Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 270415
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PDT THU MAY 26 2016

.SYNOPSIS...A large upper level low will remain anchored over
British Columbia through this weekend.  A series of small weak upper
level shortwave troughs embedded in the flow around the main low
will bring scattered showers at times to western Washington.
Continuing low level onshore flow will keep skies mostly cloudy and
day time temperatures in the 50s to mid 60s. Conditions will improve
on Memorial Day as an upper level ridge builds offshore and the
large upper level trough departs eastward. Sunnier and warmer
weather will develop Tuesday and Wednesday as the upper level ridge
moves inland.

&&

.SHORT TERM...Satellite imagery shows a large upper level low has
settled over B.C. this evening, with the low center over central
B.C. and an upper level shortwave trough extending S across the WA
coast. The shortwave trough will move over E WA early Friday
morning. The large upper level low will hang around over B.C.
through Sunday evening then finally shift E Monday morning.

The upper level low combined with continuing low level onshore flow
will keep W WA mostly cloudy and cool through this weekend. Periods
of light showers will be generated by a combination of a continuing
Puget Sound Convergence zone pattern and weak upper level
features rotating around the low and crossing W WA.

For this evening, RADAR is picking up some light showers over
Snohomish county. These should continue and probably increase a
little overnight. Both the GFS and NAM indicate convergence
continuing through tonight with the zone probably staying north. It
doesn`t look like there will be much shower activity outside the
convergence zone tonight.

GFS increases the coverage of the light showers on Friday. This
could be due to the nose of a NW-SE oriented 85 kt jet streak that
moves over the area. POPs are a mix of chance and likely, with the
higher POPs over the interior with the convergence zone.

It looks like the remnants of a weak front reach the area on
Saturday for another period of more widespread shower activity. The
associated surface trough moves over the coastal waters and ends up
disrupting the onshore flow and the convergence zone for a short
time. Once the trough moves inland Saturday night, onshore flow will
return. The upper level low will keep the air mass cool enough so
that snow levels over the mountains will range from 4000-5000 feet
through Saturday. This is low enough for light snow showers to reach
the higher passes at times.

Another upper level shortwave trough will move SE over W WA on
Sunday. Kam

.LONG TERM...Previous discussion from the 330 pm AFD...the medium
range models remain in good agreement through Sunday...keeping the
anchoring upper level trough over Central British Columbia. Models
show the pattern beginning to break down Sunday with the upper level
trough beginning to eject into the interior of Canada as an embedded
shortwave moves down the backside of the larger trough. There are
some differences in the models with the timing and details of this
transition. The ECMWF shears the anchoring low apart earlier than
the Canadian and GFS...as early as Saturday night. The GFS...and
Canadian...continue to show the embedded shortwave clipping Western
Washington during the day...providing some upper level dynamic
support and enhancing showers. The overall trend holds with
improvement over the area on Memorial Day as an upper level ridge
builds over the East Pacific.
Medium range models remain in good agreement through mid week with a
rapid warm up as the upper level ridge offshore moves over western
Washington and a thermally induced trough develops along the coast.
High temperatures will rise into the 70s Tuesday for most of the
area. The upper level ridge will shift east Wednesday with weak
onshore low level flow helping cool temperatures mainly along the
coast.

&&

.AVIATION...An upper level trough over British Columbia will extend
south over Washington through Friday. moderate northwesterly flow
aloft. Strong low level onshore flow...easing Friday. Moist in the
lower levels and somewhat unstable. There is a slight risk of a
Thunderstorm tonight across the extreme north which includes the
KBLI terminal. Scattered showers elsewhere. A Puget Sound
convergence zone is possible tonight, possibly affecting
the KPAE/KSEA/KBFI terminals with shifting winds and showers.

Ceilings in marine stratus will be mostly in the 3-5K FT range
through Friday with patchy MVFR through the morning hours. However,
MVFR cigs are likely in the vicinity of the convergence
zone...mainly affecting KPAE. Outflow from strong onshore flow in
the strait will keep the wind at KPAE mostly north-northwest. A
similar wind shift is possible at the KBFI/KSEA terminals but only
for an hour or two...with the chance for this to occur decreasing
after midnight.

KSEA...Mainly VFR cigs through Friday but MVFR is possible at times
tonight and Friday morning. Southwesterly wind 8-12 Kt...possibly
becoming north-northwesterly for a brief time until around 09z. DTM

&&

.MARINE...Strong onshore flow tonight with westerly Gales
persisting in the central and east strait. Outflow from the strait
will likely give advisory strength winds to the northern inland
waters and Admiralty Inlet. The onshore flow is also strong enough
to give small craft advisory level wind to the coastal waters
tonight.

The onshore pattern will repeat Friday but with less strength. A
weak front will move into the area Saturday, bringing the onshore
flow to a temporary halt. Onshore flow will resume Sunday and
Monday. On Tuesday northerly and offshore flow is possible. DTM/CHB

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Gale warning central and east strait.
     Small craft advisory coast, west entrance, northern inland
waters, Admiralty Inlet.

&&

$$

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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