Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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159
FXUS66 KSEW 191026
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
326 AM PDT THU MAY 19 2016

.Synopsis...An upper low over Vancouver Island will bring showers
to western Washington as it moves across the area today and into
western Oregon tonight. The air mass will dry out somewhat on
Friday on the north side of the low, but a chance of showers will
remain in the forecast through the weekend. Another upper trough will
drop southeast into the region early next week.

&&

.SHORT TERM...A 5510 meter upper low is over southern Vancouver
Island early this morning, and the result is a cool moist and
weakly unstable air mass over western Washington. Scattered
showers continue across the forecast area, especially from around
Seattle northward in a rather disorganized Puget Sound
convergence zone. Temperatures at 3 am were in the mid 40s to
lower 50s.

The upper low will move south across western Washington today and
western Oregon tonight, reaching northern California late
tonight. Moist westerly flow and weak instability should produce
showers across the forecast area. The finer scale models agree
that the convergence zone will move south, probably making it all
the way to SeaTac this afternoon. Showers will likely decrease
tonight as the low moves south, and the convergence zone will
probably dissipate. Moisture wrapping around the low will keep a
chance of showers in the forecast though.

The snow level will be around 4000 ft today and 4500 ft tonight,
with roughly 2 to 5 inches of new snow above the snow level.

The upper low will move slowly east across northern California on
Friday, with an upper trough axis extending north across western
Oregon and Washington. Friday will probably begin cloudy, but the
air mass should dry in light easterly flow. Most of the western
Washington lowlands should have dry or nearly dry weather Friday,
with morning clouds giving way to partly sunny skies. Weak
instability will produce some light showers mainly over the
mountains during the afternoon and evening. The snow level will be
4500 ft to 5000 ft, but only another inch or so of new snow is
expected.

The upper low will probably turn northeast into southeast Idaho
Friday night and then travel north into eastern Washington on
Saturday. As a result, the air mass over western Washington will
remain somewhat unstable with moisture and showers increasing
Saturday and Saturday night, with most of the showers again over
the mountains. McDonnal

.LONG TERM...Models are not in very good agreement for the
extended forecast period Sunday through Wednesday. However it looks
like the long wave upper trough position will remain over the
region through at least Monday, with another shortwave trough
probably dropping southeast over Pacific Northwest. The upper
trough should begin to move east of the region on Wednesday, with
a weak upper ridge moving into the Pacific Northwest offshore
waters. MCDONNAL

&&

.AVIATION...An upper level low will remain over the area through
Friday. Variable flow aloft becoming northeasterly tonight. Low
level onshore flow weakening today and dissipating tonight.

Convergence zones forming and dissipating between KPAE and KSEA
today with the last of the convergence dissipating near KSEA
tonight. Ceilings in the convergence zones ranging from 1000-2000
feet.

Outside of the convergence zone multiple cloud layers between 3000
and 8000 feet with clearing over the south sound this morning.

KSEA...Convergence zone near but mostly north of the terminal today
with ceilings 1500 to 3000 feet. Convergence dissipating tonight.
Winds mostly southerly 4 to 8 knots. Felton

&&

.MARINE...Onshore flow will weaken today with falling surface
pressure offshore. A weak surface low will develop off Vancouver
Island tonight and drift over the coastal waters on Friday. The low
will dissipate Friday night. Low level onshore flow will develop on
Saturday and continue into Sunday. Felton

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 2 AM PDT Friday for Central U.S.
     Waters Strait Of Juan De Fuca-Coastal Waters From Cape
     Flattery To James Island 10 To 60 Nm-Coastal Waters From
     James Island To Point Grenville 10 To 60 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater 10 To 60 Nm-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PDT this afternoon for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm.

&&

$$

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