Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 260435
Area Forecast Discussion
National Weather Service Seattle WA
935 PM PDT Tue Apr 25 2017
.SYNOPSIS...A weak upper trough will maintain showery weather
through Thursday. High pressure will dry the area out Friday and
possibly Saturday. A front will bring a little rain Sunday. High
pressure aloft will likely be in control early next week.
.SHORT TERM...A weak frontal system will fall apart over the area
tonight. There will be some light rain but it looks spotty and
more off than on. Showers will linger on Wednesday with a baggy
upper trough overhead - mainly coast, mountains, and in a Puget
Sound Convergence Zone. Thursday will be about the same as
Wednesday as another weak upper trough swings through for a chance
of showers, mainly in the afternoon. High pressure aloft will try
to build over the area on Friday for some drying. There will be
enough residual moisture and lingering instability for isolated
showers again, mainly in the afternoon. Highs will be a few
degrees below normal each day. Schneider
.LONG TERM...Previous discussion...It looks like dry weather on
Saturday may be asking too much of Mother Nature. The GFS was
faster in bringing the precipitation back into the area than the
ECMWF but models indicated a threat of precip. The question was:
How far east? Decided to compromise between the faster and slower
solutions, keeping the threat confined to the western part of the
CWA during the day Saturday.
The upper flow pattern will remain progressive during this period
for continued unsettled conditions. There will be periods of dry
weather between systems. Overall confidence in the forecast was
somewhat low for Saturday and beyond.
Not to sound like a broken record but temps during this period are
expected to be below normal.
.AVIATION...A weak upper trough over the region through Wednesday
will maintain moderate west to northwest flow aloft. The associated
weak surface front will move east of Puget Sound this evening.
Moderately strong onshore flow with a moist and showery air mass
will remain over Western Washington through Wednesday evening. The
Puget Sound convergence zone could become active Wednesday and
Wednesday evening in the prone corridor from Sea-Pae. A wind shift to
the north-northwest will occur over north Puget Sound by Wednesday
afternoon and could cause southwest winds at KBFI/KSEA to turn
westerly or switch to north-northwest for a few hours late Wednesday
afternoon and early afternoon. Opposing flow w/ wind speeds to 10
kt at KSEA/KBLI possible roughly 00z-05z early Wednesday evening.
KSEA...A front near the terminal will move east by midnight with
southwesterly winds picking up. Wind speeds were boosted a little
during an earlier amendment and still looks good for the 06z TAF
issuance. VFR cigs with showers will likely become more MVFR by
midnight through early Wednesday morning with showers decreasing. A
convergence zone could be active, nearing the KSEA/KBFI terminals by
late Wednesday afternoon. A wind shift to westerly or north-
northwesterly may occur, best chance 00z-05z early Wednesday
evening. Outside any convergence zone, cigs should generally improve
to VFR by late Wednesday morning. dtm
.MARINE...Onshore flow will increase behind a front tonight.
Expanded small craft winds to most waters for post frontal onshore
west-southwesterly winds. A stronger westerly push is anticipated in
the central/east Strait, thus a gale warning is up for Wednesday
afternoon through much of Wednesday night. Small craft winds may
continue over other waters near the east entrance to the Strait. dtm
PZ...Small Craft Advisory until midnight PDT Wednesday night for
Admiralty Inlet-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-
Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory until 11 AM PDT Wednesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of
Juan De Fuca.
Gale Warning from 11 AM Wednesday to 3 AM PDT Thursday for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until 8 AM PDT Wednesday for Puget Sound
and Hood Canal.
You can see an illustrated version of this discussion at