Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 252338 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
438 PM PDT Tue Apr 25 2017

Corrected some typos.

.SYNOPSIS...An occluded front offshore will move across the area
overnight. Expect the low pressure system aloft to move across the
region on Wednesday for unsettled conditions and locally gusty winds.
Unsettled weather will persist through Friday. High pressure aloft
will return Friday night before moving east on Saturday.

&&

.SHORT TERM...
Precipitation associated with an occluded front offshore has
overspread the southwest part of the forecast area. Expect the
precipitation to gradually overspread the rest of the CWA tonight.
Although, some areas (mainly downwind of the Olympic range) will
probably end up shadowed. The front will move across the area
tonight, and is expected to be east of the Cascades by sunrise.
Expect locally breezy conditions overnight ahead of the front.

The upper level trough of low pressure will move across the region
during the day Wednesday for scattered showers, more numerous
over the mountains and within a Puget Sound Convergence Zone
(PSCZ). The PSCZ will likely develop between KAWO and KPAE and
then drift south to its typical location near the Snohomish/King
county line during the afternoon. The heavier showers within the
PSCZ will probably contain small hail.

Meanwhile, the onshore surface pressure gradient will tighten
Wednesday afternoon for locally windy conditions, especially over
those areas adjacent to the central and eastern Strait of Juan de
Fuca. Strong westerly flow across the Strait of Juan de Fuca coupled
with gusty southwest winds over the central and south Puget Sound
region, in particular, will result in the development of the PSCZ
mentioned in the previous paragraph.

As far as snowfall in the Cascades was concerned, it now appeared
that amounts will stay below advisory criterion (6-11 inches in
twelve hours) through Wednesday afternoon. However, will need to
keep an eye on the areas (above 4K ft) that will be impacted by
the PSCZ.

Cyclonic flow aloft will persist Wednesday night and Thursday for a
continued threat of showers across much of the CWA. The best chance
will be over the mountains and within the PSCZ. The PSCZ may weaken
or mostly dissipate late Wednesday night or early Thursday before
redeveloping or strengthening Thursday afternoon. The PSCZ is
anticipated to be over the central Puget Sound region by Thursday
evening.

High pressure aloft will build into the region from the west during
the day Friday. There will be enough instability and moisture for a
threat of showers (albeit low) over mainly the mountains and within
a weak PSCZ. Dry conditions will return Friday night.

Temperatures during this period will be below normal.

.LONG TERM...
It looks like dry weather on Saturday may be asking too much of
Mother Nature. The GFS was faster in bringing the precipitation back
into the area than the ECMWF but models indicated a threat of
precip. The question was: How far east? Decided to compromise
between the faster and slower solutions, keeping the threat confined
to the western part of the CWA during the day Saturday.

The upper flow pattern will remain progressive during this period
for continued unsettled conditions. There will be periods of dry
weather between systems. Overall confidence in the forecast was
somewhat low for Saturday and beyond.

Not to sound like a broken record but temps during this period are
expected to be below normal.

&&

.AVIATION...A front just off the coast will spread areas of rain
into Western Washington this evening. The front looks a little
disorganized, so there will be rain at times through tonight but it
does not look wet enough to really get the cigs down or drop the
visibility except over the mountains. The flow will switch from
southeast to westerly in the wake of the front on Wednesday, with
showers and a classic strong puget sound convergence zone pattern by
afternoon.

KSEA...current TAF seems to be in the ballpark, there will be a
southerly breeze today and tonight that should not shift til
Wednesday afternoon when a strong PSCZ pattern will develop.

&&

.MARINE...Pretty good southerly winds will spread
inland with the front this evening--strong enough for small craft
advisories in the normal breezier spots. The frontal passage will
reach the coastline this evening and westerlies will move down the
Strait of Juan de Fuca overnight. The westerlies in the Strait
should reach gale force Wednesday afternoon. There will be a strong
PSCZ wind pattern in place Wednesday afternoon and night, and the
flow will ease somewhat on Thursday,

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory in effect for the coastal waters, east
 entrance to the Strait of Juan de Fuca, northern inland
 waters, and Admiralty Inlet until 5 AM PDT Wednesday.

  Gale Watch in effect from Wednesday afternoon through late
 Wednesday night for the central and eastern Strait of
 Juan de Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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