Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 262231
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
331 PM PDT TUE JUL 26 2016

.SYNOPSIS...High pressure aloft will prevail over the region
through Friday. Onshore flow will also persist. Expect
temperatures to be above normal through Friday. Cooler conditions
are expected this weekend due to low pressure aloft settling over
the area.

&&

.SHORT TERM...
Low clouds lingered over the coast this afternoon while elsewhere
skies were mostly sunny. There were scattered thunderstorm just
east of the far north Cascade crest at this time. There is still a
possibility of an isolated thunderstorm developing over the north
Cascade crest this afternoon.

Meanwhile...High pressure aloft will prevail over the region
tonight through Friday. The flow aloft will become more zonal on
Friday. Onshore flow will persist but will be weaker Wednesday
and Thursday. This will allow max temperatures to climb several
degrees above normal. Since the flow will remain onshore during
the period, expect night and morning stratus mainly over the
coast.

Expect the low level onshore pressure gradient to strengthen
late Friday afternoon or early evening in response to an upper
level trof digging over British Columbia.

.LONG TERM...
Confidence in this period was not very high due to timing and
strength issues between the models concerning the upper level
trofs that will impact the region during this period. Have chosen
to lean toward the ECMWF solution since, overall, it has been
performing better during the extending period. Therefore, went
with idea of a weaker trof over the Pacific Northwest on Saturday.
The main impact this system will have on our weather will be
stronger onshore flow, a deeper marine layer, and cooler
conditions.

Anticipate an upper level trof to be over the region on Sunday.
Decided to throw in a mention of low PoPs in the coastal and
mountain zones as well as the area typically affected by a Puget
Sound Convergence Zone. A risk of showers does not appear outside
the realm of possibility.Otherwise, expect temps to be below
normal on Sunday.

Beyond Sunday, confidence in the forecast becomes lower. The
models were out of phase due to timing differences. Decided to
stick with the idea of near normal temperatures and continued
onshore flow.

&&

.AVIATION...A weak upper level trough over the area this afternoon
will be replaced by a flat upper ridge Wednesday through Wednesday
night. Light W to NW flow aloft. The air mass is stable and dry,
except for low level marine layer moisture along the coast and in
the coastal waters. The marine layer will be shallower tonight and
Wednesday as low level onshore flow relaxes and as subsidence
further dries and warms the low level air mass. This will reduce
the temporal and spatial extent of marine clouds Wednesday morning
in the interior. Interior terminal forecast ceilings will
generally be handled with a TEMPO group. Ceilings that do develop
Wednesday morning will average 1000-1500 feet. Albrecht

KSEA...Clear through much of tonight. Expect scattered to
occasionally broken stratus with ceilings around flight level 012
from 13Z through 17Z. Light winds will turn nwly 5-7 kt before 00Z
then will veer to NE after 05Z. Albrecht

&&

.MARINE...A ridge of high pressure over the offshore waters will
give moderate to strong northwesterly flow to the coastal waters
through Saturday. Expect small craft advisories over most of the
coastal waters, especially the outer ones, as a rather long fetch
of 15-30 kt northwesterly winds produces a rather low period
"square waves" that will build from 6 or 7 feet tonight to 10 to
12 feet by Saturday.

Inflow through the Strait of Juan de Fuca will give small craft
advisory west winds during the late afternoon and overnight hours
each day. Expect the strongest inflow to be this evening, then
again on Friday and Saturday night as upper level disturbances and
daytime heating in the interior force east to west pressure
gradients to tighten. Small craft advisory conditions are likely
in Admiralty Inlet this evening, then in the inland waters
adjacent to the central and eastern portions of the strait Friday
and Saturday evenings. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for the coastal waters
  from Cape Flattery to Cape Shoalwater from 10 to 60 Nm,
 central/eastern Strait Of Juan De Fuca.and the Admiralty
 Inlet.

&&

$$

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An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html


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