Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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796
FXUS66 KSEW 280344
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
844 PM PDT Tue Sep 27 2016

.SYNOPSIS...Dry southwest flow aloft will bring partly to mostly
sunny weather Wednesday and Thursday, except for patchy morning
low clouds and fog. A large upper level trough will move closer to
the area Friday, and the trough should bring cooler showery
weather to Western Washington this weekend and early next week.

&&

.SHORT TERM...Dry southwest flow aloft developed across the region
today in the wake of the minor upper trough that moved through
late Monday night, and it continues this evening. There is some
cirrus over Western Washington, but generally skies are clear to
partly cloudy with 8 pm temperatures in the 50s to lower 60s.

Dry southwest flow aloft will continue Wednesday and Thursday, as
an upper low west of Haida Gwaii digs slowly southward into the
waters well west of Washington. Low level flow will be weakly
onshore, and there will be some stratus and patchy fog late at
night and in the mornings. But Wednesday should be mostly sunny,
and Thursday should be partly to mostly sunny. Highs will be near
to slightly above normal both days, in the 60s to lower 70s.

The flow aloft will back toward southerly Thursday night and
Friday, as the upper low continues south into the Oregon offshore
waters. The models begin to disagree at this point on the position
of the low and whether it will present a threat of precipitation
moving into or developing over Western Washington. It seems that
the best chance for some showers will be convection developing
along the Cascades crest in the marginally unstable southerly flow
aloft. McDonnal

.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- The models do not agree beginning Saturday, with
different tracks for the offshore upper low giving widely divergent
weather solutions over Washington. The GFS moves a secondary upper
low southeast into Oregon, making Saturday afternoon through Sunday
morning a wet period. The Canadian model keeps the low further
offshore, with mainly mountain showers Saturday and a surprisingly
dry Sunday and Monday. The Euro is between the two and keeps rain
confined to mainly western sections. The GFS is wetter than the
other two models on Monday and Tuesday. The upshot is a fair amount
of uncertainty through the extended period. For the forecast have
kept a chance of showers in every period, with mountain showers
likely Saturday night and Sunday. It is not going to rain every
period, but everywhere will probably get some rain at some point
during the Saturday to Tuesday period. Burke

&&

.AVIATION...Southwest flow aloft. Low level northerly flow. The air
is stable and dry except for some coastal stratus and fog that will
develop overnight and some valley fog that should form in the
interior river valleys. Otherwise it will be mostly clear with just
that strip of cirrus over NW Wa tonight.


KSEA...Clear skies except for a little cirrus, and a northerly
breeze. The wrfgfs and hrrr do not show any fog or low clouds for
Seattle Wednesday morning.

&&

.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 and
extends through Wednesday afternoon. A weather system should reach
the area Friday and over the weekend.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Wednesday 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.

&&

$$

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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