Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202131
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
231 PM PDT TUE SEP 20 2016

.SYNOPSIS...An upper trough will gradually shift south over the
region the next couple days. The Western Washington lowlands will
have mostly sunny weather Wednesday and Thursday, with scattered
afternoon and evening showers over the mountains. A weak front
could bring some rain Friday, then a strong ridge will bring dry
and warmer weather Saturday through Monday.

&&

.SHORT TERM...An upper trough extends southwest from southeast British
Columbia across Western Washington and into the Oregon offshore
waters, with a weakening southwesterly jet along the south side of
the trough. The air mass is somewhat moist in the lower levels and
slightly unstable. Light scattered showers started to develop over
the Olympics, along the coast, and in the lower Chehalis Valley
around midday; this activity should increase through early evening
and spread to the Cascades as well, then decrease after sunset.
Elsewhere it has been a mostly sunny day.

The upper trough axis will shift slowly southward over Western
Washington tonight. Skies will clear over the lowlands tonight
with light winds, and patchy stratus and fog should form in the
favored areas late tonight.

An upper low will probably develop along the trough axis over the
Oregon offshore waters tonight and move into southwest Oregon
Wednesday, and then northern California Wednesday night. This will
leave a more north-south oriented trough across Western
Washington. The air mass will remain slightly unstable but
moisture over the forecast area will be even more limited, as our
weak low level onshore flow switches to northerly. A few afternoon
and evening showers over the mountains should be the extent of the
precipitation, with another mostly sunny day for the lowlands.

The main upper low will move east into Nevada on Thursday. The
upper trough axis will probably still extend northward across
Western Washington in the morning. More stable northerly flow
will develop, though, as the trough weakens and moves east of the
Cascades during the day, and Thursday should be a dry day for the
entire forecast area.

A small upper ridge will move quickly across Western Washington
Thursday night, followed immediately by a weakening front on
Friday. The last few model runs have been in better agreement that
the front will bring some precipitation to the forecast area, at
least west of the Puget Sound. We are still keeping the PoPs in
the chance to likely categories for now. Showers should decrease
Friday night.

.LONG TERM...The models have also come into better agreement that
the next weak front will probably fizzle offshore or be shunted
northwards, as a large upper ridge begins to build over the
region. We have removed the mention of precipitation from the
forecast for Saturday and Saturday night. The ridge will build
strongly Sunday with 500 mb heights rising to around 5900 meters.
Then it will shift eastward Monday. Both days should be generally
sunny, and we have raised high temperatures considerably. Much of
the area should have highs in the mid 70s to lower 80s both days.
A weak front will probably move into the region Tuesday. McDonnal

&&

.AVIATION...Flow aloft is light as a weak upper level trough remains
over the area. At the surface, northerly flow will continue through
Wednesday. The air mass is mostly dry and stable. Conditions will be
VFR this evening. Patchy IFR fog is likely to develop late tonight
and continue into Wednesday morning, with VFR by afternoon.

KSEA...VFR, mostly clear today and this evening. Patchy fog is
possible late tonight or early Wednesday morning but will keep it
out of the TAF for now. Northerly wind 6-12 KT. CHB

&&

.MARINE...Northerly winds over the interior waters will be 10-15 KT
this evening, then ease overnight. Models no longer support small
craft advisory intensity winds on the coast this evening, so have
dropped the advisory there.

Winds will strengthen Wednesday afternoon and evening over the
coastal waters, with small craft advisory strength winds possible,
especially over the southern zones. Moderate onshore flow will
develop Thursday as higher pressure settles offshore. Models now
show a front reaching the coast Friday with small craft advisory
southerly winds possibly widespread. Winds will be lighter over the
weekend as high pressure develops aloft and at the surface. CHB

&&

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

$$

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



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