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Area forecast discussion 
National Weather Service Seattle Washington
325 am PST Tuesday 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 Delaware fuca zone
which includes Port Angeles where it seems that precipitation continues to
fall...with latest observation indicating that this precipitation is taking the
form of snow (pic0). A Winter Storm Warning remains in effect there
through 9 am and given this most recent batch of observation...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 observation from the northern tier
of the County Warning Area 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 British Columbia will allow for Fraser River
outflow to be dominant weather feature today (pic1) 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 County Warning Area will make for a rather
cold couple of days ahead with high temperatures each day only in the middle
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 (pic2) 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 European model (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 middle-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
middle-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 (pic10), and it will continue today
and tonight as an offshore upper ridge moves toward the region
(pic11). 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 (pic12). 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 feet 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 knots with higher gusts north of Bellingham and
across the northern San Juans this morning.

Ksea...north to northeast wind 5-10 knots 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 (pic13). Fraser River outflow is producing
northeast winds to around 30 knots 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 Delaware fuca and across the
northern coastal waters.

Easterly winds will begin to rise again Wednesday (pic14) 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 (pic15) through Thursday night (pic16). 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 Delaware 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...
Washington...Winter Storm Warning until 9 am PST this morning for eastern
Strait of Juan Delaware 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 Delaware fuca-West Entrance
U.S. Waters Strait of Juan Delaware fuca.

Small Craft Advisory until 3 PM PST this afternoon for central
U.S. Waters Strait of Juan Delaware fuca-West Entrance U.S.
Waters Strait of Juan Delaware fuca.

&&

$$

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