Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210401
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Tue Jun 20 2017

.SYNOPSIS...A few showers fell around Seattle this evening as a
Puget Sound Convergence Zone shifted south through King County.
Strong high pressure just offshore will shift closer to the area
through the end of the week. Low level westerly flow will become
more northerly with a warming trend. Offshore flow and rather
warm temperatures are likely for the weekend as thermal low
pressure from California shifts north toward Western Washington.

&&

.SHORT TERM...A few showers fell around Seattle this evening
as a PSCZ shifted south through the metro area. Strong onshore flow
tonight will ease Wednesday and Thursday. Strong surface high
pressure will build just offshore under an upper ridge. Areas of
morning clouds will give way to sunshine on Wednesday. Thursday will
be sunny and will mark the start a warming trend as the upper ridge
shifts closer to the area.

.LONG TERM...The 4pm discussion...The medium range solutions were in
better agreement for this weekend. Expect an upper level ridge to
dominate the Pacific Northwest. The low level flow will become
offshore as the thermally induced trough of low pressure expands
north into the area and then shifts on or just off the coast. Temps
will climb well above normal, possibly flirting with daily max temp
records in some places. The forecast for KSEA for Sunday is 89
degrees.
The record for the date is 88, set in 2006. Some places in the
southwest interior are forecast to climb into the lower to
mid 90s on Sunday.

Beyond Sunday, the models differed on how quickly the upper ridge
axis and thermally induced trough of low pressure will shift
east. It`s possible that the interior could experience additional
warming on Monday. The current forecast indicates a cooling trend
for the entire CWA, beginning Monday. Temps are forecast to
return to near normal on Tuesday, thanks to onshore flow.

&&

.AVIATION...Moderate westerly flow aloft with strong
onshore flow at low levels this evening. Low level pressure
gradients will ease after midnight. The weak Puget Sound Convergence
Zone that formed earlier this afternoon continues to shift south
over central King county and will continue to dissipate this
evening. The air mass is moist, but generally stable. The exception
is light rain associated with the weak convergence zone. Expect the
air mass to dry through Wednesday.

Ceilings have scattered out at most locations to VFR levels, except
lingering MVFR conditions in the Central Puget Sound associated with
the convergence zone. Stratus will reform at most locations 12Z-19Z
Wednesday morning with MVFR conditions and ceilings around 020 MSL.
Ceilings will lift and scatter out to VFR conditions by midday
Wednesday as the air mass dries.

KSEA...The convergence zone will drop southward into KSEA before
around 05z weakening as it moves south. Weak as it is, it will be on
its last legs by the time it reaches the terminal. Nonetheless,
ceilings may drop back down to MVFR levels briefly then improve.
Ceilings will dip again to MVFR levels around 12Z then lift and
scatter by 17Z. Northeast winds 10-14 kt will ease after 06z.
terminal with NW flow at KBFI will turn to all Nly by about 00Z.

&&

.MARINE...Strong onshore flow will continue this evening will gales
through the eastern two-thirds of the Strait of Juan de Fuca, and
small craft conditions for westerly to northwesterly winds over most
other waters (with exception to Puget Sound and Hood Canal).

Moderate onshore flow will continue Wednesday, weakening on Thursday
then becoming northerly on Friday as high pressure builds into
British Columbia. Small craft advisory conditions are expected to
continue over the coastal waters through much of the upcoming week
as strong high pressure remains offshore. The persistent
northwesterly winds over the coastal and offshore waters will result
in the development of steep short period swells that will be
difficult to distinguish from locally generated wind waves. Small
craft advisory inflow is also expected to develop over the eastern
two thirds of the strait late Wednesday afternoon and Wednesday
evening.

A thermally induced trough of low pressure will develop northward
along the Oregon coast Saturday through Sunday.
While not yet in the forecast, there is the potential that
northerly flow in Puget Sound and Hood Canal could reach Small
Craft Advisory criteria Friday or Saturday with the thermal trough
to the south developing northward.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 9 PM PDT Wednesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.

     Gale Warning until 3 AM PDT Wednesday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Wednesday for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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