Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 061719
AFDSEW

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

.SYNOPSIS...Cooler and drier air will be over Western Washington
through Wednesday and there will be Fraser River outflow in the
north. Easterly gap winds will increase Wednesday night. A front
will reach the area Thursday. Cool wet weather is likely through the
weekend.

&&

.SHORT TERM...A cooler drier air mass has arrived and that will give
Western Washington mostly clear weather through Wednesday. Fraser
outflow will keep breezy cold weather over Western Whatcom county
and into the San Juans. Easterly winds will increase Wednesday night
and Thursday--especially in the mountain foothills and passes. A low
pressure system and front will bring precip to Western Washington
starting around mid afternoon on Thursday with snow turning to rain
Thursday night--although the UW wrfgfs still has snow west of Port
Angeles out through Sekiu and Neah Bay, and areas of snow in Western
Whatcom county--with a good chance of some freezing rain for that
area. That is because the model still has breezy NE winds still
coming down the Fraser and easterlies out through the Strait. But
most of the coast by that time is in mild southwest flow behind the
front. Strangely, the model turns the precip from snow to rain
before the southerly wind breaks into the Puget Sound area, but you
can`t hang your hat on these sort of details two or three days away.

.LONG TERM...An active weather pattern is likely with a tricky
balance between the northern and southern streams--there will likely
be blocking around Alaska. This should make for a low snow level and
give the lowlands periodic flirtations with snow. Fraser outflow
could continue at times through the weekend but probably will not be
especially strong. Looking at the 12z GFS...there is a chance that a
weather system around Sunday night will be problematic for Western
Washington as the modified arctic front could push back south into
Western Washington at that time. The 00z ecmwf looks like there
could be light to moderate outflow through the weekend into the
Bellingham area, but keeps southerly gradients and milder weather
for most of Western Washington with no shift south. Then around 12z
Tuesday of next week the euro takes a southern stream low pressure
center and frontal system into Oregon--and Fraser outflow resumes as
the arctic front strengthens.


&&

.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...
.AVIATION...Dry northerly flow aloft will continue over W WA today
as the upper level shortwave trough over E WA departs and an upper
level ridge offshore approaches the region. A surface high over B.C.
will maintain cold dry northerly low level flow. The air mass is
drying out with mostly clear skies in most areas. There is still
some local stratus hanging around for spotty IFR or MVFR conditions,
mainly from Tacoma southward. The stratus is expected to be gone by
20Z, leaving good VFR conditions throughout W WA.

KSEA...Clear skies today. The FEW-SCT003 stratus should be gone by
19Z. Surface winds will be N 5-10 KT today then become NE tonight.
Kam

&&

.MARINE...A surface ridge over B.C. will gradually weaken through
tonight...allowing the northeasterly offshore flow over W WA to
gradually diminish. The gale warning for the north inland waters is
being replaced by a SCA this morning. Other SCA for the coast and
parts of the strait are set to expire later today as winds ease.
Models still indicate marginal SCA winds over the north inland
waters tonight north of the San Juans so that SCA will continue.

The big issue is the evolution of the low currently offshore W of
the N CA coast near 40N/145W. Models agree that the low will drift
slowly NE toward the Pacific Northwest over the next few days.
Models have been in disagreement and inconsistent as well. What
seems reasonably certain is that easterly offshore flow will
increase to gale levels Wednesday evening and continue through
Thursday night as the associated front moves NE across the area.
What is much less certain is the track and strength of the surface
low which will impact winds late Thursday and Friday. A gale watch
is in effect for Wednesday night through Thursday night for the
coast and parts of the strait. Kam

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.

PZ...Small Craft advisory coast...west entrance...central strait
      and north inland waters.
     Small craft advisory for rough Grays Harbor bar.
     Gale Watch coast...west entrance...central strait from
      Wednesday evening through late Thursday night.

&&

$$


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