Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 122221
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 PM PDT Sat Aug 12 2017

.SYNOPSIS...Mid and high level clouds will increase tonight
ahead of a weak cold front that will move onto the WA coast this
evening and into the interior after midnight. This system will bring
the first widespread rain to the area in nearly two months. The
overall pattern for the next week will return a more typical regime
of night/morning clouds with afternoon sunshine and near normal
temperatures.

&&

.SHORT TERM...Morning clouds have cleared back to the coast this
afternoon, with just a few lingering showers along the crest of the
Northern Washington Cascades this afternoon. Satellite and radar
imagery showing precipitation and clouds associated with the
approaching weak cold front beginning to move into the outer coastal
waters. The forecast remains on track with rain spreading onto the
north WA coast by around 02 UTC and into the interior after
06 UTC. This system will bring the first widespread rainfall to
the area in nearly two months with 0.25 to 0.5 inches of rain
expected along the coast and 0.05 to 0.25 inches over parts of the
interior. Rain will transition to showers from the west early
Sunday morning with the stronger onshore flow behind the front
helping to drive a Puget Sound convergence zone which may persist
into Sunday evening. The convergence zone will boost clouds and
rainfall into the afternoon hours, especially over the Central
Puget Sound. Most other locations will likely see some sunbreaks
during the afternoon. In addition to the return of precipitation to
the area, this system will further improve air quality around the
region. High temperatures will remain on the cool side Sunday,
reaching into the 60s to near 70 over parts of the interior.

Flow aloft will become dry and northwesterly into Monday as the
upper level trough associated with Sundays system shifts east to the
east and an upper level ridge settles over the East Pacific. Low
level flow will remain onshore, resulting in widespread morning low
clouds with afternoon clearing to partly to mostly sunny skies. High
temperatures Monday will return to near normal values in the low to
mid 70s over the interior and mid to upper 60s along the coast=.

.LONG TERM...The upper level ridge over the East Pacific will
remain in place through midweek with dry northwesterly flow aloft
and onshore flow at low levels. This is a pretty typical pattern for
night/morning clouds and afternoon clearing which will be reflected
by near normal temperatures through Wednesday. The medium range
models remain consistent in showing a system shifting into the Gulf
of Alaska midweek that will help nudge the upper level ridge onshore
Wednesday evening into Thursday. The building heights Thursday into
Friday as the ridge shifts onshore will result in warming aloft and
some reduction in the onshore flow which will equate to a slight
warming trend late in the week. Otherwise expect pretty typical
conditions for mid August.

&&

.AVIATION...A Pacific cold front approaching the region will move
across the area tonight. The trailing upper level trough will
move across W WA on Sunday afternoon then be over E WA on Monday.
Flow aloft will be SW as the front crosses the area, shifting to
W-NW behind the front Sunday afternoon through Monday.

Light rain should reach the coast by 03Z and then spread across
the interior 06Z-12Z tonight. Expect MVFR cigs to develop with
and behind the front and then continue into Sunday afternoon
before lifting to low end VFR cigs BKN030-050.

A convergence zone is expected to form over Puget Sound behind the
front, between 18Z-21Z then fade out by 06Z as it moves slowly
south from KPAE down to past KSEA.

KSEA...Good VFR conditions are expected up to about 05Z tonight,
when a cold front will be approaching the terminal. The front
should cross KSEA around 15Z. Light rain is possible from 06Z-
15Z. CIGS are expected to drop to MVFR around 06Z then continue
into early Sunday afternoon. A convergence zone is expected to
form and could impact the terminal 21Z-06Z. Surface winds will
remain S-SW 5-15 KT. The convergence zone could cause a wind shift
between 21Z-00Z Sunday afternoon. Kam

&&

.MARINE...The surface ridge along the WA coast this afternoon will
weaken this evening as a Pacific cold front approaches the coast. The
cold front will reach the outer coastal waters this evening then
move inland late tonight.

A gale warning remains in effect for the central and east Strait
of Juan De Fuca late this afternoon and this evening. Model
guidance remains divided on whether a gale will actually develop
tonight, but the GFS is indicating a UIL-BLI pressure gradient of
3.1 MB at 00Z so there is certainly a chance for a gale. The
winds should be strongest during the early evening since ridge
will weaken rapidly overnight as the cold front approaches. This
event looks like it will be weaker than last night and weaken
faster during the evening, with gales probably not making it as
far east as Smith Island in the East entrance. W winds over the
central strait are still in the low end SCA range this afternoon.
The SCA will continue until 00Z/5 PM when the gale takes effect.

Another surface ridge will rebuild over the WA offshore waters
behind the cold front Sunday afternoon. The ridge will remain
offshore through mid-week although the strength of the ridge and the
resulting onshore flow over W WA will vary from day to day. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 5 PM this afternoon to 2 AM PDT Sunday 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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