Washington State Forecast Discussion

Western Washington


885
FXUS66 KSEW 281610
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
910 AM PDT Sat Sep 28 2024

.UPDATE...Current temperatures are in the 40s and 50s this
morning. Low clouds and patchy fog evident across the SW interior
into puget Sound given recent observations. No changes made to
the inherited forecast. The previous discussion remains below
along with an updated marine/aviation section.

&&

.SYNOPSIS...Drier conditions today as zonal flow aloft
continues. A subtle trough will enter western Washington Sunday
increasing precipitation chances. An upper level ridge set up to
start the new week, however a series of weak troughs will influence
the pattern. Most precipitation should stay north of the area.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Current satellite imagery
showing some high clouds passing over W WA this early morning with
most obs sites not showing anything in the way of low clouds,
stratus or fog...save for over portions of Whidbey Island. Looking
at temp/dewpoint spreads, there is the hint that this might not be
the case throughout the remainder of the morning, as that split is
only one or two degrees in some spots. Inherited forecast carved out
an area of patchy fog running from the SW interior up northeastward
along the Sound and am inclined to leave that risk stand through
late this morning.

Otherwise, pretty uneventful weather-wise today. Zonal flow aloft
shifting southwesterly in advance of a weak upper level trough
progged to traverse the area Sunday. For today, that means
increasing clouds and slightly cooler temperatures. Daytime highs
expected generally in the lower to mid 60s for most lowland
locations, although some water-adjacent spots may find themselves a
little cooler than that in the upper 50s. As the weak trough passes
Sunday, any potential for precip should remain from Everett north
east of the Sound and perhaps just nicking the north coast on the
Olympic Peninsula. Monday sees upper level ridging return bringing
clearing skies and the possibility for slightly warmer temperatures
for the SW interior...with highs in the mid to upper 60s. The
remainder of the area may see a slight nudge upward as well...but
not enough to get the lowlands out of the lower to mid 60s.
Overnight lows in the near term will certainly feel autumn-like,
generally in the lower to mid 40s.

18

.LONG TERM /TUESDAY THROUGH FRIDAY/...The upper ridge will flatten
out as Tuesday rolls around, thanks to an upper level low moving
southward from the Gulf of Alaska. Deterministic models in
disagreement with the GFS keeping any precip north of the border for
the midweek period running Tuesday through Thursday, however the
ECMWF brings a swathe of moisture into W WA Tuesday night into
Wednesday morning before favoring dry conditions for the remainder
of the day. Ensembles do lean toward the Euro solution, however even
there the resultant precip amounts are very low. As such, NBM in
forecast has finally had its coffee when it comes to this scenario,
with PoPs generally in the 10-30 pct range, a much better reflection
of current solutions when compared to output 24 and 48 hours ago.
This PoPs, unimpressive as they are, will mainly be of interest to
the northern two-thirds of the CWA...with points Tacoma southward
likely to receive nothing. Upper level ridging builds in as early as
Wednesday night, keeping conditions dry for Thursday and much of
Friday. Deterministic models and ensemble members in agreement with
a frontal system associated with an upper low off the Canadian coast
reaching our coastline late Friday afternoon/early Friday evening
with a more convincing shot at precip for the area, although given
that is expected in the day-7 timeframe, NBM rightly maxes PoPs out
around 40 pct for now...enough to warrant some confidence but plenty
of time to move upward or down as solutions likely change.

18

&&

.AVIATION...Northerly surface flow is likely to continue for most
terminals throughout the TAF period. Locations nearest the coast and
Strait of Juan de Fuca will see an onshore component late this
evening. MVFR to IFR cigs likely to improve after 18Z this morning,
with VFR ceilings throughout the rest of the day. Mid and high
clouds will linger. Another onshore push overnight will bring in
clouds for Sunday morning, but should dissipate by midday for a
mostly clear afternoon.

KSEA...Lingering 700 ft ceilings are expected to persist until at
least 18Z, with model guidance unclear on exact solutions on
ceilings until then. VFR cigs likely after 18Z-19Z. Winds are going
to be northerly for much of the TAF period, with speeds relaxing
below 7 kt after 00Z. As another onshore push occurs overnight,
expect another round of stratus at the terminal Sunday morning.

Kristell


&&

.MARINE...Surface ridging over the interior of British Columbia
will weaken today. Seas over the coastal waters will drop
below 10 feet this morning. Onshore flow will increase late today
into tonight ahead of a weak system moving into southern B.C. This
will be sufficient to generate small craft advisory westerlies in
the central/east strait this evening and overnight. Northwesterlies
will also pick up over the coastal waters tonight, but current
forecasts are capped around 20 knots.

Surface ridging offshore will expand into the inland waters late
Sunday into Monday allowing onshore flow to relax and perhaps turn
weakly offshore. A weak front will approach the area late Tuesday
into Wednesday for possible wind and sea related headlines over the
coastal waters. 27/Kristell


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Sunday
for Central U.S. Waters Strait Of Juan De Fuca-East
Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 3 PM PDT this afternoon for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From James Island To Point Grenville 10 To
60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm.

&&