Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 122131
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
231 PM PDT Wed Aug 12 2020

.SYNOPSIS...High pressure will build into the area for the end of
the week into the weekend with much warmer temperatures and dry
weather.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Upper level trough is
departing off to the east this afternoon, leaving scattered clouds
and cooler temperatures than yesterday at this time. Cool down
will be short-lived as high pressure begins to build over the area
on Thursday and amplifies over the Desert Southwest into the
weekend. This will yield mostly clear skies by Friday and a rapid
warm-up in temperatures for the weekend. Highs on Thursday will
be in the 70s but the lowlands will be back into the 80s by
Saturday. A few spots south of Olympia may reach the 90 degree
mark. Heat risk will reach moderate levels for those areas on
Saturday so those who are sensitive to heat should begin to plan
ahead for a warm weekend.

CEO

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Next week is characterized
by a large amount of uncertainty regarding the ridge and its
subsequent break-down. Regardless, Sunday should be the peak of
the heat with temperatures well into the 80s to low 90s across the
lowlands. Areas south of Olympia and in the Cascade valleys may
see the mid to upper 90s. This will yield a moderate to high heat
risk. Overnight lows in the upper 50s to low 60s should provide a
bit of relief though.

Past Sunday, uncertainty grows as deterministic models and their
respective ensemble solutions are struggling with small-scale
features. This is largely in part to how they handle the remnants
of now Tropical Storm Elida as it rides north from it`s current
location west of the Baja Peninsula. Latest GFS runs surge some
monsoonal moisture north along the western periphery of the ridge
late Sunday into Tuesday for isolated showers (and maybe even a
thunderstorm or two along the Cascades) with cooler temperatures.
These solutions also swing an upper level trough through western
WA by Wednesday with a few showers and further cooling. ECMWF and
its ensembles are more bullish - holding onto the ridge for much
of next week with mostly dry weather and slightly warmer
temperatures than GFS/GEFS. For now, have stuck with a blended
approach which leads to cooler temperatures than the weekend and
periodic chances for showers, especially across the Cascades, into
mid-week.

CEO

&&

.AVIATION...Clouds are continuing to clear out throughout the
area. Some high level cirrus near KBLI and between KPAE and KOLM are
prolonging the dissipation. The nightly marine push of moist air is
expected to begin around 0800 UTC and with that clouds will be
moving into the area for the night and into tomorrow morning.
However, a ridge of high pressure is moving into the area so
ceilings are expected to be higher then the past few nights have
brought. VFR and MVFR conditions are expected at all airports
tonight and into Thursday morning. The lowest overcast ceilings are
should be around 1500 ft and only at airports near the coast.
Airports inland should experience higher ceilings of 5000 ft and
above. Thursday mornings clear out is expected to start around
1800UTC for areas inland and closer to 2000 UTC near the coast.
Winds are to be less than 10 knots for all terminals in the area.

KSEA...Conditions will continue to improve during the rest of this
afternoon and evening. Stratus will begin to move in around 0800 UTC
tonight. At their lowest ceilings are should be around 5000 ft and
occur around 1200 UTC. That being said, sky conditions are expected
to remain in VFR.

Butwin

&&

.MARINE...The daily push down the Strait is expected to begin around
0000 UTC and will be at its peak in the East Entrance Zone around
0600 UTC. Wind speeds will be range from 15 to 20 knots with gusts
close to 30 knots. As a result a small craft advisory has been
issued for this time period. Winds will decrease come 1200 UTC and
remain down until 0000 UTC Friday. Following that winds will begin
to increase with the next push down the Strait. Because of the high
pressure system also moving in wind speeds are expected to be closer
to 30 knots. Wind gusts are likely to be at Gale force. Northerly to
northwesterly flow will persist across the offshore waters through
Saturday before flow becomes offshore briefly on Sunday. Onshore
flow will resume late Sunday into Monday. Wind speeds will be less
than 15 knots in these offshore waters during this entire period.

Northwesterly swell will continue to diminish tonight and into
Thursday. Swells will be falling from 5 to 2 feet over the course of
the next three days. Swells will increase again come Sunday with
heights between 5 and 6 feet.

Butwin

&&

.FIRE WEATHER...Rapid-warm up expected this weekend with
temperatures well into the 80s and 90s by Sunday. Dry and unstable
conditions are expected to develop across the Cascades and lowland
areas south of Olympia which could lead to critical fire weather
conditions on Sunday. Minimum relative humidity values in those
areas will drop into in the teens to 20s with mid-level Haines
values of 6. Will continue to monitor for the possibility of any
headlines needed over the next few days. There is some uncertainty
regarding the weather pattern past Sunday but cooler conditions
are likely for Monday and Tuesday.

CEO

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 6 AM PDT Thursday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion