Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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371
FXUS66 KSEW 260407
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
906 PM PDT WED MAY 25 2016

.SYNOPSIS...Continuing low level onshore flow help keep western
Washington mostly cloudy with highs in the 60s through this weekend.
At the same time an upper level trough will maintain a chance of
showers over the area. An upper level ridge is expected to bring
more sunshine and warmer weather Tuesday and Wednesday.

&&

.SHORT TERM...The large surface high over the NE Pacific will
maintain cool moist low level onshore over W WA through Friday
night. In addition, an upper level trough will remain over the
region Thursday through Sunday. Both features will contribute to
keeping W WA weather close to normal for late May and June.

This is actually kind of our doldrums period where weather systems
are quite weak and the dominant feature is most often the onshore
flow. The day-to-day weather usually stays pretty much the same,
marine stratus keeping the area mostly cloudy with some afternoon
sun breaks. Passing upper level troughs may bring a chance of light
showers every once in a while, and max temperatures get stuck in the
60s.

In the details for the next few days, the upper level low over the
AK panhandle this evening will slide SE over Vancouver Island
Thursday afternoon, then over W WA Thursday evening. Between the
increasing onshore flow and the approaching trough the moisture over
W WA will be deeper and shower activity will increase. It looks like
a Puget Sound convergence zone will set up by Thursday evening,
starting out around Everett then sagging S overnight. This doesn`t
look like a widespread shower event so although there is a chance of
showers everywhere, the best chance will probably be in the
convergence zone. Could see nudging up POPs to the low end likely
range to cover the convergence zone.

There might be a minor lull in the light shower activity Thursday
night and Friday morning as the departing upper level trough moves
over E WA. However, the GFS shows shower activity increasing again
Friday afternoon as the left exit region of an 80 kt upper level
jetstream moves over the area. The convergence zone will probably
remain active Friday and Friday night as well. It looks like a weak
upper level shortwave trough will maintain chance of showers on
Saturday. Kam

.LONG TERM...Previous discussion from the 315 pm AFD. 500mb heights
fall a bit Sunday and Sunday night as the upper trough redevelops
for a short period. That is probably going to mean a showery
Memorial Day in some areas--once again the PSCZ and Cascades more
likely to see showers. The 12z GFS shows remarkably little
precipitation on Monday despite what looks like a good PSCZ pattern
and the ECMWF shows faster upper ridging next week, so the forecast
may yet change for the better. By Tuesday and Wednesday sunnier dry
weather looks likely.

&&

.AVIATION...An upper level trough along the B.C. coast will dig
southward tonight and brush Washington Thursday and Thursday night.
Light westerly flow aloft...becoming moderate northwesterly flow
Thursday afternoon and night. Low level onshore flow persist. The
air mass is stable with moisture mostly in the lower
levels...becoming somewhat unstable by Thursday evening. A Puget
Sound convergence zone is likely Thursday night...possibly affecting
the KPAE/KSEA/KBFI terminals with shifting winds and showers.

Cigs from KSEA northward are mostly VFR 3-3500K FT this
evening...with a mix of MVFR 2-2500K FT or VFR 3K FT elsewhere. MVFR
12-1800 FT becoming more widespread after 08z. Patchy drizzle
possible in the morning with vis temporarily down to 4-6SM...mainly
toward the coast. Cigs lift to mainly VFR stratus by Thursday
afternoon.

KSEA...Cigs mainly 3-3500K FT through midnight. MVFR cigs around
1500K FT developing 08-09z. Southwest wind 7-10 KT this
evening...then 8-12 KT w/ gusts 20 KT. Winds swinging around to west
or even Northwest Thursday evening with convergence zone showers
over or just north of the terminal. DTM

&&

.MARINE...Varying degrees of onshore flow will continue through the
end of the week. Gale force westerlies are likely through at least
midnight tonight in the Central-East Strait and again Thursday
evening. Small craft advisory winds are likely in the south part of
the North Inland waters and the north part of Admiralty Inlet during
periods of stronger westerly flow in the Strait. The onshore pattern
will repeat Friday at least and could continue into next week. DTM

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory for the Outer Coastal Waters...Central and
     Eastern Strait...Northern Inland Waters...Admiralty Inlet.

&&

$$

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