Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 300441
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
940 PM PDT Sat Apr 29 2017

.SYNOPSIS...Rain will change to scattered showers by late this
evening after a cold front moves across Western Washington. Onshore
flow will maintain scattered showers on Sunday, mainly over the
coast, mountains, and in a Puget Sound convergence zone. On Monday,
a weak weather system will brush the south part of the area with a
few light showers. An upper level ridge will build over the region
Tuesday through Thursday for warmer generally dry weather, but a
little light rain Tuesday night and Wednesday is still possible. A
weak front may bring some rain Thursday night, followed by an upper
level Friday and Saturday.

&&

.SHORT TERM...RADAR shows the rain band from the cold front crossing
the area over central Puget Sound at 830 PM. At this rate the rain
band should exit the lowlands by midnight with just a few showers
moving inland behind the band. Surface obs and models still show the
surface front near the coast. It will move across Puget Sound late
this evening but with nothing more significant than a wind shift to
W-SW and slightly stronger winds.

The shift to westerly onshore flow will bring Puget Sound
convergence zone conditions late tonight into Sunday night. The
mesoscale models don`t really have much consensus regarding movement
of the PSCZ, but it will probably spend a bunch of time over N King
county and then eventually work it`s way down over Seattle. Outside
of the convergence zone showers are more likely over the coast and
mountains. The showers should diminish later Sunday evening as a
surface ridge moving over the area weakens the onshore flow.

Models are still having trouble with a weak system on Monday. The
strongest part of this weak system still looks like it will move
over N Oregon, but one or more very weak upper level shortwave
troughs moves across W WA. There is a chance of showers Monday as
the system moves through, precip is more likely over the S half of W
WA but the latest GFS shows a little over the N half of W WA as well.

Models are in good agreement that an upper level ridge will build
over W WA on Tuesday and then remain over the region through
Thursday. Bad news is that the models are vacillating between light
rain or dry late Tuesday through Wednesday morning. Despite warming
from the ridge over the area, the 00Z GFS has enough flow through
the ridge to allow an area of warm advection to bring some light
rain across W WA. The older 12Z ECWMF keeps the warm advection just
offshore, barely brushing the NW part Wednesday morning. The 00Z NAM
teams up with the GFS. Kam

.LONG TERM...Previous discussion from the 330 PM AFD...An upper
ridge will probably keep Wednesday and Thursday dry. However the
models are not quite dry so have kept slight chance pops in some
areas. On Thursday night and Friday an upper trough, seen in both
the GFS and Euro, will lower heights and bring moisture. There may
be some drying on Saturday.

The good news is the upper ridge will probably bring some highs in
the 70s on Wednesday and Thursday, possibly including the first 70
degree day this year at KSEA. The upper trough will lower temps to
around 60 Friday and Saturday. Burke

&&

.AVIATION...An upper trough over the region will shift east on
Sunday. The associated surface front near the coast at 9 PM will
push through Puget Sound roughly around midnight. Moderate southwest
flow aloft becoming light west-northwesterly Sunday. Moist air mass
and stable...becoming weakly unstable later tonight and Sunday. Some
drying aloft on Sunday but low and mid level moisture may produce
isolated showers with a possible Puget Sound convergence zone from
KPAE to just north of KSEA/KBFI Sunday...possibly reaching the KBFI
terminal early Sunday evening before dissipating. Mostly MVFR with
patchy IFR and rain until the front passes, then areas of MVFR
generally improving to VFR by late Sunday morning.

KSEA...Winds should switch from westerly to more southerly to 10 kt
through midnight, then switch to southwesterly 12-15 kt w/ gusts 24
kt behind the front. Winds will ease slightly on Sunday. The
convergence zone may approach the terminal from the north by early
Sunday evening but only has a 25 percent chance of causing a
northerly wind shift. dtm

&&

.MARINE...A front near the coast this evening will push through the
inland waters roughly around midnight. Small craft winds along and
behind the front are likely for all waters. Post frontal flow will
cause westerly winds in the central/eastern Strait to reach up to 30
kt later tonight/Sunday. A brief gale is possible but small craft
strength winds will dominate. Winds in Admiralty Inlet and the north
inland waters will be strongest near the eastern entrance to the
Strait of Juan De Fuca. Small craft winds in Puget Sound will
probably on occur for a few hour near and behind the front tonight.
Winds in the Sound will ease on Sunday and switch to a northerly
direction north of Seattle in the afternoon. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory all waters.

&&

$$

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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