Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 270344
Area Forecast Discussion
National Weather Service Seattle WA
843 PM PDT TUE JUL 26 2016
.SYNOPSIS...Upper level ridging and low level onshore flow will
remain in place over the region through Friday. This pattern will
produce sunny and dry weather with above normal temperatures through
Friday. An upper level trough will bring cooler conditions for the
weekend with a slight chance of showers Sunday.
Low clouds remained pinned to the coast today with mostly sunny
skies through the interior with the exception of some limited
cumulus development along the Cascade Crest. The thunderstorms along
the East Slopes of the Cascades remained there this afternoon and
have all but died out this evening. Onshore flow will continue to
strengthen tonight before relaxing around daybreak. Expect the
marine stratus along the coast to work into the interior by morning,
likely just reaching into the Central Puget Sound. Expect much less
coverage over the interior than Tuesday morning, with an earlier
burn off. High pressure will continue to build over the region
Wednesday, which coupled less morning cloud cover will result in a
slight warmer day Wednesday. High temperatures will rise into the
low 70s along the coast and strait, mid to upper 70s in the north,
and low 80s from the central interior southward.
High pressure aloft will continue to build Wednesday night into
Thursday morning before the flow aloft becomes more zonal during the
afternoon and evening hours. Weaker onshore flow Thursday, will also
help bump high temperatures several degrees above normal reaching
into the mid 80s especially over the south interior. Conditions
will remain similar Friday.
.LONG TERM...from previous discussion...
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
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
.AVIATION...Light WNW flow aloft, the air mass is stable and dry
except for low level marine layer moisture along the coast and over
the coastal waters. The marine layer cloudiness will be shallower
and the areas of morning low clouds that push into the interior less
extensive for Wednesday than they were Tuesday morning.
KSEA...Clear skies and then just a few hours of low clouds are
expected Wednesday morning.
.MARINE...High pressure is over the offshore waters with low
pressure inland. The westerly pressure gradient in the Strait of
Juan de Fuca is only 2.1mb this evening, yesterday it peaked at
3.5mb--a small craft advisory is in effect there. The small craft
advisory for the outer coastal waters is delayed until Wed
afternoon, there were no ship obs out there today with a wind speed
greater than 15kt, but the models show the wind increasing.
The small craft for Admiralty inlet for this evening has been
cancelled, winds are quite a bit lighter than expected.
PZ...Small Craft Advisory central/eastern Strait Of Juan De
Fuca...Small Craft Advisory outer coastal waters starting at 3pm Wed.
An illustrated version of this forecast discussion can be seen at