Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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892
FXUS66 KSEW 171527
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
827 AM PDT Sun Sep 17 2017

.SYNOPSIS...A front will bring rain later today. An upper trough
and additional weather systems will keep the weather cool and
showery into the middle of the week. Drier weather is likely late
in the week.

&&

.SHORT TERM...A front will move through the area today, bringing
rain to the coast this morning and to the interior this afternoon.
Highs will be in the 60s which is 5 to 10 degrees cooler than
yesterday.

An upper trough will move over the area Monday with snow levels
lowering to 5000 to 5500 feet. It will be a showery day and the
higher locations in the mountain could pick up a few inches of
snow. There will also be a slight chance of thunderstorms Monday
with cold unstable air aloft over the area. Highs will be about 10
degrees below normal.

Models show an upper short wave moving through the area Monday
night causing an uptick in showers, especially over the south.
This could bring 6+ inches of snow to Paradise which would meet
winter storm criteria for the first event of the season. If the
12Z models continue to show this, a winter storm watch will likely
be issued for the Cascades of Pierce and Lewis counties later
today.

Showers will linger into Tuesday but probably not as prevalent as
Monday as the upper trough weakens over the area. Highs will be
about 10 degrees below normal again, similar to Monday. Schneider

.LONG TERM...A vigorous upper short wave will move onshore
Tuesday night, clipping the south part of the forecast area. This
should bring steadier precipitation with heavier amounts south of
Seattle. An associated surface low will track inland just to the
south of the area. This should keep windy conditions south of the
area but it is close enough that it needs to be monitored.

An upper trough will be over the area Wednesday for more showers.
Drier northerly flow aloft will develop over Western Washington
Thursday. It looks like most of the area will be completely dry by
Friday and Saturday. Schneider

&&

.AVIATION...A cold front will move through Western Washington today.
The radar shows some rain just off the coast at 8am. There is some
low stratus on the coast this morning at Hoquiam but that will lift
up and merge with the increasing frontal clouds. The low clouds made
it to shelton this morning but most of the interior of Western
Washington just has mid and high clouds so far--but cigs will come
down this afternoon and the rain pushes in. Showers are on tap
tonight in onshore flow and a PSCZ will probably form for a few
hours. The smoke will go away, replaced with showers and clouds
obscuring the mountains at times for the next several days as an
upper trough moves slowly across the region with cool unstable
air.

KSEA...The current TAF shows the first front of fall, a little rain
later today, southerly winds. Sometime this evening, probably around
04z, a PSCZ will probably develop and could cause the winds at Sea-
Tac to become VRB05kt for several hours

&&

.MARINE...The first real front of fall should give many areas
breezy weather for awhile today. The cold frontal passage and wind
shift from 20-30kt southerlies to more moderate westerlies over the
coastal waters should occur midday to early afternoon, reaching
Grays Harbor around 3pm. Westerlies will come down the Strait of
Juan de Fuca mid to late afternoon. For the inland waters southerly
winds 15-25kts will turn to a convergence zone wind pattern and ease
this evening. Looking at the wrfgfs, small craft advisories might
have to stay up for awhile this evening in the north and Strait for
winds behind the front. Looking at the 06z GFS...a low pressure
trough moves through the area Monday night, and then another system
with a deeper low moves through the area Tuesday night--with a 992mb
low moving ashore over NW Oregon, so those are systems to watch for
some blustery weather.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 11 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 6 PM PDT this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet-Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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