Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 121632
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
932 AM PDT Sat Aug 12 2017

.SYNOPSIS...A weather disturbance brushing the region this morning
will keep showers and possibly thunderstorms mainly over the
Cascades through midday. The onshore flow that brought abundant low
clouds to the area overnight will result in a cooler day today with
some clearing in the afternoon. A weak cold front will move onshore
late tonight into Sunday bringing the first widespread rain in
almost two months to the area. The pattern for the next week will
return a more typical regime of night/morning clouds with afternoon
sunshine and near normal temperatures.

&&

.SHORT TERM...Satellite and radar imagery this morning showing
showers with isolated thunderstorms moving north through the Central
Washington Cascades. Expect limited showers with the potential for
isolated lightning to remain over the Cascades through the morning,
then taper off as the upper level feature that is supporting them
lifts north and east of the area in the early afternoon. The strong
low leve onshore flow that has allowed marine stratus to push inland
overnight will keep highs in the mid 60s to low 70s this afternoon.
Stratus will scatter out over much of the interior mid afternoon to
partly sunny skies, but remain over the coast and western Strait of
Juan de Fuca.

The front approaching the region from the west remains on track to
move onshore early Sunday morning and weaken as it moves inland.
This will bring the first widespread rainfal 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. Stronger onshore
flow behind the front may support a central Puget Sound convergence
zone that will persist into Sunday evening, further boosting showers
there. 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 assocated with Sundays system shifts east into
the Intermountain region. Low level flow will remain onshore which
will result in widespread morning low clouds over the interior
clearing into the afternoon to mostly sunny skies over the interior
and partly sunny skies along the coast during the afternoon.

.LONG TERM...from previous discussion...An upper ridge will be
offshore on Tuesday with west-northwest flow aloft over the Pacific
Northwest and low level onshore flow into Western Washington. Latest
long range model solutions hint at passing ridging aloft midweek,
along with a reduction of onshore flow for some warming and maybe a
reduction in late night and morning cloudiness in the interior. A
weak front and an increase in onshore flow is possible as we move
toward the end of the week.

For now, the extended forecast was left unchanged. Later
forecasts may need to warm the middle of the week somewhat, but we
will wait for more consistency in model solutions before making
changes. Albrecht

&&

.AVIATION...An upper level shortwave trough over W WA at 16Z will
move over NE WA by 00Z. Mid/Hi level moisture and isolated
thunderstorms mainly over the Cascades will continue moving N and
eventually out of W WA by 21Z.

Low level onshore flow overnight brought a moist marine layer to the
lowlands of W WA this morning. A mix of IFR and MVFR cigs below 2000
feet predominate although visibilities were generally 10SM or
better. The smoke problem from the past week may be over with. One
greatly appreciated PIREP from an SR22 near KBFI at 1519Z put the
tops of the marine layer at 2100 feet, so it still looks like the
stratus is shallow enough to burn off this afternoon. Best guess is
still sometime 19Z-21Z.

A cold front approaching the region will move across the area
tonight. Rain should reach the coast by 03Z and then spread across
the interior after 06Z. Expect MVFR cigs to develop with the front
and then continue through 21Z Sunday.

KSEA...IFR stratus is still expected to dissipate around 20Z, with
VFR conditions and just high clouds after that. Surface winds will
remain SW 5-12 KT. Kam

&&

.MARINE...The surface ridge over the WA offshore waters this
morning will be squished and pushed to the coast this afternoon as
an offshore Pacific cold front approaches the region. The cold front
will reach the outer coastal waters this evening then move inland
late tonight.

A gale warning has been issued for the central and east Strait of
Juan De Fuca late this afternoon and this evening. Model guidance is
divided on whether a gale will actually develop tonight, but the GFS
is indicating a UIL-BLI pressure gradient of 2.9 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. W winds over the strait are still in
the low end SCA range this morning, so a 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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