Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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636
FXUS66 KSEW 252226
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
326 PM PDT SUN SEP 25 2016

.SYNOPSIS...A strong upper level ridge will give dry and warm
weather to the area though Monday. A cold front will move through
the area Monday night. This front will bring a small chance of
precipitation Monday night and Tuesday morning. Expect dry
conditions Tuesday afternoon through Thursday. An upper level
trough will bring showery and cool weather to the region Friday
through next weekend. Mountain snow levels by next weekend will
fall to around 5000 feet.

&&

.SHORT TERM...Strong upper level ridging now sits across the
region with 500 mb heights above 588 dam. With the ridging,
cloudiness has eroded back to the north coastal portions of
Washington and the northern half of the Cascades. Clouds will
continue to erode this afternoon.

The air mass is mild and dewpoint temperatures are now mainly in
the 50s. With the moist lower levels, expect radiational cooling
and the longer nights to result in the development of some fog in
the more fog-prone valleys and in the south interior. Fog that
forms should clear out by midday Monday.

Short term models continue to show warm temperatures aloft Monday
afternoon with 850 mb temperatures in Seattle around 16C. With
some mixing in the afternoon, expect high temperatures to get well
into the 70s and possibly into the lower 80s across the southern
half of the interior. The record high temperature of 78 degrees at
SeaTac airport will be threatened on Monday.

As the upper ridge over the area shifts eastward, expect a cold
front now extending south-southwest from a 1008 mb low near 50N
139W to move across the area Monday night. This front will bring a
chance of a little rain, mainly to the northern half of the
forecast area, late Monday night. There will also be a chance of
morning showers on Tuesday as westerly flow interacts with the
terrain and possibly develops a Puget Sound convergence zone.

A deep trough will form around 140W early Wednesday then will
slowly shift eastward with time. Light to moderate southwest flow
aloft with 500 mb heights around 574 dam will result in dry and
seasonably mild conditions with some late night and morning fog.
Albrecht

.LONG TERM...Deep troughing around 135W late Wednesday will shift
slowly east through the period, finally moving across Western
Washington On Sunday. Long term models bring rain chances into the
area late Thursday night into Friday morning. Showers are likely
across much of the area next weekend. Snow levels will lower to
around 6000 feet on Friday and 5000 feet Saturday and Sunday.
Higher mountain passes like those on the North Cascades highway,
may see some snow next weekend. Albrecht

&&

.AVIATION...High pres aloft will remain over the region for
westerly flow aloft. The low level flow will be light northerly
thru tonight. Anticipate localized LIFR CIGs/VSBYs after 0900 UTC
from about KOLM swd and wwd.

KSEA...VFR with northerly winds of 5-10 knots.

&&

.MARINE...Light northerly flow will prevail tonight due to higher
pressure over British Columbia with lower pressure over Oregon.
The flow will become onshore or westerly Monday afternoon and then
strengthen Mon night in the wake of a cold front. This will result
in gale force westerlies over the central and eastern portion of
the Strait of Juan de Fuca. For this reason, a Gale Watch was
issued for these areas. The onshore pressure gradient will weaken
on Tuesday.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Gale Watch Monday night for the central and eastern portions
     of the 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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