628
FXUS66 KSEW 231056
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
356 AM PDT Mon Sep 23 2024

.SYNOPSIS...Upper level ridge building over Western Washington
today pushing weak system near the Canadian border further north
this morning. Ridge continuing to build tonight before shifting
east Tuesday. Thermally induced surface trough moving north along
the Oregon coast today reaching the Washington coast late tonight
with the low level flow going briefly offshore. The thermally
induced trough will move inland Tuesday. With the ridge well to
the east door open for front to arrive Wednesday with another
frontal system Thursday into Friday. Drying trend this weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
cloudy skies over Western Washington. None of the surface
observations reporting rain but doppler radar has some echoes
near the Eastern Strait of Juan de Fuca and the San Juan Islands.
With cloud ceilings 2000 to 3000 feet some of this precipitation
is reaching the ground. Due to the cloud cover and being in the
warm sector temperatures at 3 am/10z were mild, in the mid 50s to
lower 60s.

Upper level ridge building over Western Washington today. This
will push the weak front near the Canadian border north ending the
chance of showers for the North Coast, Strait of Juan de Fuca and
the Northwest Interior this morning. By 00z Tuesday 500 mb
heights in the mid 580 dms. Surface gradients northwesterly this
afternoon which will put a little damper on the highs. Increasing
sunshine will get highs to near normal, mid 60s to mid 70s.

Thermally induced surface trough moving up the Oregon coast today
will reach the Washington coast late tonight wit with the low
level flow turning briefly offshore. The low level offshore flow
will keep temperatures elevated overnight with lows only in the
mid 50s to lower 60s. Some places could set record high minimum
temperatures ( the Seattle record is 60 set in 2011 ).

Upper level ridge shifting east Tuesday with thermally induced
trough moving inland. Tuesday the warmest day of the week and
could be the warmest day until next spring. Highs in the 70s with
the Southwest Interior reaching the lower 80s. Warmest location
likely to be the Cascades foothills where mid 80s are possible.

Upper level ridge continuing to move east Tuesday night with the
thermally induced surface trough now east of the Cascades. Frontal
system approaching overnight spreading clouds over the area making
for another mild night with lows in the mid and upper 50s. Like
Tuesday morning record high minimums are possible. Rain out ahead
of the front reaching the North Coast after midnight with a chance
of rain mainly west of Puget Sound and over the Northwest
Interior.

Front moving through Western Washington Wednesday with rain across
the area. Highs in the lower 60s.

Front east of the Cascades by early Wednesday evening. Stability
indexes not that impressive with lifted indexes above 0, lapse
rates 5 to 6C and CAPE for the most part less than 200 J/KG. A
little extra lift from the Cascades and a convergence zone over
King and Snohomish county could add enough lift to produce a
thunderstorm Wednesday evening.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Extended models continue to
show good run to run consistency. Next system right on the heels
of Wednesday`s front with a warm front creating a chance of rain
Thursday ( looks like it will mostly go by to the north ).
Trailing cold front arriving Friday morning with rain out ahead of
the front Thursday night. Post frontal showers Friday afternoon
into Friday night.

Weekend looks dry at this point with the models indicating some
weak trofiness over the area but little in the way of moisture to
create any shower activity. Temperatures will be on the cool side
with highs only in the lower to mid 60s. Felton

&&

.AVIATION...Northwesterly flow will persist aloft today, before
transitioning to more westerly overnight into Tuesday as an upper
level ridge over the region flattens. Surface winds generally
persisting out of the S/SW at 3-6 kt early this morning, increasing
to 4-8 kt by this afternoon. Winds then look to transition to the
N and persist at 3-7 kt by this evening.

A weak frontal system moving into British Columbia has continued
to bring some light shower activity to the area overnight, mainly
along the coast and for terminals north of KPAE. Cigs have
started to lower towards more widespread MVFR, though a few spots
have had cigs dip down to IFR. Expect widespread MVFR/IFR cigs to
persist through the morning hours and to lift towards 3000-4000
ft by the late afternoon. Cigs look to lower towards MVFR again
heading into Tuesday.

KSEA...MVFR cigs with winds persisting out of the S at 4-7 kt.
Probabilistic guidance shows a 25 percent chance of cigs dipping
down into IFR at times, primarily between 12-18Z. Cigs will
gradually lift and scatter towards VFR again by the late afternoon.
Winds will transition to northerly at persist at 4-7 kt this
evening. 14

&&

.MARINE...A weak frontal system will continue to move into British
Columbia this morning. High pressure will then build back into the
coastal waters today into Tuesday as a thermally induced trough
expands northward along the Oregon coast. This will promote northerly
and weakly offshore flow tonight through Tuesday. A frontal system
will then move into the coastal waters on Wednesday, allowing for
flow to transition to southerly and back to onshore. Westerly
winds will increase and approach small craft criteria in the wake
of the front along the central and eastern Strait of Juan de Fuca
Wednesday night into Thursday. A stronger front then looks to move
into the region Thursday afternoon into Friday and will bring
more widespread small craft winds to the western Washington
waters.

Seas 4-7 ft will persist across the coastal waters today, before
building towards 7-9 ft Tuesday afternoon into Wednesday. Seas may
even briefly build to 10 ft across portions of the outer coastal
waters Tuesday night into early Wednesday. Seas will then build
towards 10-13 ft Thursday afternoon into Friday, with latest GEFS
probabilistic wave guidance indicating roughly a 50 to 70 percent
chance of seas building to greater than 12 ft over portions of
the coastal waters. 14

&&

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

$$