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030
FXUS66 KSEW 281634
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 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...There is a thick patch of stratus along the east side
of Puget Sound but otherwise skies are mostly sunny. The stratus
should evaporate by noon. Highs today will be mostly in the 60s
with some low 70s over the southwest interior. Thursday will be
similar to today with some morning clouds and fog and a mostly sunny
afternoon. Highs Thursday will be a few degrees lower than today.

Friday will begin similarly but upper level flow will turn more
southerly and higher clouds will start to increase in the afternoon.
The forecast is still basically dry but there is a chance of showers
in the mountains and possibly the south coast. Burke

.LONG TERM...Previous discussion...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...An upper level trough offshore will dig southward
just outside 130W through Thursday. Light northwest flow aloft
becoming southwesterly Thursday. Stable conditions. Dry in the mid
levels and somewhat moist above 20k ft today with cirrus over the
area at times. Moist in the low levels with areas of MVFR or locally
IFR stratus and fog along the I-5 corridor from KBLI to KOLM. Low
clouds and fog have also formed in some river valleys and the
Chehalis Gap. Cigs/vis should improve after 17z and dissipate by
noon.

KSEA...Stratus has formed along the I-5 corridor just east of Puget
Sound. Satellite shows little movement. Cigs around 1000-1500 ft may
last as long as noon before dissipating and clearing. Brief IFR
stratus just below 1000 ft cannot be ruled out until after 18z.
North-Northeast winds 4-7 kt becoming northwest after 18z.

&&

.MARINE...High pressure offshore and lower pressure east of the
Cascades will produce enough gradient for small craft northwesterly
winds over the coastal waters through this evening. A weak westerly
push in the Strait could bring small craft winds to 25 kt in
central/east portion this evening. 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 and the central and eastern
strait of Juan De Fuca.


&&

$$

www.weather.gov/seattle

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