FXUS66 KSEW 041011
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
311 AM PDT Wed Aug 4 2021

.SYNOPSIS...A ridge will build north into the area today,
bringing some smoke aloft to eastern portions of the forecast area
today. A front will then approach on Thursday, eventually passing
through the area on Friday and bringing the chance for some light
rain. Another system off the Pacific could bring some light rain
to the area on Saturday, with conditions drying out again towards
the middle of next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Water vapor imagery early
this morning continues to show areas of troughing just off the
West Coast of the US; a strong area of closed low pressure was
noted over the Gulf of Alaska and nearby northern BC Coast, while
another trough was noted well offshore the California Coast. Just
inland over the PNW, a vigorous shortwave trough continues to
push east of the Cascade crest, while a ridge of high pressure
continues over the desert SW and deeper cyclonic flow remains over
the Eastern CONUS. At the surface, onshore flow remains dominant
over the PNW as an anticyclone remains anchored over the Pacific
and weak troughing continues over Eastern WA and OR. Farther
downstream, broad high pressure encompasses much of the middle of
the country, while a stationary boundary continues along the Gulf
Coast and SE Coast.

The aforementioned vigorous shortwave trough currently crossing
the Cascade crest was responsible for sufficient synoptic scale
lift that was able to work in tandem with peak daytime heating and
favorable orographic flow to spark a few thunderstorms yesterday
afternoon and evening. As of 3AM Wednesday, all remaining thunderstorms
were located well east of the local area, as the continued
eastward progression of the shortwave takes lift and moisture with
it. All is currently quiet across the CWA, and expect conditions
to remain benign as we approach daylight hours, with marine
stratus being the only feature of note. Marine stratus currently
encompasses the Coast, Strait, North Sound, and Lower Chehalis
Valley. Expect stratus to continue to expand eastward through
mid-morning, likely reaching the Everett and Olympia areas before
lifting and retreating back west into the afternoon.

The desert SW ridge will be building into the region through the
day today, helped in part by the eastward progression of the
offshore California trough. This should help raise temperatures a
few degrees this afternoon. With the ridge building into the area
from the south, flow aloft will also be southerly. This will help
pull some of the smoke from the California wildfires northward,
however, recent guidance is suggestive that onshore flow may
largely save most of the area from hazy skies. That being said, do
think areas south and east of Puget Sound will see some haze to
the skies today, so have opted to make mention in the forecast.
The smoke will remain aloft and will not impact AQ.

Overnight tonight and into Thursday, the ridge will be pushed east
as the California trough finally makes it to the CA Coast while
the strong Gulf of Alaska closed low mosies southward ever so
slightly. This slight southward progression of the closed low will
be enough to push a frontal boundary towards the area. The front
will remain well offshore through most of the day Thursday, but
slightly stronger onshore flow should help push the smoke east.
Thursday looks dry, with highs dropping back into the upper 70s
(60s along the Coast).

For Friday, what had looked like a promising wetting rain is
beginning to look a bit more moisture starved. The closed low
responsible for our frontal system is now expected to take a track
further north than previous model trends had indicated. The front
is still poised to push through the area on Friday and is still
likely to bring rain to many locations, but QPF amounts have
dropped quite noticeably. Current forecast now calls for anywhere
from a couple hundredths of an inch to perhaps a few tenths of
inch in the lowlands to around a quarter of an inch or so for the
higher terrain.

Kovacik


.LONG TERM /SATURDAY THROUGH TUESDAY/...To start the long term
period, the upper level pattern will feature broad cyclonic flow
over the Gulf of Alaska into the Western Canada (BC and Alberta),
with a stronger belt of westerly flow located to the south of this
flow, influencing the PNW. Current model guidance is suggestive of
this flow containing a subtle perturbation that may contain enough
moisture to bring a quick shot of rain to the area on Saturday.
Again, QPF amounts look generally light, but combined with any
rainfall from Friday it would certainly be helpful.

Ensemble guidance then appears to be in reasonable agreement with
deepening the trough into the PNW for Sunday, with the potential
for some showers to continue. Thereafter, the trough will slowly
migrate eastward as a strong ridge of high pressure develops just
upstream over the Pacific. It is possible this ridge will attempt
to build into the area towards the middle of next week, but that
is getting out a little beyond this current forecast period. If
current trends continue, residual troughing could keep some
showers in the forecast for Monday, with conditions drying out for
Tuesday.

Kovacik

&&

.AVIATION...Southwest to west flow over western WA today with an
upper level ridge over the West. Near the surface, onshore flow
continues with stratus clouds along the coast, south sound and
through the strait (IFR conditions expected). Low clouds will burn
back to the coast this afternoon for VFR conditions. 33

KSEA...VFR conditions expected today with N wind around 5 kt
increasing to 5-10 kt around 00z this afternoon. 33

&&

.MARINE...Onshore flow continues across western WA today with
strongest wind and waves through the central and eastern Strait of
Juan de Fuca. May see westerly gales through the strait Thursday
night with a stronger onshore push. A pair of weak frontal systems
will push inland Friday and Saturday. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Thursday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.

&&

$$

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