Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181005

Area Forecast Discussion
National Weather Service Seattle WA
305 AM PDT Mon Sep 18 2017

.SYNOPSIS...Chilly low pressure aloft will bring wet and unsettled
conditions to the region through Tuesday. There will be a slight
risk of thunderstorms over parts of the area today. Conditions
will gradually improve on Wednesday as high pressure aloft begins
moving in from the west. Temperatures will be below normal through


There were scattered showers over mainly the southwest half of the
CWA at this time. There have been a few lightning strikes with the
cells over the offshore waters.

Expect the thunderstorm threat to increase over the WA coastal
waters this morning as the cold pool aloft (temps near -25 degree
Celsius) moves over the area. The air mass will destabilize over
much of the CWA during the day, but especially over the coast,
southwest interior, and the southern Puget Sound Region where
isolated thunderstorms are a possibility. The heavier showers will
likely contain small hail, possibly enough to cover the ground.
The thunderstorm threat should end after sunset except over the
coastal waters.

Meanwhile, a chilly upper level trof will prevail over the Pacific
Northwest for cool, showery weather today. The showers will be
most numerous during the afternoon but since the air mass will
begin destabilizing as early this morning, thought it would be a
good idea to not try to time them. There should be glimpses of
sun between showers.

A disturbance will move across the CWA tonight. The main impact
will be a subtle increase in the shower activity the first half of
tonight or ahead of the disturbance. There is a possibility of
locally heavy snow in the south central Cascades above the 5000
foot level tonight into Tuesday morning. Paradise on Mount
Rainier could receive nine inches of new snow tonight; therefore,
a Winter Storm Watch for the Cascades in Pierce and Lewis Counties
will remain in effect.

The flow aloft will become zonal or westerly on Tuesday. The models
were in disagreement concerning the details, so confidence in the
forecast was not high. The GFS was drier than the ECMWF;
therefore, chose to compromise between the two and broadbrush
the forecast.

Another upper level low will approach the region Tuesday night.
The models have been taking this system farther south with each
cycle. The GFS was the farthest south and was drier for Tuesday
night. Chose to compromise between the farther northward and
wetter ECMWF solution and the GFS. It did appear that the occluded
front will move across the far southern part of the CWA, and this
should keep the strongest winds south of the area.

The models were in agreement that conditions will improve on
Wednesday as the upper trof moves slowly east.

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...An upper level trough moving through the area will allow
for scattered showers over W WA this morning. Southwesterly flow
aloft will turn more westerly by this afternoon as the upper level
low draws closer and passes just to the north. Low level onshore
south to southwesterly flow will will remain in place today. The air
mass will remain moist and will gradually destabilize this morning
and into the afternoon and early evening...allowing for scattered
showers to continue as well as the possibility for an isolated
thunderstorm. This activity will make cigs tricky today...but
general trend looks like most locations will either be low-end VFR
or MVFR...however locations that see showers may dip down into IFR
conditions occasionally.  SMR

KSEA...Scattered showers will still be possible near the terminal
today with best chances being this afternoon as the atmosphere
becomes more unstable. Winds light and variable this early morning
but otherwise will generally remain southerly at speeds ranging 7-11
kts. SMR


.MARINE...Onshore south to southeasterly flow will continue today as
low pressure approaches the coast. Winds look to be strongest over
the coastal waters where SCA speeds are expected to develop this
afternoon. Speeds for the inner coastal waters are expected to drop
off tonight while said speeds are expected to linger over the outer
coastal waters into Tuesday. Thus...SCAs from evening forecast will
remain in place. More breezy weather is possible Tuesday and
Wednesday at times--as disturbances move through the upper level
trough over the area. Models remain on track that the strongest
gradients and more blustery weather will head into Oregon, but
additional advisories may be needed--mainly for the coastal waters.


WA...Winter Storm Watch from this evening through Tuesday morning
 for the Cascades of Pierce and Lewis Counties above 5000

PZ...Small Craft Advisory from 11 AM this morning to 11 AM PDT
     Tuesday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 11 AM this morning to 11 PM PDT this
     evening for Coastal Waters From Cape Flattery To James
     Island Out 10 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater Out 10 Nm.



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