532
FXUS66 KSEW 131514
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
814 AM PDT Mon May 13 2024

.SYNOPSIS...Weak upper level trough moving through the area today.
Upper level ridge building tonight and Tuesday with the ridge
remaining over the area until at least Thursday. Another upper
level trough moving down from the north could reach Western
Washington Friday through the weekend.

&&

.UPDATE...Onshore flow and marine stratus overhead will result in
much cooler conditions today. There will be a slight chance of a
weak convergence zone this afternoon and evening, with a 15-25%
chance of showers over the central Cascades and 10-15% over the
central Puget Sound lowlands. Otherwise mostly dry conditions are
expected. The rest of the forecast detailed below is on track and
no further updates are expected this morning. -Wolcott-

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
weak upper level trough over Vancouver Island moving southeast.
Stratus along the coast moving inland through the Lower Chehalis
Valley. With the trough approaching middle level clouds are
beginning to increase. Temperatures at 3 am/10z were in the upper
40s to mid 50s.

Upper level trough moving through Western Washington today. Little
in the way of dynamics with this feature with just increasing
cloud cover this morning. Could squeeze out a shower over the
Cascades but that is about it for precipitation with the trough.
Cloud cover and low level onshore flow will keep high temperatures
in the upper 50s to mid 60s.

Trough east of the area tonight with an upper level ridge building
offshore. With the ridge centered well offshore middle level cloud
deck remaining over Western Washington overnight. Low level flow
becoming northwesterly which will restrict the amount of stratus
forming especially over the interior. Lows in the 40s.

Upper level ridge continuing to build Tuesday. As the ridge begins
to shift a little east more subsidence will dissipate the middle
level cloud deck later in the day. Northwesterly flow in the lower
levels will keep temperatures from warming up too much with highs
in the 60s and lower 70s.

Upper level ridge remaining over the area Tuesday night and
Wednesday. Temperatures aloft warming with 850 mb temperatures
going from around plus 8C Tuesday night to plus 10 to 12C
Wednesday afternoon. Low level flow remaining northwesterly
keeping highs in the mid 60s to mid 70s with the cooler locations
near the water. Lows Tuesday night in the mid 40s to lower 50s.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Models still wrestling with
inconsistency in the extended period especially run to run. Both
the operational GFS and ECMWF have a weak shortwave moving by to
the north Thursday. Ensemble solutions not in agreement with the
operational runs with high temperatures just a couple of degrees
cooler Thursday and the shortwave further north. Zonal flow aloft and
low level onshore low cooling high temperatures a few more
degrees Friday and Saturday. Operational runs bring a stronger
trough into the area Sunday but once again the ensembles are not
on board with this idea keeping highs a little above normal with
not much cloud cover. Will add a chance of showers to the Cascades
Sunday but keep the lowlands dry through the period. Highs mid 60s
to lower 70s Thursday and Sunday with highs mostly in the 60s
Friday and Saturday. Felton

&&

.AVIATION...Upper trough axis over Western Washington this morning
will shift eastward with upper ridge rebuilding into the region
later today and tonight. Westerly flow aloft will turn northwesterly
tonight. The low level flow remains onshore. IFR ceilings along the
coast in stratus will extend locally inland this morning. The
onshore flow will produce some weak convergence over the interior.
This has been primarily MVFR around 2,200 ft into Puget
Sound/Interior terminals (with a couple pockets of IFR around KPAE
and KPWT). Improvement is not expected until early this afternoon
(around 21Z) to a broken VFR deck. Surface winds will be out of the
southwest this morning at 5 to 10 kt, shifting to the northwest mid
afternoon.

KSEA...Upper trough passing through the region combined with some
weak low level convergence is expected to bring in MVFR broken decks
around 2,200 ft this morning (based on southward trends of ceilings
up at KBFI). The ceilings may not improve to VFR until early this
afternoon (~21Z), but once they do, will hover around 5,000 ft.
Southwest winds at 5 to 10 kt will become northwesterly around 00Z
this afternoon (possible earlier once the convergence subsides).

27/HPR

&&

.MARINE...Late spring type of pattern expected with broad surface
ridging remaining centered well offshore with low pressure inland
throughout the week ahead for varying degrees of onshore flow. This
will lead to periods of small craft advisory strength
northwesterlies over the coastal waters as well as small craft
advisory strength westerlies in the central/east strait each
afternoon and evening. A couple of systems passing to the north of
the area late in the week could increase the onshore flow enough to
produce westerly gales in the central/east strait.

Seas will hold around 6 to 8 ft through Friday before increasing 8
to 10 ft by Friday into the weekend.

27/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...Small Craft Advisory until 3 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to midnight PDT
     tonight for Admiralty Inlet.

&&

$$