Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 282200
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Wed Sep 28 2016

.SYNOPSIS...Dry southwest flow aloft will bring mostly sunny weather
through Friday, except for patchy morning low clouds and fog. An
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...Skies have cleared nicely though a patch of stratus in
King and Snohomish counties persisted longer than expected. 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--mostly in the 60s.

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
afternoon showers in the mountains and on the coast. High
temperatures will decrease again, into the low to mid 60s.

An upper level low will move south well off the coast Saturday, then
into northern California and Oregon Sunday. Models are in reasonably
good agreement. The low will spin moisture north toward Washington,
but now it appears that Saturday is more likely to be wet than
Sunday. The low is perhaps far enough away on Sunday that showers
will stay in Oregon, or at any rate over southwestern parts of the
CWA. There is a hint of offshore surface flow on Sunday in the
models, which would tend to keep things dry. Have changed the
forecast a bit to give likely pops to the coast and Olympics on
Saturday and Saturday night, then chance pops everywhere Sunday. It
will be cooler over the weekend than it has been lately, with highs
in the lowlands in the low to mid 60s, and the snow level around
5000 feet in the mountains. Burke

.LONG TERM...Models all have moist northwest flow aloft beginning
Monday after the upper low has passed to the southeast. There could
be another front about Wednesday (the GFS) or Tuesday (the Canadian
and Euro). For now have broad brushed a chance of showers each day.
Conditions will be on the cool side with highs in the low to mid 60s
and the snow level 5000-6000 feet. Burke

&&

.AVIATION...An upper level low off the central coast of British
Columbia will drop southward off the Washington coast near 130W by
Thursday afternoon. Light northwest flow aloft becoming
southwesterly. Stable conditions and dry in the mid and upper
levels. Residual low level moisture will again allow areas of MVFR
or local IFR stratus and fog to form late tonight and Thursday
morning, especially in sheltered river valleys and lower lying
areas. Cigs/vis should improve after 17z and dissipate by noon.

KSEA...north winds 6-10 kt becoming northeast 4-7 kt by late this
evening into Thursday morning. MVFR stratus cigs are possible with
the best chance between 12-17z Thursday morning. There is also about
a 25 percent chance of IFR cigs/vis due to fog within that time
period. dtm

&&

.MARINE...High pressure offshore and lower pressure east of the
Cascades and south over Oregon will produce enough gradient for
small craft northwesterly winds over the coastal waters through this
evening. A weak westerly push in the central and eastern Strait
could bring small craft winds to 25 kt this evening. Gradients will
ease across the area after midnight with winds below 20 kt
throughout the waters.

A low pressure system and surface trough will affect the waters
Friday into the weekend. This will maintain light to moderate
onshore flow. Small craft winds are possible in some of the waters
outside Puget Sound. Calmer conditions expected Monday with light
onshore flow. dtm

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small craft advisory coastal waters.
     Small craft advisory for 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



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