Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222151
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
251 PM PDT Thu Jun 22 2017

.SYNOPSIS...A building upper ridge offshore will move into Western
Washington by Sunday then east of the Cascades early next week.
This ridge, combined with low level offshore flow, will provide
dry weather and a warming trend through the weekend. Onshore flow
will develop Sunday night or Monday ushering some cooler marine
air into the area. Dry weather will persist through the middle of
next week, but night and morning clouds will return Monday or
Tuesday.

&&

.SHORT TERM...A building ridge along 135W combined with developing
northerly flow is resulting in clear skies across the region. Low
level offshore flow is resulting in the development of a low level
thermal trough over Northern California and Southern Oregon. This
trough will develop northward along the Oregon and Washington
coasts Friday through Saturday, then shift into the interior of
Western Washington Sunday afternoon as offshore flow increases
northward and the upper ridge moves across the area.

Northwest to north flow today has kept the air mass rather cool
after this morning`s chilly start. Good radiational cooling and a
dry air mass will result in another chilly night tonight
(especially in outlying valleys). Significant warming will begin
on Friday as northerly pressure gradients gradually shift to
northeasterly. High temperatures on Friday will be in the 70s,
except lower to mid 80s several miles east of Puget Sound and in
the interior south of Seattle.

Warming will really ramp up on Saturday as adiabatic compression
does its job at lower levels and temperatures aloft warm with the
incoming upper ridge. Offshore flow will be aided by the
development of a thermally induced trough to the south and
southwest of the area. Temperatures will rise well into the 80s
most areas with some lower 90s seen inland from the beaches on the
north coast, a few miles east of Puget Sound, and in the interior
south of Seattle.

Continued offshore flow Saturday night will result in a warm night
across much of the area, especially in the Seattle area and in the
foothills of the Cascades and Olympics. Lows will be in the 50s,
with some portions of Seattle possibly staying near 60.

Sunday will be the hottest day of this mini heat wave as the
thermal trough shifts into the Western Washington interior. With
850 mb temperatures rising to 21C in Seattle, the forecast high
of 92 at Seatac airport should be easy to hit. From Shelton and
Tacoma southward along the I-5 corridor and westward through the
Chehalis Gap, many locations will see mid 90s. The coast will see
its maximum temperature earlier in the afternoon as pressure
gradients transition to onshore, so highs there will be a bit
cooler than on Saturday.

Models are generally consistent with previous solutions, and only
minor updates were made to the forecast. Albrecht

.LONG TERM...The long-wave upper ridge axis shifts east of the
area Sunday night and Monday. Many models are rather robust in
shifting pressure gradients onshore as the ridge moves eastward
Sunday evening, resulting in significant cooling on Monday.
However, there is the hint of a short wave ridge offshore at the
mid levels that would only result in a partial marine push Sunday
night, with a more significant one Monday evening. Our current
forecast reflects the latter scenario and results in cooling for
Monday, but still warm in the Puget Sound area and near the
Cascades.

After a stronger marine push on Monday, a weak trough of low
pressure aloft combined with moderate onshore flow will result in
night and morning clouds and temperatures near to slightly below
average Tuesday onward. Albrecht

&&

.AVIATION...An upper level ridge will slowly move eastward this
afternoon and through this weekend...keeping northwesterly flow
aloft in place over the area. This will allow for continued dry and
stable conditions as well as persisting clear skies.

KSEA...Clear to mostly clear skies expected to remain over the
terminal for the entirety of the TAF forecast period. Winds
remaining northerly at 6-12 kts.  SMR

&&

.MARINE...A northeast-southwest oriented surface ridge will remain
over the Pacific through Saturday. Meanwhile, a strong heat low over
California and southern Oregon will set up northerly gradients over
the waters. Winds over the coastal waters look to increase back into
SCA territory by late this afternoon/early this evening...and as
such will be putting up a small craft advisory with the afternoon
forecast package. Gradients will weaken on Sunday, then onshore flow
will quickly increase on Sunday night. Haner/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Friday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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