Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 262237 CCA
AFDSEW

Area Forecast Discussion...corrected
National Weather Service Seattle WA
306 PM PDT Fri May 26 2017

Corrected the 2nd paragraph in the long term section regarding the
marine push.

.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 a little
toward mid next week. Moist south flow aloft could bring a few
sprinkles to the area from Monday onward.

&&

.SHORT TERM...Afternoon visible satellite imagery shows clear
skies over the W WA lowlands with just scattered cumulus clouds
over the mountains.

The strong upper level ridge centered offshore along 130W will
move slowly inland over W WA on Saturday and will continue to
build slowly through Sunday. The ridge will slide slowly E over E
WA Sunday afternoon but won`t make it over W MT until Tuesday
night.

With the ridge in place high temperatures will be well above
normal, in the mid 70s to mid 80s, but not within realistic reach
of record highs. This is probably because weak onshore flow will
continue along the coast and offshore flow never really cranks up
over the interior.

The GFS show some weak W-SW onshore flow Sunday night and Monday
night which could bring a shallow marine air mass part way inland,
but it will probably have little effect on inland temperatures.
The ECWMF is not as strong with this minor onshore flow.

Once the ridge axis moves over E WA on Sunday, the flow aloft will
switch to SSW over W WA. Isolated convection could develop along or
E of the Cascade crest Sunday and Monday afternoon per the GFS. The
other models are much weaker with this. In addition, areas of mid
and high level moisture will move up over the area from Oregon and N
CA from Monday onward. This could cut back a little on warming on
Monday.

The current forecast has Sunday as the warmest day with highs in
the 70s to mid 80s. Monday may be a little cooler due to the weak
low level onshore flow and the minor increase in clouds over the
area. However, confidence is low, and Monday could easily be the
warmer day. Kam

.LONG TERM...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 pressure centered just offshore will
shift eastward directly over Washington on Saturday. Light northerly
flow aloft becoming southerly Saturday afternoon. Weak low level
onshore flow. VFR skies, except isolated IFR stratus or fog near the
coast late tonight/early Sunday morning.

KSEA...VFR skies. Northwesterly winds 4-7 kt, becoming northeast 3-5
kt by midnight. dtm

&&

.MARINE...Light onshore flow with lower pressure east of the
Cascades and surface high pressure offshore. Diurnal westerly winds
may reach up to 20 to 25 kt this evening in the central/eastern
Strait before easing. Slightly stronger onshore flow may develop by
Sunday with more solid small craft winds possible in the Strait. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.

PZ...Small Craft Advisory from 6 PM this evening to 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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