763
FXUS66 KSEW 091057
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
357 AM PDT Thu May 9 2024

.SYNOPSIS...Upper level ridge centered offshore nosing into
Southern British Columbia today. The ridge will remain in place
Friday then begin to weaken Saturday. Low level flow turning
offshore today. Offshore flow continuing into Friday with the
flow turning onshore Saturday. Weak upper level trough moving
through later Sunday. A weaker ridge will temporarily rebuild
Monday. Another trough will arrive late Tuesday into Wednesday.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Satellite imagery shows
clear skies over Western Washington with the exception of a small
area of stratus along the north coast. Temperatures at 3 am/10z
were in the 40s and lower 50s.

The process leading to the warmest days of the year for Western
Washington is already underway this morning. Upper level ridge
offshore building into Southern British Columbia. Cross cascade
gradients down to plus 2 mb and will go negative later today.
Surface gradient has already turned negative in the Strait of Juan
de Fuca. In addition to the lower level flow turning offshore
winds at 850 mb and 700 mb are also turning easterly this morning.
With the ridge building temperatures aloft warming. 850 mb
temperatures near plus 8C this morning warming to plus 12 or 13C
by late this afternoon. All of this adds up to significant warming
today across the area with inland temperatures about 10 degrees
warmer than Wednesday and temperatures along the coast 15 degrees
or so warmer versus yesterday. Highs in the 70s with the warmer
locations getting into the lower 80s.

Little change in the pattern overnight into Friday. Temperatures
aloft continue to warm with 850 mb temperatures in the plus 13 to
plus 15C range by late Friday afternoon. Lows tonight in the upper
40s to mid 50s, close to record high minimum temperatures in some
locations. Highs Friday a little warmer with 70s to mid 80s. While
not expecting record highs Friday, temperatures this warm are
unusual for the first half of May. Seattle has only had 3 days in
the 80s on May 10th in 79 years of records.

Even with the surface temperatures warming quickly waters around
Western Washington are still very cool. Water temperatures this
morning, Lake Washington 54 degrees, Lake Sammamish 56 degrees and
Puget Sound in the lower 50s. The rivers are even colder with the
Cedar River at 48 degrees, the Skykomish and Tolt 46 degrees and
the Skagit checking in with a cold 45 degrees. Cold water shock
can be deadly. If you are going to be on the water wear a life
jacket.

Upper level ridge weakening Friday night into Saturday. Low level
offshore flow turning weak onshore. Winds at 850 and 700 mb turn
west northwesterly. With the ridge weakening slightly
temperatures aloft cool a touch with model 850 mb temperatures
late Saturday afternoon plus 12 or 13C. Another night of mild low
temperatures, close to record high minimums in places, in the
lower to mid 50s. Highs Saturday much cooler along the coast, in
the mid 60s with highs over the interior in the 70s and lower 80s.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Models in good agreement
Sunday with the ridge continuing to weaken and an upper level
trough riding over the ridge into British Columbia late in the
day. This feature doesn`t have much punch with just a chance of
showers in the Olympics and Cascades north of about Snoqualmie
Pass Sunday night. Ensembles are weaker with the upper level
trough versus the operational runs. Operational runs have highs
only in the 60s for the interior Sunday. Will lean a little
towards the ensembles and go for highs in the 60s and lower 70s.

Upper level ridge tries to rebuild Monday but the ridge axis is
well offshore and low level onshore flow continues through the day.
Temperatures aloft continue to cool. This combination will lower
high temperatures Monday back into the 60s.

Model solutions become inconsistent Tuesday and Wednesday with
the ECMWF building a stronger ridge over the area while the GFS
has a front approaching from the northwest Wednesday. High
temperature spreads by Wednesday rather large. For Seattle 80
percent of the solutions have the high between 78 and 58 degrees.
With the large amount of uncertainty will split the difference and
have highs in the 60s and introduce a slight chance of showers
Tuesday night and Wednesday. Felton

&&

.AVIATION...Offshore/northeasterly flow aloft today with a ridge
building off the Pacific coast, and inland over Washington today.
Skies remain clear this morning, with offshore flow keeping stratus
well off the coast. A couple isolated areas may see patchy fog with
the cool temperatures this morning. Otherwise, light
northerly/northeast winds should be enough to keep skies clear
during the morning rush. Couple terminals (particularly around Puget
Sound/Central Strait of Juan de Fuca) may see gusts to 20 kt during
the afternoon, but winds generally at 6 to 12 kt in the forecast
with VFR skies.

KSEA...VFR/clear skies expected in this forecast. Northeast winds
this morning will transition to northerly by 18Z at 08-12 kt with
gusts to 20 kt possible.

HPR

&&

.MARINE...High pressure has built up some over the waters, and
building will continue over the State of Washington today. With a
thermal trough underneath the ridge up the coast, flow is
predominately offshore with north/northeast winds. Winds for
Thursday may gust up to 20 kt in the coastal waters, as well as
Puget Sound and Central Strait of Juan de Fuca (but are expected to
remain below hazardous levels for small craft). This pattern will
continue into the weekend, until a trough moves through the region
Saturday/Sunday. Gusty winds above 20 kt are possible in the Strait
of Juan de Fuca and the outer coastal waters - will continue to
monitor for possible headlines.

Seas of 5 to 8 feet through late week into next week.

HPR

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$