Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231635
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
935 AM PDT TUE AUG 23 2016

.SYNOPSIS...Upper level high pressure over the eastern Pacific
will continue to expand its influence into western Washington
through Thursday night. In combination with weak offshore wind
flow, a warming trend will take place through Friday. Cooling
onshore flow will pick up on Friday evening and persist through
next Monday, with an upper level trough brushing by the area this
weekend.

&&

.SHORT TERM...An upper level high pressure ridge near 135W will
amplify and strengthen through Wednesday, resulting in northerly
flow aloft and rising upper heights over western Washington. From
Wednesday night through Friday, the upper ridge will retrograde
gradually to near 150W, strengthening all the while. The net
effect over western Washington will be upper heights and
temperatures aloft holding nearly steady on Thursday and Friday.

This evolution will support the initial development of a flat
thermal pressure trough near the coast from Wednesday through
Friday morning. It will sharpest and most distinct on Thursday
morning. With low-level flow trending weakly offshore during this
time, a substantial warming trend will ensue over western
Washington from today through Friday. Cooling onshore flow will
resume on Friday afternoon, but it will come too late over the
interior to keep Friday from being the warmest day inland from
coast.

Otherwise, marine stratus will be confined to locations adjacent
to Grays Harbor on Wednesday morning, with no marine stratus on
Thursday and Friday mornings. With a dry air mass and large-scale
subsidence through Friday, no rain expected. Haner

.LONG TERM...From Previous Discussion: At this point...one should
be thankful that at least the models agree on the broad strokes of
the idea of a trough for this weekend...ish. Looking at them
individually...however...may lead to excessive hair pulling. The
GFS remains fairly consistent with its solution...bringing an
upper level trough through the area starting Saturday...the
associated low never really dropping below the Canadian
border...placing the best chance for precip Saturday night into
Sunday. The Canadian has had a bit of a shift in tone...keeping
the upper level low over the southernmost portions of Alaska and
just introducing a broad trough-y pattern to the area for much of
next week before eventually bringing the low center down into the
area. The ECMWF remains the more frustrating solution allowing for
just the tip of an upper level trough to move through the area
before bringing in a bit of weak ridging Sunday then giving way to
a broad trough-y pattern into the start of next week.

While it is nice to see agreement for the start of next week and the
fact that it will result in continued cooler weather...this does not
help when attempting to forecast for the weekend. Given the general
lack of consensus...have opted to maintain trend of inherited
forecast: generally cooler conditions...the chance for showers
saturday and into sunday and again at times into the new week.
Hopefully future runs coalesce on a slightly more unified solution.
SMR

&&

.AVIATION...Northerly flow aloft will prevail today and tonight
with an upper ridge offshore. At the surface, northwest flow will
prevail today with high pressure offshore and lower pressure
inland. Low level flow will become more north to northeast tonight
as a thermal trough develops along the coast. The air mass is dry
and stable.

An area of low clouds and fog over the Central Coast and partly
inland into the Chehalis gap should burn off today. Patchy valley
fog over the interior should burn off in the next few hours. There
could be some low clouds or fog again on the coast Wednesday
morning but this should be even more limited with offshore flow
developing.

KSEA...Clear skies. Northerly wind 5-10 knots, strongest in the
afternoon and early evening. Schneider

&&

.MARINE...Weak northwest onshore flow will switch to offshore tonight
as a thermal trough develops along the coast. The thermal trough will
remain along the coast through Friday morning then shift inland
Friday afternoon with onshore flow returning. Small craft advisory
west winds are expected in the Central and Eastern Strait of Juan De
Fuca Friday night as onshore flow strengthens. Schneider

&&

.FIRE WEATHER...Will need to keep an eye on Thursday for a
critical combination of low RH below 30 percent and moderate
breezes in excess of 12 mph. With N-NE gradients, the most prone
areas will be the northwest Olympic Peninsula near Ellis Mtn. and
Forks, as well as the interior lowlands from Seattle and Bremerton
on south.Haner

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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