Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 281005
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
305 AM PDT Wed Sep 28 2016

.SYNOPSIS...Dry southwest flow aloft will bring mostly sunny weather
today through Friday, except for patchy morning low clouds and fog.
A large upper level trough will bring showery and cooler weather
Friday night through the weekend. Unsettled weather will likely
continue early next week in northwest flow aloft.

&&

.SHORT TERM...Dry southwest flow aloft will continue over W WA
today through Thursday. Onshore flow will remain weak, but still
have just enough strength to make isolated low clouds or fog
possible each morning.

The large upper level low W of the B.C. coast and near 52N/140W at
09Z/2 AM will continue moving SSE the next few days, eventually
reaching 45N/130W or W of the Oregon coast on Friday. As the low
nears the area on Friday, the flow aloft over W WA will shift to
southerly. The 00Z models were in good agreement through Friday and
keep the area dry through Friday afternoon. Model RH progs and time-
height sections kept the air mass dry enough through Friday to call
it mostly sunny. I kept a slight chance of showers over the Cascade
crest Friday afternoon, even though the 00Z models were backing away
from the showers. High temperatures will gradually cool through
Friday as the large low approaches the region and 500 MB heights
fall. Kam

.LONG TERM...The 00Z GFS ECMWF and Canadian solutions begin
diverging a little Friday night. The models show a shortwave trough
ejecting out of the large trough just offshore sometime Friday night
or Saturday but differ on timing. The ECMWF is the fastest and the
Canadian slowest. Depending upon how fast the shortwave ejects,
showers may not make it very far inland Friday night, but should
spread across W WA on Saturday.

The feature that is kicking the shortwave inland is another small
low dropping S into the bottom of the main trough. The main trough
along with the small low eventually move inland across the region on
Sunday. This could end up being the wettest day of the period. The
air mass will cool significantly this weekend allowing mountain snow
levels to drop down to around 5000 feet. NW flow aloft develops
early next week behind the departing upper level trough. It looks
like there could be one or more weak shortwaves embedded in the flow
so the chance of showers will continue. Kam

&&

.AVIATION...Southwest flow aloft. Low level northerly flow. The air
is stable and dry. Some patchy valley fog may for around daybreak
and there is a chance that some fog or low clouds will set up around
Grays Harbor, although the hrrr has shown less now. There is some
cirrus passing over early this morning from a weak upper trough.

KSEA...Some cirrus and a northerly breeze. Only the sheltered
valleys away from the metro area have a chance to see any fog.

&&

.MARINE...High pressure offshore and to the north of the area with
lower pressure inland and to the south will give northerly breezes.
A small craft advisory remains in effect for the coastal waters but
as a weak low drifts toward the area the gradient will decrease
tonight and Thursday. By the end of the week there should be a slow
moving area of low pressure just offshore and some rain is likely
for the weekend.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory coastal waters.

&&

$$

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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