Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 252215
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 AM PDT WED MAY 25 2016

.SYNOPSIS...An upper trough will bring some showers at times to
Western Washington through the Memorial Day weekend. An upper
ridge with sunnier warmer weather will arrive starting next Tuesday.

&&

.SHORT TERM...An upper trough will work over the region for the
holiday weekend and it will bring stronger onshore flow, deeper
moisture, and a good chance of showers. The Puget Sound convergence
zone area and the mountains and foothills should have showers at
times. The UW wrfgfs has the most showers north and east of Seattle
late Thursday night through Friday morning and then some PSCZ activity
at times. There could be a general increase in showers for the coast
on Saturday, with showers increasing and spreading to the interior,
including the Seattle metro area, later Saturday afternoon through
evening. That is what the mesoscale model shows. There are fewer
showers in the GFS but the idea of some sort of disturbance arriving
on the coast Saturday morning and then showers developing around
the metro area later in the day is is also there. Looking just at
the 500mb heights would suggest the best chance of showers would
be Thursday night with the trough axis and fewer showers on Saturday
with just westerly flow aloft and a hint of a little pause in the
troughiness, but there is an odd surface low pressure area that
comes ashore Saturday afternoon and evening so that may very well
be the wetter period.

.LONG TERM...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
remarkalby 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...Weak westerly flow will persist over Washington tonight
until an approaching upper trough sharpens the flow on Thursday.
Onshore flow will continue at the surface. The air mass is stable
with low level moisture over most areas. Marine stratus is breaking
up somewhat this afternoon and some areas, mostly north, will have
several hours of just scattered clouds this evening.

Remaining ceilings are VFR around 3k ft. Further lifting and
clearing will occur this evening, then marine stratus will spread to
all areas over night with MVFR ceilings.

KSEA...Ceiling 3-4k ft may scatter out for a few hours this
evening, then increasing marine stratus will bring the ceiling down
to MVFR by 10z. Southwest wind 8-12 kt will swing around to west or
even northwest late in the afternoon, then return to southerly
overnight. CHB

&&

.MARINE...Onshore flow will continue through the end of the week.
Gale force westerlies are possible this evening in the strait but
for now have kept the small craft advisory with a forecast of 20-30
kt. Small craft advisories are also in effect this evening in
Admiralty Inlet and the northern inland waters.

Gradients increase a bit Thursday night so have issued a gale
watch for the strait. The onshore pattern will repeat Friday at
least and could continue into next week. CHB

&&

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