Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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793
FXUS66 KSEW 231018
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
318 AM PDT Wed Aug 23 2017

.SYNOPSIS...Low level onshore flow will bring clouds into the
interior this morning. An upper level trough offshore will move
through Western Washington on Thursday. A very weak system out
ahead of the trough will bring a slight chance of showers to the
area tonight. An upper level ridge will build quickly into the
region on Saturday with much above normal high temperatures
through Tuesday.

&&

.SHORT TERM...Satellite imagery shows stratus already as far east
as the Puget Sound at 0930z/230 am with the Southwest Interior
beginning to fill up with stratus as well. Stratus also coming
down the Strait of Juan de Fuca but the stratus there has not made
as much eastern progress as the southern branch of the stratus.
Temperatures are in a narrow range, mid 50s to lower 60s.

Decent marine push this morning with KUIL-KBLI gradient peaking
over plus 4 mb, KOTH-KSEA near plus 4 mb and KHQM-KSEA near plus
3 mb. Even the cross Cascade gradient is now up to plus 6 mb. With
onshore gradients this strong expect the stratus to get into the
Cascade foothills this morning south of the Strait of Juan de
Fuca. Model cross sections show the tops of the marine layer in
the 2000 foot range. Not thick enough to last the whole day over
the interior with the onshore gradients weakening a little. Even
after the stratus burns off plenty of high cloud cover so will
word the afternoon part of the forecast as partly sunny. With the
marine push highs will be much cooler today versus Sunday with
mid 60s to mid 70s common.

Upper level trough offshore approaching the Coast tonight. Weak
shortwave out ahead of the trough axis moving into Western
Washington tonight. Can`t rule out a light shower west of Puget
Sound tonight and over the entire area as the shortwave moves
through overnight. Lows will be in the 50s.

Upper level trough axis moving into Western Washington on
Thursday keeping a slight chance of showers in the forecast for
the morning hours. The main effect of the trough will be to
increase the mid and high level moisture over the area. Low level
onshore flow will increase behind the trough axis Thursday
afternoon. This will keep high temperatures below normal, mostly
in the 60s and lower 70s.

Not much going on Thursday night into Friday with zonal flow
aloft. Low level onshore flow becomes more northwesterly Thursday
night into Friday morning. This will reduce the stratus coverage
over the interior Friday morning with mostly sunny skies forecast
for the entire day. Stratus along the coast will burn off by
afternoon. Highs will be a little warmer on Friday, back into the
mid 60s to mid 70s.

.LONG TERM...Extended models are in good agreement with a strong
upper level ridge building quickly over the area from the
southeast on Saturday. Highs warming into the 70s to mid 80s. The
ridge will remain over the area Sunday and Monday before shifting
slightly east on Tuesday. 500 mb heights with the ridge near 590
dms. Low level flow turns offshore on Monday making it the warmest
day in the extended forecast. Warmest locations on Sunday will get
into the lower 90s with more 90s on Monday south of Seattle. No
records highs forecast at this time with the highs on Sunday and
Monday just a few degrees short of the records. With the ridge
shifting slightly east on Tuesday and the low level flow turning
onshore during the day will go for a little cooler high
temperatures. Felton

&&

.AVIATION...An upper trough approaching the region today will shift
across the waters later tonight and Thursday morning. Strong low
level onshore flow with light southwest flow aloft. Stable
conditions. Moist in the lower levels through Thursday morning and
somewhat moist aloft later today and tonight as the trough passes.

Low Mvfr or IFR status has spread from the coast to parts of the
southwest interior, Strait, and south Puget Sound early this
morning. Strong onshore gradients suggest widespread MVFR stratus
will continue to fill across the lowlands early this morning with
areas of IFR, especially at terminals with a few hundred feet
elevation. Cigs should lift to a VFR stratus most interior spots by
around 18z. Some partial scattering is possible later today then
MVFR develops again later tonight across the area.

KSEA...S-SW wind 3-6 kt becoming SW 4-8 kt. Winds shifting northerly
to 5 kt early this evening, then NE overnight. There is fairly high
confidence that stratus will move into KSEA around 11-12z, probably
with ceilings OVC008-012. Stratus should lift to a VFR deck by 18z
and could scatter at times during the afternoon. MVFR is likely to
develop again late tonight/early Thursday morning. dtm

&&

.MARINE...Strong onshore gradients will ease with solid Small
Craft winds in the central/east Strait falling below 20 kt during
the morning. After a lull through early afternoon, westerlies will
pick up again in the central/east Strait reaching gale force by late
today or early evening. Winds will reach small craft strength in
waters adjacent to the east entrance tonight.

Weaker onshore or northerly flow will prevail Friday through this
weekend, as high pressure remains off the coast with lower pressure
east of the Cascades. dtm

&&

.CLIMATE...The current streak of highs 70 or greater in Seattle is
now at 55 days. The record is 62 days from June 13 to August 13,
2015. Thursday is the best chance to break the streak. If Seattle
hits 70 or higher on Thursday looks like the record will be broken
a week from today on August 30th. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 8 AM PDT this morning for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 3 PM this afternoon to 5 AM PDT Thursday 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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