Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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345
FXUS66 KSEW 230336
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
836 PM PDT SUN MAY 22 2016

.SYNOPSIS...An weakening upper trough will result in a mix of
scattered light showers and sunbreaks through Monday. A weak upper
ridge will shift through the area Tuesday and Wednesday. Another
upper trough will move down over the Pacific Northwest for Thursday
through Saturday.

&&

.SHORT TERM...Radar shows showers currently fading with the loss
of daytime heating and becoming more confined to the mountains.
Monday looks similar to today. A weak upper trough over the area
will bring a few showers and some afternoon sunshine. Weak high
pressure aloft should give a mainly dry day Tuesday although
isolated showers will probably bubble up over the Cascades in the
afternoon. Wednesday looks about the same as Tuesday. Temperatures
will be near normal through Wednesday. Schneider

.LONG TERM...Previous discussion...Another upper trough will drop
into the region after the middle of the week--for a return of some
shower activity. The GFS and ECMWF are in pretty good agreement
through Thursday but diverge a great deal over the weekend on how
the upper low evolves and where it drifts.

&&

.AVIATION...The lingering upper level trough will remain the
dominant weather factor over W WA tonight and into Monday as the
associated upper level low slowly treks eastward...currently
centered in southern Alberta crossing into Saskatchewan Monday
morning. As it does so...this should weaken the influence of the
trough.

Cigs generally VFR this evening and breaking up in some
locations...revealing some peaks at the sunset. This will actually
be the case into early tonight...but a weak shortwave disturbance
coupled with a secondary low off the Pacific coast and dipping down
into OR will import another round of moisture...resulting in
increasing clouds overnight and possibly some isolated light showers
or sprinkles. Locations that do see precip may see their cigs drop
into upper-end MVFR...but that condition is not expected to be
widespread. SMR

KSEA...VFR conditions are expected to continue through 06Z with a
predominant SCT-BKN050-070 layer. Surface winds will be SW 5-12 kt.
Kam/SMR

&&

.MARINE...A weak surface trough just W of the WA coast this
evening will drift S over the NW Oregon coastal waters Monday
morning then dissipate. The presence of the surface trough just
offshore will weaken the onshore flow tonight and this should help
to rule out any SCA winds within the CWA. The exception...as
always...may prove to be the strait as obs at Race Rocks are inching
close to SCA criteria...so that may need to be watched...but current
thinking is that winds will still fall short...even if just barely.
While no SCA is planned...would not rule out a short-fused one for
at leas the central strait.

Once the surface trough dissipates Monday afternoon a surface ridge
centered offshore along 140W will become the dominant feature. The
offshore surface ridge will build closer to the WA
coast...increasing the W-SW onshore flow Monday night. This onshore
flow pattern will continue the rest of the week, with SCA W winds
expected to return to the strait during the evenings the rest of
this week. Kam/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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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