Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 112257
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
357 PM PDT Fri Aug 11 2017

.SYNOPSIS...A low pressure system offshore will move across the
region on Sunday for cooler weather. A weak disturbance ahead of
it will bring a threat of thunderstorms to the interior tonight
and the Cascades Saturday morning. A cold or occluded front will
bring a decent shot of rain to the area Saturday night, likely
ending Seattle`s dry streak. Onshore flow will prevail on Monday.

&&

.SHORT TERM...
A few sprinkles fell from a batch of northward-moving altocumulus
castellanus this afternoon. It`s possible that an isolated
thunderstorm could develop over the Cascades, near the crest, in
Lewis and Pierce Counties late today. Otherwise, skies were mostly
cloudy over the coast and Olympic range while elsewhere it was
mostly sunny to partly cloudy. Temperatures over the interior were
5-15 degrees cooler than at the same time yesterday.

Regarding the wildfire smoke: Per the monitoring sites, the air
quality was better than at the same time yesterday, with most
readings in the good to moderate range. The smoke will be a
non-issue this weekend.

Meanwhile, a low pressure system offshore will continue moving
east and will be over the region Sunday. A weak disturbance or
short wave trough ahead of this system will bring a threat of
thunderstorms to parts of the interior overnight, and the Cascades
(near the crest) Saturday morning. The cold or occluded front
associated with the offshore upper level trough will sweep across
the CWA late Saturday night or early Sunday morning for a decent
shot of steady rain. Seattle`s dry streak will likely come to an
end Saturday night. Expect the steady rain to taper off to
showers behind the front.

Look for the showers to diminish during the day Sunday, with the
threat becoming confined to mainly the Cascades and within the
Puget Sound Convergence Zone by late in the day. Northwest flow
aloft or weak cyclonic flow aloft will prevail on Monday.

Temperatures during this period are forecast to be near normal on
Saturday and slightly below normal Sunday and Monday.

.LONG TERM...
The models were more or less in agreement during much of this
period. Anticipate the upper ridge axis to stay offshore,
resulting in northwest flow aloft and continued low level onshore
flow of varying strength across the CWA. Temps will be near normal.
The ECMWF and GFS solutions indicated that a weak upper level
trough may move across the area on Day 7. The main impact would
be stronger onshore flow and a deeper marine layer. However, this
is Day 7 and is subject to change with the next model cycle. Thus,
chose to lean towards climatology for now.

&&

.AVIATION...An upper level ridge is centered over E WA with SW
flow aloft over W WA. The airmass is generally dry but potentially
unstable. The increasing SW flow aloft will gradually push the
leftover forest fire smoke out of W WA. Visibilities have been
gradually improving although some spots are still down around 5 SM
to 6 SM.

A weak upper level shortwave trough will move NE across W WA
06Z-12Z. There is a chance that the trough will be strong enough
to generate some isolated high-based thunderstorms.

IFR stratus from this morning has dissipated almost all the way
back to the coast. However, KHQM still has an IFR CIG. The
onshore flow tonight will spread IFR/MVFR conditions back across
most of the W WA interior late tonight and Saturday morning. With
stronger onshore flow there is a chance that the marine layer will
be a little deeper which would mean less coverage of IFR
conditions.

KSEA...Skies have mostly cleared this afternoon with just some
high clouds and smoke over the area. SW flow aloft should continue
to gradually erode the smoke with visibility a little better than
in previous days. Onshore flow will increase again tonight for
another round of IFR/MVFR conditions Saturday morning 12Z-19Z.
There is a very small chance that isolated thunderstorms could
develop late tonight as a weak upper level trough moves across
W WA. Surface winds will be SW 4-8 KT. Kam

&&

.MARINE...A surface ridge just offshore will generate strong
onshore flow tonight. Gale force winds are expected in the strait
of Juan De Fuca. SCA winds are also expected tonight in the
waters adjacent to the Strait, i.e. Admiralty inlet and the
portion of the north inland waters over the southern San Juan
Islands.

A cold front will be approaching the coast on Saturday then move
across W WA Saturday night.

Behind the front the offshore surface ridge will rebuild on
Sunday, allowing the standard onshore flow pattern to return. This
pattern could continue through early next week. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 5 AM PDT Saturday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
  Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Saturday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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