Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 210342
Area Forecast Discussion
National Weather Service Seattle WA
842 PM PDT FRI MAY 20 2016
.SYNOPSIS...Showers will probably increase over the weekend as a
disturbance moves into the area. An upper level trough will remain
over the region for typical May weather into next week.
.SHORT TERM...An upper low over southern Oregon will move slowly
east and then northeast. The low will wind up over the southern
prairie provinces of Canada by Sunday and Monday. Wrap around
moisture from this upper low and slightly unstable air will bring
showers at times to western Washington.
Currently, showers over the Cascades are generally fizzling as
they move west into the lowlands. There has also been a
noticeable decreasing trend on radar in the last hour or so as
diurnal heating ends.
Models show showers increasing from the west after midnight
tonight as an apparent upper disturbance rotates through. Showers
will be likely from about Puget Sound eastward Saturday morning.
There will be a general decreasing shower trend Saturday afternoon
and evening as the short wave exits.
After that, the upper low will keep a chance of showers in the
forecast through Monday but there does not appear to be anything
to focus convection for any particular time or location. The best
chances will be afternoons and early evenings over the Cascades.
.LONG TERM...Previous discussion...Upper troughiness and low
level onshore flow will keep a chance of showers in the forecast.
There will be sunny periods, mainly in the afternoon, and most of
the showers will be over the Cascades. High temps will mostly be
in the 60s for the lowlands.
.AVIATION...A large upper level low will remain over the
PacNW through Sunday. The low center will remain down over N CA and
S Oregon through tonight with an upper level trough extending N over
WA. Flow aloft over W WA will be light E-SE. The airmass will remain
moist but stabilize as the night progresses. Earlier afternoon
cumulus have diminished more into flat stratus to provide a visual
indicator of this increase in stability.
A weak upper level shortwave trough will spread moisture westward
across the Cascades late tonight with light rain or showers possibly
reaching the Puget Sound lowlands very early Saturday morning. This
will result in areas of MVFR cigs redeveloping after 12Z.
The upper level low will weaken Saturday night and Sunday as the
center drifts up over the northern Rockies, but W WA will still
remain under the west part of the broad low.
KSEA...Afternoon cumulus layer continues to dissipate this evening
as the air mass becomes more stable. Surface winds will remain N-NW
4-10 kt through 06z, then become NE 3-6 kt late tonight.
A weak trough will spread mid level clouds westward over the area
around 07Z with CIGS gradually lowering to around BKN050 around 10Z.
Some lower stratus is expected to form around 14Z with MVFR cigs.
Some light rain or showers should develop. Kam/SMR
.MARINE...A weak surface trough will form over the N interior this
evening with slightly higher pressure along the coast. Models agree
that weak westerly flow will develop in the Strait of Juan de Fuca
this evening with speeds reaching borderline SCA levels in the
central and east strait. At this time...obs from that area are not
near SCA levels. Will leave inherited SCA up as a safety measure
should winds increase...but with obs where they are...may update to
remove headline before early morning forecast package issuance. Weak
W-SW onshore flow will develop across the rest of W WA tonight. The
surface trough will deepen into a weak low over Vancouver Island by
Saturday evening with W to SW onshore flow continuing over W WA. The
onshore flow should be a little stronger Saturday evening with high
end SCA winds likely in the central and east strait. Higher pressure
offshore will continue through early next week allowing varying
degrees of onshore flow to continue.
PZ...Small Craft Advisory until 5 AM PDT Saturday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at