Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 261623
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
923 AM 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 a little
toward mid next week. Moist south flow aloft could bring a few
sprinkles to the area from Monday onward.

&&

.SHORT TERM...Morning visible satellite imagery shows clear skies
over W WA. Pressure gradients are weakly offshore. With the strong
upper level ridge centered offshore along 130W still building over
the region, highs today will remain mostly in the 70s. There
could be a few spots that break 80, but the rest of the area will
have to wait for this weekend.

The axis of the offshore ridge will move over W WA Saturday
morning while drifting every so slowly E. The axis won`t reach the
ID border until Sunday night. Pressure gradients across the area
will remain weak Saturday and Sunday. Low level flow through the
interior will be northerly and the coast will see a little weak
onshore flow in the afternoon. So, with the low level flow not
really going offshore, afternoon highs will not get excessively
hot, but will stay in the 70s to mid 80s.

The GFS is showing slightly stronger onshore flow Sunday night
which might push a some shallow marine air part way inland, but
not really far enough to reach the interior.

The GFS suggests there will be a little weak convection
developing over the Cascades Sunday afternoon, but the NAM has no
convection developing and the ECMWF barely has any. Kam

.LONG TERM...The upper level ridge axis is still hanging over the
ID border on Monday and doesn`t shift over W MT until Tuesday
night. The shift of the thermally induced surface trough is tied
to the movement of the ridge axis, which makes it a good
indicator of when a marine push will develop. The 06Z GFS was
more in line with the ECMWF. The present thinking is that there
will be some minor onshore flow both Sunday and Monday nights, but
not enough to qualify as a full blown marine push. Substantial
cooling over the interior will have to wait until the marine push
really develops Tuesday night.

With the ridge axis over E WA, S to SSW flow aloft will prevail
over W WA. It looks like there will be some higher level moisture
embedded within the flow that drifts up over W WA Monday and
Tuesday. The moisture will probably be debris from convection over
the Oregon Cascades and Siskiyou`s and could contain some light
showers. Kam

&&

.AVIATION...A ridge of high pres will prevail over the region
through Saturday. Light northerly flow aloft becoming southerly
Saturday afternoon. Weak low level onshore flow. VFR skies today.

KSEA...VFR. Winds will be light north-northwesterly, up to 8 kt
after 18z. dtm

&&

.MARINE...Light onshore flow with lower pressure east of the
Cascades and surface high pressure offshore. Typical diurnal
westerly winds reaching low-end small craft strength will develop
again this evening in the central/eastern Strait. 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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