276
FXUS66 KSEW 150937
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
237 AM PDT Thu Aug 15 2024

.SYNOPSIS...Weak troughing over the region will maintain cooler
and partly cloudy conditions through the rest of the week. A low
will deepen along the coast through the weekend, increasing the
threat of thunderstorms across western Washington on Saturday and
Sunday. Unsettled conditions will persist into next week with a
continuation of cooler temperatures and chances of showers.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...A weak low pressure system
will continue to push inland across Oregon and lift northeastward
throughout the day today, maintaining cloudy conditions and below
normal temperatures in the mid 60s to mid 70s across western
Washington. A wave of lofted smoke will be advected northward into
the region, but is expected to stay above the surface, resulting
in little more than hazy skies. As the low exits the state, wrap
around moisture will generate isolated showers over the Cascades
later this afternoon and evening. While confidence is low, a
lightning strike or two cannot be ruled out on the western side of
the Cascade crest.

The weakening low will move northward into British Columbia by
Friday, maintaining a chance of showers and isolated thunderstorms
over the North Cascades. Elsewhere, weak onshore flow will
maintain cloudy skies and cooler temperatures with highs across
the lowlands peaking once again in the low to mid 70s.

A low pressure system will deepen off the Pacific Northwest coast
on Saturday, which will bring the next round of weather to western
Washington. Moisture will surge inland Saturday afternoon and
evening, generating widespread showers and thunderstorm activity
that will linger into early Sunday. Instability will increase as a
frontal system lifts northward across the state later on Saturday,
with probabilistic guidance showing a 15% to 30% chance of
thunderstorm activity into the evening hours. Nocturnal thunder
also cannot be ruled out with favorable dynamics in place. Latest
ensemble guidance suggests roughly half an inch of rainfall across
western Washington over the weekend, though this will be highly
dependent on where showers set up and where any heavier showers
and thunderstorms develop. While forecast models continue to hone
in on thunderstorm potential for the weekend, ensembles still
show some uncertainty of the track of the upper level low, which
could lead to differences in rainfall totals, timing, and
thunderstorm chances. As for now, the threat for storms continues
to be monitored as new guidance becomes available.

.LONG TERM /MONDAY THROUGH WEDNESDAY/...Forecast models continue
to favor troughing into next week, maintaining cooler temperatures
and chances for lighter shower activity across the region.
Temperatures are on track to stay several degrees below normal
through the period, giving us a preview of early fall-like
conditions.

Lindeman

&&

.AVIATION...A weakening block of high pressure will move east as an
upper level low and surface low/cold front push through the region
Thursday. Flow aloft will be weak during the transition between high
and low. The onshore flow along with the front/low will bring cloud
coverage into much of the region early Thursday morning, ranging
from MVFR to IFR CIGs via pushes through the Strait of Juan de Fuca,
and through the Southwest Interior. These pushes will bring stratus
into inland areas (from Shoreline to Alger, and as far east as the
West Slopes of the Cascades in the north, and from Chehalis up to
Bethel/Kent). Lower LIFR is possible closer to the coastlines. The
ceilings may not lift until after 19-22Z, but will improve to VFR
during and after this timeframe (except the coastline which will
improve to MVFR). Winds will become primarily southwesterly at 6 to
10 kt (higher gusts to 15 kt or higher near the Strait of Juan de
Fuca and in the Southwest Interior between Shelton and Aberdeen).

KSEA...Conditions remain VFR with light north winds as of early
Thursday morning. As the front approaches, winds will calm and turn
southwesterly at 6 to 10 kt after 12Z. Expect marine stratus to
arrive after 12Z. Confidence remains highest in MVFR CIG coverage
from 15Z to 21Z, but there remains a 20% chance of IFR CIGs making
it into KSEA from the south (this will be very brief if it lowers to
IFR).

HPR

&&

.MARINE...A weak cold front will move through the region early this
morning. While remaining dry across the region Thursday, areas of
low stratus and patchy fog have been observed in the coastal waters,
and onshore flow will push some of the stratus through the Strait of
Juan de Fuca this morning, covering portions of Puget Sound,
Admiralty Inlet, and the Northern Inland Waters. The low stratus/fog
may reduce outward visibilities at times through Thursday afternoon.

Gusty winds up to 25 kt are expected with this push through the
Strait of Juan de Fuca today, which will be hazardous for small
craft (thus an advisory was issued Thursday afternoon through Friday
morning for Central & East Strait of Juan de Fuca). Winds over the
coastal waters will remain light and turn northwesterly Thursday
afternoon into the weekend. Winds will also be light during this
time out of the south in all inner waterways (with west winds in the
Strait of Juan de Fuca). There is a slight chance for thunderstorms
Saturday afternoon through Sunday morning for inner waters as a
strong upper level low moves in. Threat for gusty sustained winds
remains low with this system (except for possible gusts from
thunderstorms).

Seas will generally remain 3 to 5 feet through next week.

HPR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 AM this morning to 5 AM PDT Friday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$