Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 111643
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
943 AM PDT Fri Aug 11 2017

.SYNOPSIS...Changes in the weather pattern were underway. The
high pressure system responsible for the dry, warm weather was
moving off to the east, allowing a low pressure system offshore to
approach. This system will move across the region Saturday night
and Sunday for wet and cooler weather. In the very near term, look
for the smoke to finally exit the area later today.

&&

.SHORT TERM...
The air quality was better this morning compared to yesterday.
This morning, most air quality monitoring sites had readings in
the good or moderate range. Expect the air quality to continue
improving as the smoke is gradually pushed out of the area due to
increasing westerly flow in the low to mid levels of the
atmosphere. Will allow the Air Quality Alert to expire at noon
today.

Meanwhile, the axis of the upper level ridge of high pressure
that has dominated the weather pattern over the Pacific Northwest
has finally moved east of the area. It was over eastern WA at this
time, and will continue moving east. This will allow an upper
level trough offshore to approach the Pacific Northwest. Expect
the onshore pressure gradient to tighten ahead of the trough and
the flow aloft to strengthen from the south. Max temperatures will
be cooler at most places today, particularly across the interior
lowlands.

The rapidly weakening Low that was over northern CA yesterday was
over Oregon at this time. It was being booted northward by the
approaching the upstream kicker (upper trough). The remnants of
this feature will bring a risk of thunderstorms to mainly the
mountains late today through tonight. There is also a possibility
of isolated high-based thunderstorms over the lowlands between
the Olympic range and Cascades tonight. The thunderstorm threat
may persist over the northern Cascades into Saturday morning.

A strong low pressure system and associated cold or occluded front
will bring steady rain to much of the CWA Saturday night. It looks
like the dry streak for Seattle will finally end Saturday night.
Look for the front to exit early Sunday, with the rain tapering
off to showers behind it. The showers should become mainly
confined to the Cascades and within a Puget Sound Convergence Zone
Sunday afternoon. Temperatures on Sunday will be below normal for
much of the area.

.LONG TERM FROM THE PREV DISCUSSION...
The upcoming week will see mainly zonal flow aloft with 500 mb
heights rising back into the mid 570 to lower 580 DAM range and
low level onshore flow. Temperatures may creep back to 2-4 degrees
above normal by the end of the week. There is a chance that weak
disturbances embedded in the zonal flow aloft could produce some
spotty light showers, but forecast solutions are generally showing
little consistency in their handling of these systems. The
forecast from Monday night onward was left unchanged. Albrecht

&&

.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 help push the leftover
forest fire smoke out of W WA. This will be a gradual process so
some areas will still have visibilities 6-12 miles in haze and
smoke.

Onshore flow overnight was a little stronger than previous days
and IFR stratus has pushed inland over Puget Sound as far N as
KSEA, and over the North interior up to KBLI. The stratus should
dissipate by 18Z-20Z.

A weak upper level shortwave trough was just W of the N Oregon
coast at 16Z and will spread mid and high level clouds over the SW
half of W WA through 00Z. An upper level trough approaching from
offshore will push the shortwave trough and the associated cloud
cover inland over the remainder of W WA tonight. There is a chance
that the shortwave will trigger some isolated showers or
thunderstorms, in the potentially unstable air mass, E of Puget
Sound after 06Z.


KSEA...IFR stratus is hanging at the edge of the terminal this
morning with conditions oscillating from SCT002 to BKN002 with
occasional VIS down to 1/2SM FG. This will probably dissipate by
19Z. Mid/high level clouds from a trough just W of the N Oregon
coast will spread over the terminal after 19Z.

Leftover forest fire smoke and haze is still around but increasing
SW flow aloft will allow for better VIS, 6-12 SM today. Surface
winds will be W-SW 4-7 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. Onshore flow will also be fairly strong Saturday
night but the winds could either be in the high end SCA range or
low end Gale range. 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, which will
help to maintain the strong onshore flow Saturday night. The
front is expected to 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 from 5 PM this afternoon to 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 PM PDT this afternoon for Central
 U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
  Strait Of Juan De Fuca.

 Small Craft Advisory from 5 PM this afternoon to 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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