Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251550

Area Forecast Discussion
National Weather Service Seattle WA
850 AM PDT Tue Jun 25 2019

.SYNOPSIS...An upper level low will approach the area through
today and move over the Pacific Northwest on Wednesday. The low
will be over the area through Friday. The low will slide east on
Friday into Saturday with a weak trough over the area on Sunday.


.SHORT TERM /TODAY THROUGH THURSDAY/...Satellite imagery shows a
swath of mid to high level clouds sliding north across W WA this
morning. An upper level low located near 48N/136W will continue to
slide southeast through today with the low positioned offshore of
the OR/WA coast by late this afternoon. Expect a mix of clouds
and sunshine through this afternoon with increasing clouds with
the next system on approach. A few showers may develop, especially
across the mountains and southern portions of the CWA, late this
afternoon with the destabilizing airmass. Temperatures will be in
the mid 60s along the coast with low to mid 70s across the

The upper level low will slide east tonight and be positioned
along the OR coast by Wednesday afternoon. Flow aloft will become
more south to southeasterly. Increasing instability on Wednesday
with daytime heating will increase POPs for Wednesday afternoon.
Models soundings show lifted indices in the Cascades varying from
the -1 to -5 range with CAPE values near 1000 J/kg, this would
suggest at least the chance for afternoon thunderstorms across the
Cascades. Upper level winds from 700mb to 500mb will generally be
south to southeasterly but only around 150-160 degrees, so will
keep thunderstorms in the Cascades. If winds trend more
southeasterly, at least a slight chance of thunderstorms will be
possible in the foothills. Some showers will otherwise be around
the interior Wednesday with high temperatures in the 60s to low
70s in spots.

The upper level low will slowly slide east Wednesday night into
Thursday but continue to be over the Pac NW. Showers will continue
to be around. Thursday will be the coolest day of the week with
the upper level low overhead with highs in the low to mid 60s
across the area. JD

.LONG TERM /FRIDAY THROUGH MONDAY/...Extended models have a little
bit different solutions but the same weather over Western
Washington. The ECMWF keeps the upper level low in the vicinity
through the extended period with the low weakening over the
weekend into Monday. The GFS kicks the low off to the east on
Saturday with a weak upper level trough coming in behind the low
for Sunday and then a weak upper level ridge trying to build on
Monday. Both solutions keep the threat of showers over the area
Friday and Saturday with a drying trend for Sunday and Monday.
Highs only in the 60s Friday and Saturday. Low level flow
remaining onshore Sunday and Monday so highs just a little
warmer, in the 60s to lower 70s perhaps a few mid 70s by Monday.


.AVIATION...Southwest flow aloft becomes southerly late today as
an upper trough digs southward over the offshore waters. Moisture
will be increasing in the mid and upper levels through the day. The
air mass will become marginally unstable this afternoon with a
chance for a thunderstorm mainly over the mountains. The air mass
will continue to moisten overnight, but MVFR ceilings look to be
confined to areas near the coast on Wednesday morning. VFR ceilings
expected for the remainder of the region through midday Wednesday.

KSEA...VFR with increasing high and mid level moisture. Surface
winds light and variable this morning becoming W-NW less than 10
knots this afternoon. Wind backing to light southerly after 06Z
tonight.   27


.MARINE...An upper level low will move into the offshore coastal
waters and will linger in the region throughout the week. This
will maintain onshore flow across the area over the next several
days. Flow will be strong enough to produce small craft strength
westerlies along the central and eastern Strait of Juan de Fuca
during most afternoons and evenings. Other waters will remain below
advisory criteria. 14


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


PZ...Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.


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