Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 270418
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
918 PM PDT Fri May 26 2017

.SYNOPSIS...A strong upper level ridge will remain over the
Pacific Northwest through the Memorial Day weekend, with sunny
skies and afternoon high temperatures well above normal.
Increasing onshore flow will finally cool temperatures starting
Monday night or Tuesday. Moist south flow aloft could bring a few
sprinkles to the area from Monday onward.

&&

.SHORT TERM...With the exception to just a few clouds in the
Olympics and Cascades, skies across the area are clear this
evening. Stratus has developed over the outer portions of the
coastal waters, and some stratus has moved onto the Grays Harbor
County beaches.

A strong upper level ridge is centered just outside of 130W this
evening. This ridge will move slowly eastward into Western
Washington on Saturday then will shift east of the Cascades later
Sunday into Monday. The problem today concerns the speed of the
eastward progress of the ridge with the new 00z runs appearing
faster for a earlier push while the 12Z ECMWF was rather slow with
the progression of the ridge. At this time it appears that the
best course of action is to only make minor adjustments through
the Saturday part of the forecast with high temperatures 2-4
degrees warmer than today. Low temperatures will fall into the
50s, with typically colder river valleys falling into the upper
40s and warmer urban areas staying in the mid and upper 50s.
Temperatures on Sunday are indicated to be another 2-4 degrees
warmer than Saturday; this is based on a combination of previous
GFS solutions and the ECMWF.

As the ridge moves off to the east of the Sunday into Memorial
Day, expect low level pressure gradients to increasingly become
onshore. Also, expect an increase in mid and upper level moisture
on Memorial Day. Temperatures from around the Puget Sound westward
should see at least a few degrees cooler than Sunday. Of course,
a faster push per the 00Z runs would result in another 5 deg
cooling on Monday. Confidence in the timing and strength of a
push is low.

A quick update was made this evening to increase clouds on the
beaches tonight and to raise maximum temperatures a degree or two
on Saturday. Otherwise the short term forecast is okay for now.
Albrecht

.LONG TERM...From the previous long term discussion: The GFS
shows a larger batch of clouds moving up over the area on Tuesday,
enough to have a slight chance of showers in the forecast. The
combination of the cloud cover, the ridge sliding E, and another
minor shot of onshore flow could drop Tuesday`s highs back into
the 70s.

With the ridge axis finally shifting over W MT Tuesday night and
an upper level shortwave trough approaching the coast, models are in
reasonable agreement that Tuesday afternoon will probably be the day
when the main marine push is triggered. This should spread a cooler
marine air mass across the interior with widespread low clouds on
Wednesday. Highs should drop back to the mid 60s to near 70 which is
just slightly above normal.

An upper level trough will remain over the area Thursday and
Friday for normal weather. There could be some light showers at
times from a passing shortwave trough. Kam

&&

.AVIATION...A ridge of high pres will remain over the region. Light
nly flow aloft will become variable. The low level flow will remain
weak onshore. Areas of LIFR CIGs/VSBYs are expected over the coast
after 0600 UTC.

KSEA...VFR with light nly winds.

&&

.MARINE...High pressure offshore with lower pressure inland will
result in light onshore flow through this weekend. Expect the
onshore flow to strengthen on Monday for the possibility of gale
force winds over parts of the Strait of Juan de Fuca.

&&

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

&&

$$

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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