489
FXUS66 KSEW 202323
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
423 PM PDT Sat Jul 20 2024

.SYNOPSIS...Mostly dry and warm conditions will continue across
western Washington through the weekend. A weak shortwave bring
spotty showers to the Olympic and Cascade mountains on Sunday
along with the potential for elevated thunderstorms. An upper
level trough moving into British Columbia early next week will
cool temperatures closer to seasonal norms and increase night and
morning cloudiness.


&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...An upper-level ridge is
amplifying over the Inland Northwest and bringing the warmest
temperatures we`ll see for the foreseeable future this afternoon.
Upstream, an upper-low is spinning offshore. We`re right in
between these features under south-southwesterly flow aloft. For
tonight, overnight lows will drop into the 50s areawide but urban
heat island should keep the Seattle metro in the lower 60s.
Looming offshore, a shortwave ejecting out of the aforementioned
upper-low will bring increased chances for elevated convection,
possibly producing thunderstorm for the Olympic and Cascade
mountains.

As mentioned, a shortwave will bring the chance for elevated
convection on Sunday. There isn`t bountiful moisture but
instability is present as model derived mid-level lapse rates are
7-8 C/km. With orographic enhancement overlaid with diurnal
heating, the potential isn`t out of the question from being
realized. A dry slot in the lower levels should keep rainfall
from making it to the surface for lowland locations. Hi-res CAMs
have this activity pushing from west to east throughout day on
Sunday, possibly lingering late into Sunday night over the
Cascades. With dry fuels the concern for new fire starts must be
expressed (see the fire weather section down below). Going into
Monday and Tuesday, the pattern looks to flip. Stout low-level
onshore flow looks to set up bringing temperatures down to near
average for the first time it was has been weeks! High temps look to
generally be in the mid to upper 70s for the interior and low 60s
for the coast. Can`t rule out some lower 80s for the Cascade
valleys.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The broad upper level trough
will continue to influence our weather through the long term, with
periods of onshore flow, morning stratus, and temperatures near
season normals. Temperatures look to stay in the 70s for the
interior and the 60s for coastal locations. As of right now,
there looks to be nothing in the way of any meaningful
precipitation.

McMillian


&&

.AVIATION...A strong upper ridge axis over Washington will
continue to shift eastward on Sunday as an upper level trough and
a preceding shortwave approaches offshore. Southerly flow will
continue aloft over Western Washington tonight and through the
day Sunday. Satellite does show some lingering stratus over the
coastal waters this afternoon, but mostly clear skies across the
interior. Conditions at area terminals are widespread VFR for the
as a result. A marginally stronger onshore push tonight will to
allow for the marine layer to make it further inland Sunday
morning. Guidance has stratus making it just west of Puget Sound
by early the early Sunday morning hours. Terminals in stratus
will likely see a few hours of IFR conditions. Mid to high level
cloud cover will then filter overhead during the day Sunday as a
shortwave skirts the area. Northerly surface winds look to
transition to south/southwesterly Sunday morning, most likely
between 12-15Z.

KSEA...VFR conditions. Surface winds north to northeasterly,
persisting at 7-10 knots. Winds will become light overnight and
transition to the south/southwest between 12-15Z. Stratus looks to
stay west of the terminal Sunday morning. Mid to high level
clouds streaming overhead on Sunday.

&&

.MARINE...Broad surface ridging remains situated over the offshore
waters with lower pressure situated across the interior of western
Washington. Thermally induced lower pressure will shift eastward
tonight, allowing for a marine push to bring small craft advisory
westerlies to the central and eastern Strait of Juan de Fuca.
Onshore flow will increase on Sunday and could bring the potential
for small craft advisory strength northwesterlies across portions
of the coastal waters in addition to westerly gales for the
central and eastern Strait Sunday afternoon and evening. Onshore
flow will then continue through much of the next week as a low
pressure system gradually moves across British Columbia.

Seas 2-4 ft will build towards 3-6 ft on Sunday. Seas will then
subside and hover between 3-5 ft by late Monday and persist at
this range through much of the week.

&&


&&


.FIRE WEATHER...A Red Flag Warning has been issued for zones
652,661,658, and 659 which goes into the effect on Sunday. A weak
disturbance will bring a threat for scattered elevated thunderstorms
over receptive fuels. Precipitation should end by late Sunday night
into early Monday morning. Will continue to monitor the situation as
the conditions evolve. A cooler and more humid pattern develops
Monday as an upper level trough settles over the region.

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 5 AM to 6 PM PDT Sunday for East Portion
     of the Olympic Mountains-West Portion of the Olympic
     Mountains.

     Red Flag Warning from 8 AM Sunday to 5 AM PDT Monday for West
     Slopes of the Central Cascades Generally above 1500 Feet-
     West Slopes of the North Cascades Generally above 1500 Feet.

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

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

&&

$$