier rain early to
middle of next week. JD


.AVIATION...An upper ridge axis will shift east of the region
today with light south to southwest flow aloft developing. The low
level flow is light offshore. Substantial low level moisture remains
in place across the interior of Western Washington from the Chehalis
Gap northward through Puget Sound as well as the Strait of Juan de
Fuca. Areas of LIFR/IFR in stratus and/or fog will persist through
midday before scattering out. However, much like yesterday, the low
clouds may not entirely scatter out from south Puget Sound
southward. Widespread low clouds and fog are expected to develop
again tonight.

KSEA...LIFR/IFR in shallow fog expected through at least mid-morning
with surface vsbys below 1 mile. Surface gradients are similar to
those of 24 hours ago so will likely follow a similar timeline with
fog gradually lifting around 18Z then scattering out by around 20Z.
Flat gradients support the development of LIFR conditions again
tonight. UW-WRF model low level cloud water output would suggest
that fog will develop earlier tonight with more extensive coverage
for Wednesday morning. Surface winds light and variable. 27


.MARINE...Surface ridge remains centered east of the Cascades
with low pressure offshore producing weak offshore flow. The ridge
will weaken through Wednesday for light winds across area waters.
The light surface winds will remain conducive to widespread
visibility restrictions in night and morning fog across interior
waters. A relatively weak frontal system will move into the coastal
waters Thursday morning then push onshore in the afternoon. A more
vigorous system is expected to follow on Friday with headlines
likely for most of the area waters. A series of systems are
then expected to follow for an extended period of active weather.

Coastal seas will remain below 10 feet through Thursday then build
to 10+ feet Friday and likely remain hazardous for the foreseeable
future. 27


.HYDROLOGY...With the upcoming pattern change suggesting more rain
in the forecast, area rivers are likely to see rises through the
weekend. This will continue to be monitored and appropriate
hydrologic products will be issued as confidence grows in the
rainfall forecast.


WA...Freezing Fog Advisory until 10 AM PST this morning for Bremerton
     and Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Whatcom County.

PZ...Dense Fog Advisory until 10 AM PST this morning for Northern
     Inland Waters Including The San Juan Islands-Puget Sound
     and Hood Canal.


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