Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191541
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
841 AM PDT MON SEP 19 2016

.SYNOPSIS...An upper trough to the north will bring scattered
showers and sun breaks today, with thunderstorms possible. The
trough will move southeast over the region Tuesday and Wednesday,
with showers limited mainly to the mountains in the afternoon and
evening. The trough will move southeast Thursday, then a weak
front could bring some rain Friday or over the weekend.

&&

.SHORT TERM...Strong westerly flow aloft continues over Western
Washington this morning with a 130+ jet extending along the
Canadian/United States border from Montana to Washington. A 5550
meter upper low is over northern Vancouver Island, with its trough
axis south into the forecast area. At 8 am, we had a mix of
showers and sun breaks with temperatures in the 50s. Some of the
showers along the north coast are producing lightning.

The flow aloft will ease later today and tonight as the jet streak
moves east, and the flow will back slightly as the upper trough
moves slowly southeast. The air mass will remain somewhat unstable
today, and given what we`re seeing on radar the slight chance of
thunderstorms for the forecast area looks good. The threat of
showers and thunderstorms should end quickly this evening.

The upper trough will be over the region Tuesday, but the models
agree on generally dry weather until daytime instability produces
some showers over mainly the coast and mountains in the afternoon
and evening. The upper trough will continue to inch southeastward
Tuesday night and Wednesday. The chance of precipitation looks to
be even less on Wednesday, limited to diurnal afternoon and
evening showers over the mountains. McDonnal

.LONG TERM...Here is the long term section from the early morning
forecast discussion -- The closed upper low finally drifts south
on Thursday as another fetch of moist westerly flow approaches the
region. Models area still not consistent on the timing and
strength of the initial frontal band to arrive sometime late
Friday. The latest GFS shows the first front dissipating with a
second front decaying but bringing in some light rain by Saturday.
Other models show more rainfall. Will leave the chance pops in the
forecast for the weekend. Mercer

&&

.AVIATION...Moderate westerly flow aloft over Western Washington
will continue today then weaken and turn more southwesterly tonight.
Southerly surface flow over inland areas will weaken this afternoon
as the flow turns more onshore. The air mass is moist and unstable.

Showers today will diminish tonight. There is a slight chance of
thunderstorms over the area today but the chance is not high enough
to include in the tafs. Ceilings are generally in the 2-3k ft range
in showers but there are higher ceilings and even some clearing
between showers. Conditions will improve late in the afternoon.

KSEA...Discussion above applies. South to southwest winds 8-14 KT
today. CHB

&&

.MARINE...Onshore flow will continue through today. With an unstable
air mass over the area gusty winds are possible over all the waters.
A weak surface low will develop over the coastal waters tonight,
then dissipate on Tuesday. High pressure will build over southern
British Columbia for light northerly flow on Wednesday. Light
onshore flow will develop Thursday. A front may move into the
forecast area on Friday. Felton/CHB

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 2 PM PDT this afternoon for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 6 PM PDT this evening for Puget Sound
     and Hood Canal.

&&

$$

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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