Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182130
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT SUN SEP 18 2016

.SYNOPSIS...A couple of frontal systems, rotating around an upper
low over the Gulf of Alaska that is moving toward the area, will
move though Western Washington tonight and Monday. The upper low
will then park over the area Monday night through Wednesday,
giving just a few showers and cooler temperatures. The upper low
will move off to the east of the area on Thursday. A weakening
front will bring clouds and a threat of rain to the area Friday
through next weekend.

&&

.SHORT TERM...A weak upper level ridge near 160W this afternoon
will build and slow its eastward progress tonight through Monday.
The amplification of this ridge will force the upper low now near
53N 145W into the Pacific Northwest later Monday and stall it
over the area through mid week.

Two systems rotating around the incoming upper low will impact the
area tonight and Monday. The first is an instant occlusion feature
that extends southward from about northern Vancouver Island into
the offshore waters. This will move inland as it weakens
overnight. There has been quite a bit of lightning with this
feature, so a chance of thunderstorms has been added to the
forecast for the north portion of coastal Washington and the
western Olympics overnight tonight. The second system is
associated with the wave that now is seen just west of 140W. This
system appears that it will develop into another front that will
move east across the area on Monday. The current forecast is in
line with earlier model runs that indicated that the nature of
the precipitation will be rain. However, satellite imagery seems
to indicate that the precipitation may be more showery in nature as
it moves through. This second front may also be coming in quicker
that earlier expected and cold cloud tops hint at another chance
of thunderstorms with this second feature. Will allow the next
shift to take another look at latest guidance and satellite
imagery before making changes as this system rapidly morphs.

The upper low will settle in over the area Monday night through
midweek as it weakens. Skies will generally be partly cloudy
through the period and there will be a small chance of showers
that can pop up any time of day as difficult to time features rotate
around the circulation. Temperatures will warm into the 60s during
the day and will fall mainly into the 40s at night. Albrecht

.LONG TERM...Models are generally consistent in moving the upper
low off to the east and southeast on Thursday. The ECMWF keeps the
low near the area a bit longer than the GFS. Still, Thursday
appears that it will be the driest day of the next several as we
fall between the departing low and an incoming frontal system.

The 12Z long term models have gone back to splitting the next
frontal system as it moves into the area and weakening it over the
forecast area. The models then show a very broad ridge building
across the eastern Pacific and western North America next Saturday
and Sunday as weak impulses move across the area. Will keep clouds
and a chance of a little rain at times in the far extended along
with milder temperatures. Albrecht

&&

.AVIATION...Westerly flow aloft over Western Washington this
afternoon will continue tonight and Monday. A weak front will move
into the forecast area on Monday. The air mass will be moist in
the low levels tonight, then become moist to about 10,000 feet and
weakly unstable on Monday.

The low cloud cover has been gradually breaking up over the past
few hours, leaving SCT018-25 to locally BKN, and that slow trend
should continue this evening. The weak front will probably bring
ceilings mainly 025-045 Monday with areas of light rain.

KSEA...South to southwest winds 5-12 kt through Monday. The weak
front on Monday will probably result in OVC035 with occasional
light rain. McDonnal

&&

.MARINE...Light onshore flow will continue through Monday. A weak
low will develop over the coastal waters Monday night, then
dissipate on Tuesday. Light onshore flow will develop Wednesday
and continue through Thursday. A front could move into the
forecast area on Friday. McDonnal

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...None.
&&

$$

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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