189
FXUS66 KSEW 301644
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
944 AM PDT Sun Jun 30 2024

.SYNOPSIS...Weak upper level trough over Western Washington today
with a convergence zone over mainly Snohomish county. Trough moves
east tonight and convergence zone dissipates Monday morning. Upper
level ridge building offshore Monday will slowly move east for the
remainder of the week ending up near Western Washington by late in
the week.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Weak upper level trough
over Western Washington today. Convergence zone developing over
Snohomish county this morning with the convergence zone continuing
into the afternoon hours. Main story today will be the cloud
cover. With the weak trough will have a slight chance of showers
for most of the lowlands in the morning. By afternoon, shower
chances become confined to the convergence zone and the Cascades.
Air mass unstable enough over the North Cascades for a slight
chance of a thunderstorm this afternoon. Highs today in the mid
60s along the coast and upper 60s to mid 70s inland.

Upper level trough moving east tonight but the weak convergence
zone will remain intact into the early morning hours. Showers
chances pretty much confined to the convergence zone into early
Monday morning. Some clearing early but with increasing low level
onshore flow cloud cover increasing again overnight. Lows in the
50s.

Upper level ridge building offshore Monday. Weak shortwave
rippling down the back side of the ridge nicking the Northern
portion of the Cascades keeping a chance of showers in the
forecast for that location. For the remainder of the area morning
clouds will give way to afternoon sunshine. Highs in the mid 60s
coast and upper 60s to mid 70s inland.

Upper level ridge slowly starting to move east Monday night into
Tuesday. Low level onshore flow continuing through Tuesday keeping
the late night and morning clouds with afternoon sunshine forecast
going through Tuesday. With the onshore flow highs Tuesday will be
similar to Monday, mid 60s coast and upper 60s to mid 70s inland.
Lows Tuesday morning in the 50s.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Extended models in good
agreement with the upper level ridge continuing to move east
Wednesday. The ECMWF brings the ridge over Western Washington
Thursday with the ridge remaining over the area into Saturday. The
GFS keeps the ridge offshore Thursday through Saturday. Both
models do not turn the low level flow offshore going to more flat
gradients beginning Thursday. Slow warming trend through the
period with highs Wednesday in the mid 60s along the coast and 70s
inland warming to the mid 70s to lower 80s along the coast and
upper 70s to near 90 inland Saturday. Ensembles show a big spread
in the solutions for the high temperature Saturday. Model blend
puts highs in the mid 70s along the coast and mid 70s to upper 80s
inland. Ensemble means off both the GFS and ECMWF were a few
degrees warmer then the blend. Given the possible HeatRisk issues
have gone with the warmer ensemble mean highs for Saturday. Felton

&&

.AVIATION...An upper level trough is currently centered over western
Washington this morning, with a surface low west over the Pacific
Ocean, and east of the Cascades. Very moist air has worked its way
into the region this morning (with high 50 to low 60 dew points).
Low CIGs and VIS have been observed across several terminals
(primarily up and down Puget Sound with IFR from Arlington to
Puyallup, and east into the Cascades - also LIFR at KPAE where dense
fog has been observed). As temperatures increase, CIGs will increase
to MVFR in Puget Sound areas mid to late Sunday morning, and VFR
late Sunday afternoon/early evening (may take longer at KPAE to
clear). The moist air/post frontal convergence zone may produce
shower activity in Snohomish County and vicinity this afternoon.
Very slight chance of a thunderstorm, but the best chance will be
towards the Cascades. Surface winds out of the southwest this
morning at 6 to 10 kt (gusts to 15 kt possible), becoming variably
northwest in convergence zone areas this afternoon along
Snohomish/King County. Chance for MVFR as well Monday morning in
Puget Sound, and IFR along the coast over to the Olympics.

KSEA...IFR CIGs this morning with moist air in place, will improve
to MVFR late Sunday morning and scatter out late Sunday
afternoon/evening. Convergence zone activity will stay north of the
terminal, however variable winds may drift close or at the terminal
Sunday evening. Otherwise winds will remain southwesterly at 6 to 10
kt (potential for gusts to 15 kt late this morning and afternoon).
MVFR possible again Monday morning.

HPR

&&

.MARINE...An upper level trough remains over western Washington this
morning, with high pressure beginning to build over the Pacific.
Significant moisture has lead to low cloud coverage and areas of fog
this morning over some of the waterways - especially in the east
Strait of Juan de Fuca and Puget Sound. This will improve late this
morning as the trough begins to depart the region. A slight chance
for convergence zone showers in and near Snohomish County exists for
this afternoon (with the very low risk of a thunderstorm within).
This activity will likely initiate over the Admiralty Inlet and move
over Puget Sound/Northern Inlet Waterways.

Onshore flow will begin to build behind the trough. This will lead
to gusty westerly winds of 15 to 25 kt in the central/east Strait of
Juan de Fuca (small craft advisory from 00Z-10Z Monday). A stronger
push is likely in the same area Monday afternoon and evening,
potentially increasing from 25 to 25 kt (gale watch in effect 00Z-
10Z Tuesday). Gusty northwesterly winds of 15 to 25 kt are also
expected in the outer coastal waters Tuesday into Wednesday.

Seas of 4 to 6 feet will increase to 6 to 10 feet Tuesday into
Wednesday. Shorter periods on Wednesday down to 9 seconds may lead
to choppy seas.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 3 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Monday afternoon through late Monday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$