Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191031
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
331 AM PDT Tue Sep 19 2017

.SYNOPSIS...Low pressure aloft will remain over the region through
Wednesday for cool and unsettled conditions. High pressure aloft
will gradually move over the area from the west the latter part of
the week for a drying and warming trend.

&&

.SHORT TERM...
There were quite a few showers over the CWA at this time. A few of
these showers were heavy and likely contained small hail. There
has also been lightning strikes off the coast of Grays Harbor
County. Therefore, went on ahead and put thunderstorms in the
forecast for the far southwest part of the forecast area today.
The air mass should stabilize by early this evening.

The temperature has been hovering at 32 degrees at Paradise for
the past several hours. It is likely that the precipitation has
been falling as snow during these hours. Unfortunately, the snow
sensor appeared to be out of service. It looks like about four
inches, possibly five inches, of snow has fallen at Paradise thus
far, per the liquid accumulation. The temp should stay near
freezing through much of this morning, so went on ahead and issued
a Winter Storm Warning for the Cascades above 5000 feet in Pierce
and Lewis Counties.

Meanwhile, a broad area of low pressure aloft will keep the air
mass over the region cool and unsettled through Wednesday.
Overall, it appeared that the heaviest precipitation will occur
over the southern half of the CWA during this time frame. Temps
will remain below normal.

Heights/thicknesses will gradually rise Wednesday night through
Thursday as the upper level trof slowly slides off to the east.
This will allow northerly flow aloft to become established over
the region. However, the flow will remain weakly cyclonic. This
combined with lingering moisture will result in a chance of
showers through Thursday.

.LONG TERM...
There might be a few light showers over the interior on Friday
under continued weak cyclonic flow aloft; otherwise, expect an
upper level ridge to build over the area from the west on Friday
for a drying trend.

An upper ridge over the northeast Pacific will give dry northerly
flow to Western Washington Saturday through Monday. Highs will
warm back to near normal. Schneider/05

&&

.AVIATION...Upper level trough over the area with northwesterly
flow aloft. Air mass moist and unstable with onshore flow.

Multiple cloud layers over Western Washington between 2000 and
6000 feet with ceilings near 2000 feet becoming predominate after
12z. Ceilings improving into the 3500 to 4500 foot range this
afternoon into the evening hours. Thunderstorm possible near the
Coast throughout the day.

KSEA...Ceilings near 2000 feet improving to near 4000 feet this
afternoon. Could see visibilities briefly lower to 3-5sm in
showers. Southerly winds 5 to 10 knots through Tuesday afternoon.
Felton

&&

.MARINE...Low pressure will remain over the region through
Thursday. A trough will move the area late tonight and Wednesday
morning. Strongest winds with this trough will be south of the
area. Winds will becoming light northerly on Wednesday as low
pressure passes to the south. High pressure will build into the
area Friday into Saturday.

Swell over the coastal waters in the 12 to 14 foot range will
subside below 10 feet tonight and remain below 10 feet into the
weekend. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 5 PM PDT this afternoon for Cascades
     of Pierce and Lewis Counties above 5000 feet.

PZ...Small Craft Advisory until 11 AM PDT this morning 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 for hazardous seas from 11 AM this morning
     to 5 PM PDT this afternoon 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 for rough bar until 8 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 5 PM PDT this
     afternoon 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-West Entrance U.S. Waters 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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