Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191052
AFDSEW

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

.SYNOPSIS...A cool and unstable air mass will produce showers with a
slight chance of thunderstorms today. An upper level low will settle
over the region tonight through Wednesday. The low moves southeast
Thursday, then a couple of week fronts will move through the area
Friday and over the weekend.

&&

.SHORT TERM...An upper level low with a -26C cold pool at 500mb is
centered near the north tip of Vancouver Island early this morning.
IR satellite imagery shows widespread cold top cu streaming
southeast toward Western Washington as the low approaches. Lightning
detection is picking up numerous strikes just off the Washington
coast and over Vancouver Island. Convection is being driven by
general air mass instability but also some strong dynamic lift.
Models show a 135 kt jet streak in the region of greatest diffluence
aloft just northwest of the area which should move into Western
Washington later this morning. Forecast soundings show a backing
wind profile at the time of strongest divergence aloft. However,
lapse rates are not that impressive with marginal LI`s and
convective temperatures may be tough to reach. But the combination
of the very strong jet streak and cells with lightning offshore
warrants including thunderstorms across the entire area today. Areas
near the coast and north of Everett may have a slightly better
chance that the south interior, but thunder can`t really be ruled
out anywhere today.

Showers will decrease this evening and mostly be confined to the
mountains after midnight as the upper low moves over Washington.
Models shift the strong jet and moisture to the southeast so the
general trend should be a decrease in shower activity Tuesday and
Wednesday. Diurnal instability will still trigger a few showers over
the mountains, especially in the afternoon. It will remain cool the
next few days remaining in the 60s.

.LONG TERM...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...Westerly flow aloft over Western Washington will
continue today. The air mass will be moist to about 10,000 feet and
unstable.

Ceilings generally in the 6000-9000 foot range early this morning
with isolated 2000 foot ceilings. Ceilings will lower into the 2500-
4000 foot range later this morning with showers. There is a slight
chance of thunderstorms over the area. The chance is not high enough
to include in the tafs. Conditions will improve late in the
afternoon.

KSEA...South to southwest winds 8-14 knots today. Ceilings near 6000
feet with a 2000 scattered to broken layer early this morning.
Ceilings lowering to near 3500 feet with showers by mid morning.
Conditions will improve late in the afternoon. There is a slight
chance of thunderstorms at the airport through late afternoon. Felton

&&

.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 will move into the
forecast area on Friday. Felton

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small craft advisory Puget Sound and Hood Canal and Camano
     Island to Point Roberts.

&&

$$

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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