Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 061122
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
325 AM PST Tue Dec 6 2016

.SYNOPSIS...Fraser River outflow will be responsible for colder and
drier weather today and Wednesday. A system Thursday could produce
windy conditions along with the possibility of some lowland snow
before the precipitation changes to rain. Cool and showery weather
will continue through the end of the week but temperatures will
remain above freezing.

&&

.SHORT TERM...Radar this early morning is generally clear with one
exception...that being over the East Strait of Juan de Fuca zone
which includes Port Angeles where it seems that precip continues to
fall...with latest obs indicating that this precip is taking the
form of snow. A winter storm warning remains in effect there through
9 AM and given this most recent batch of obs...looks good to leave
that warning up for its duration. One last lingering problem this
early morning are scattered reports of fog and whether or not to
include them in forecast. While...as stated...it is indeed occurring
now...a combination of current obs from the northern tier of the CWA
and model trends suggest that as northerly air from Fraser outflow
pushes further into the area...conditions dry quickly. Case in point
being Arlington airport which went from 1/2 mile visibility to 10
miles in the span of an hour. Expecting similar results across the
area...will likely leave wording out of morning forecast package as
the next two to three hours are likely to be the last gasps of said
fog before drier and clearer conditions prevail.

Upper level ridging over the Pacific...surface low pressure over
Oregon and surface high pressure over BC will allow for Fraser River
outflow to be dominant weather feature today and Wednesday. The good
news is that skies will clear and the denizens of Western Washington
will once again catch a glimpse of the mythical giant orb in the sky
that provides light. One ordinarily adds heat to that phrase...but
alas...arctic air over the CWA will make for a rather cold couple of
days ahead with high temps each day only in the mid 30s and
overnight lows in the 20s.

Next system starts to push into the area from the south starting
early Thursday morning. Given recent mentioning of snow in
forecasts...there appears to be a fixation on the prospect of
lowland snow...which will be addressed shortly...at the risk of
overlooking the potential...and more likely...wind threat for the
day. Pressure gradients over Western Washington remain very tight
for much of the day Thursday as a strong surface low rides up the
Pacific coast and thus...has the potential to create windy
conditions. That said...there is something a bit off about it as the
GFS almost seems as though there is a frame missing in the data as
gradients very suddenly relax Thursday evening. The ECMWF does not
help this uncertainty given its lack of spatial and temporal
resolution during this particular time period. That all being
said...winds still a forecast concern for much of the day Thursday.
All right...now to the prospect of lowland snow. With warmer air
likely overriding the cold arctic air at the surface...there is a
chance for some lowland snow when things get started by mid-morning
Thursday. But with 500mb heights rising through the day...the threat
looks to be short-lived. The biggest impact looks to be that this
mid-morning threat of lowland snow looks to coincide with morning
commute. Plenty of questions remain unanswered regarding this system
and thus will take no action with AM package...will leave that for
day shift.  SMR

.LONG TERM...Winds look to relax late Thursday night through the
remainder of the week and temperatures look to rebound back into the
40s. The trade-off being that the sun goes away for another
prolonged stretch of showery weather that looks to linger for the
bulk of the weekend as a series of systems pass through the area.
These systems look to result in several inches of new snow in the
mountains through the period. SMR

&&

.AVIATION...Northerly flow aloft has developed over Western
Washington early this morning, an it will continue today and
tonight as an offshore upper ridge moves toward the region. The
air mass is moist, especially below 5000 ft, but it will gradually
dry today and tonight.

There is a wide range of conditions across Western Washington
early this morning. Low level moisture is resulting in areas of
low-end MVFR to local IFR ceilings, especially along the north
side of the Olympic Peninsula where showers have been persistent
overnight. Other areas have scattered out during the night, with
patchy fog starting to develop in some of the wind-protected
spots. Low clouds and patchy fog should persists -- or even
become a little more widespread -- for the next few hours, but on
the whole it will be short-lived as high pressure over British
Columbia continues to push cool dry air southward across the
forecast area. Widespread VFR conditions with ceilings at or above
12000 ft by midday is a good bet. Patchy freezing fog could
develop late tonight in wind-protected spots.

Also, the high over British Columbia will result in east to
northeast winds 15-25 kt with higher gusts north of Bellingham and
across the northern San Juans this morning.

KSEA...North to northeast wind 5-10 kt today and tonight. Low
clouds this morning should give way to VFR conditions by midday,
as cold dry northerly flow dries the air mass. The threat of snow
at the terminal has all but ended until the next system arrives
late Wednesday night or more likely Thursday morning. McDonnal

&&

.MARINE...A cold dome of high pressure over British Columbia and a
weak 1016 mb low along the north Oregon coast early this morning
has resulted in northerly offshore flow across Western Washington
and its waters. Fraser River outflow is producing northeast winds
to around 30 kt north of the San Juans. The Fraser outflow will
peak this morning, with gales still likely over the northern
inland waters and small craft advisory east to northeast winds
spreading through the Strait of Juan de Fuca and across the
northern coastal waters.

Easterly winds will begin to rise again Wednesday as a strong
Pacific frontal system approaches the region. The frontal system
will probably produce an extended period of strong gales over the
coastal waters, and possibly even storm force winds, Wednesday
night through Thursday night. There are some important model
differences, with the GFS showing deeper surface development. The
UW WRF-gfs has areas of storm force winds, but we will keep that
out of the forecast for now. I have issued a long-lead time gale
watch though for the coastal waters, west entrance Strait of Juan
de Fuca, and central Strait.

A surface trough will follow the front on Friday, and weak high
pressure will build over the region on Saturday. McDonnal

&&

.HYDROLOGY...No river flooding is expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 9 AM PST this morning for Eastern
     Strait of Juan de Fuca.

PZ...Gale Watch from Wednesday evening through late Thursday night
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until noon PST today for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm.

     Small Craft Advisory for rough bar until noon PST today for
     Grays Harbor Bar.

     Gale Warning until 9 AM PST this morning for Northern Inland
     Waters Including The San Juan Islands.

     Gale Watch from Wednesday evening through Thursday afternoon for
     Central U.S. Waters Strait Of Juan De Fuca-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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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