Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 101610
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
910 AM PDT Thu Aug 10 2017

.SYNOPSIS...The dry, warm pattern that has persisted for more than
a week will finally end this weekend. The high pressure system
responsible will move east of the area on Friday. This will allow
a low pressure system to approach from the west for a cooling
trend. Meanwhile, the smoke should finally exit the area on Friday
due to increased southerly flow aloft.

&&

.SHORT TERM...
Similar to yesterday, smoke blanketed the area this morning.
Overall, the air quality appeared similar to yesterday`s, with
some areas over the interior (such as the Puget Sound region)
having air quality readings in the unhealthy range. Conditions
should begin improving late today or tonight but more likely on
Friday as southerly flow aloft strengthens.

Meanwhile, the stratus/fog was a little more widespread than
yesterday due to slightly stronger onshore flow. This morning,
the low clouds and fog made it to KSEA`s doorstep. The onshore
pressure gradient will be stronger this afternoon compared to
yesterday; thus, look stratus to blanket much of the lowlands
Friday morning.

The upper level block that has plagued the Pacific Northwest for
nearly a week will finally begin breaking down today. As the
ridge axis slides off to the east, it will allow a low pressure
system over the northeast Pacific to approach the region on
Friday. This will cause the onshore pressure gradient to
strengthen further and the marine layer to deepen, resulting in
cooler temps on Saturday. Expect a short wave trough to move
across the area on Saturday. There will be a risk of thunderstorms
over mainly the Cascades Friday night ahead of this trough.

.LONG TERM FROM THE PREV DISCUSSION...
Models are showing rather good consistency today, and they
continue to advertise a rather good chance of some light rain
across portions of the area Saturday night into Sunday night or
Monday morning as a weak front moves across the area and is
followed by onshore flow and potential convergence zone activity.
The best chance for a wetting rain of 0.25 to 0.5 inch
accumulations will be on the northern coast and possibly over the
western slopes of the Cascades east of Arlington to Seattle. Their
is a better than even chance that Seattle will finally end its
record long dry streak Saturday night or Sunday.

Rain chances in the latest long term models decrease after early
Monday as flow aloft become more zonal and 500 mb heights rise.
The onshore flow will keep temperatures near normal through about
Tuesday, then temperatures will likely rise into the 70s to lower
80s by the end of next week. Albrecht

&&

.AVIATION...An upper level high pressure ridge axis will slowly
move from Western Washington this afternoon to Eastern Washington
on Friday afternoon. Once the ridge axis moves east, light
southwest flow aloft will develop. The air mass will be dry and
stable today. Mid-level moisture and instability will increase on
Friday. Marine stratus spread a little further inland this
morning, reaching PWT and almost reaching SEA. Marine stratus will
quickly burn back to the coast around mid-day. Otherwise,
visibility reductions from wildfire smoke will continue today,
with northerly surface flow this afternoon reinforcing the
situation by bring in more smoke from B.C.. The smoke situation
will improve over the Olympic Peninsula on Friday, then over the
Puget Sound region on Friday night.

KSEA...Fog and stratus on the southwest doorstep of the airfield
right now, but it will retreat and dissipate any time now. Smoke
models show particulate concentrations (and visibility by proxy)
should be as bad or a little worse today then yesterday, so no
improvement in the smoke situation today or tonight. The trend in
recent days has been for smoke-related visibility to bottom out
during the afternoon hours and improve a little at night. More
substantial improvement expected around Friday evening.
Otherwise, a westerly seabreeze will develop around mid-day at the
terminal, gradually becoming more northerly during the course of
the afternoon. Haner

&&

.MARINE...Moderate low-level onshore flow will lead to another
round of small craft advisory strength westerlies through the Strait
this evening. A marine push will further strengthen onshore flow
on Fri PM, with the 06z GFS model forecasting the UIL-BLI gradient
to peak near +3.6 mb on Fri evening. This should be strong enough
for westerly gales through the Strait at that time.

A cold front will cross the waters on Sunday morning, followed by
another round of strong onshore gradients on Sunday evening.
Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 7 AM PDT
 Friday for Central U.S. Waters Strait Of Juan De Fuca-East
 Entrance U.S. Waters Strait Of Juan De Fuca.

 Gale Watch from Friday afternoon through late Friday night
 for Central U.S. Waters Strait Of Juan De Fuca-East
 Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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