Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 180329
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
829 PM PDT Sun Sep 17 2017

.SYNOPSIS...Onshore flow will continue tonight with showers mainly
in the Cascades and north sound. An upper trough and additional
weather systems will keep the weather cool and showery into the
middle of the week. Dry weather is likely late in the week.

&&

.SHORT TERM...A big change in the weather today as a cold front
brought measurable rain to the region. Totals so far ranged from
0.50-1.00" along the coast to 0.10-0.25" in the interior. We`re in
post-frontal onshore flow now with showers mainly in the Cascades
and north sound (where there`s low level convergence). Showers
will taper down late tonight.

The air mass will remain cool, moist and unstable through Monday
as an upper level low moves over the Pac NW. The cooler air mass
will keep temps in the 50s to lower 60s. Expect more showers with
snow levels in the mountains dropping to around 5,000 feet.
Models are showing an increase in lift and moisture with a trough
axis Monday night. Places like Paradise at Mount Rainier may see
up to 6 inches of snow. This snowfall total satisfies our early
warning criteria and a Winter Storm Watch is in effect.

The next system moves inland Tuesday night into Wednesday but
mainly passing just to our south. However, we should still see
some precip clipping the area. Temperatures will continue to trend
below normal. 33

.LONG TERM...Previous discussion...Drier northerly flow aloft
will develop over Western Washington Thursday. It looks like
things will dry out entirely around Friday and remain dry into the
weekend with northerly flow aloft continuing. Schneider

&&

.AVIATION...A cold front will continue to move eastward this
evening. Current IR satellite imagery puts the front on the east
side of the Cascades at the time of this writing while post frontal
rains remain over the Cascade foothills and up into the Cascade
crest. For W WA in the wake of the front...current radar shows
scattered showers starting to pop up over the coastal waters and
move inland and there looks to be a weak Puget Sound Convergence
Zone trying to set up over Snohomish county. This activity will make
cigs tricky tonight...but general trend looks like most locations
will either be low-end VFR or high-end MVFR...however locations that
see showers may dip down into IFR conditions occasionally.

Southwesterly flow aloft will turn more westerly by Monday afternoon
as the upper level low draws closer to W WA and passes just to the
north. Low level onshore flow will gradually ease overnight
tonight...but will remain in place for Monday. The air mass will
remain moist and will gradually destabilize Monday morning and into
the afternoon and early evening...allowing for scattered showers and
the possibility for an isolated thunderstorm.  SMR

KSEA...Scattered showers will still be possible near the terminal
tonight...although it looks like the PSCZ will remain to the north.
Scattered showers remain on tap for Monday. Winds will generally
remain southerly at speeds ranging 7-11 kts. SMR

&&

.MARINE...Onshore flow in the wake of the cold front will ease
overnight...allowing for the majority of SCAs to expire with the
evening forecast package. Inherited SCA for coastal waters and
Central and Eastern Strait goes out til 3 AM Monday morning...but
current obs actually support taking those down as well. More breezy
weather is possible Monday through Wednesday at times--as
disturbances move through the upper level trough over the area. Will
evaluate incoming 00Z model data to see if additional headlines are
warranted in the near term. It looks like the strongest gradients
and more blustery weather will head into Oregon, but advisories will
probably be up at times--mainly for the coastal waters.  MM/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Monday afternoon through Tuesday morning
     for Cascades of Pierce and Lewis Counties.

PZ...None.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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