Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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466
FXUS66 KSEW 052311
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
311 PM PST Mon Dec 5 2016

.SYNOPSIS...Fraser River outflow will bring colder air tonight,
and the snow level will lower to sea level with a threat for more
snow showers. Colder and drier weather will follow Tuesday and
Wednesday. A system Thursday could also produce 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...The threat of snow will continue tonight across
Western Washington. An upper level trough will keep the air mass
cool and unstable with snow levels at the surface. Temperatures
will drop below freezing with lows in the upper 20s to lower 30s.
Most areas will only see a trace to 1 inch of snow but there is a
chance for higher snow amounts, especially in the eastern Strait
of Juan de Fuca. The setup is strong high pressure over interior
B.C. which will bring Fraser river outflow winds. This is a N/NE
upslope wind for the eastern Strait of Juan de Fuca with heavy
snow possible along the foothills of the Olympics. A Winter Storm
Warning is in effect with snow amounts up to 4 inches possible.

The Fraser outflow will bring cooler air into the north interior
with lows dropping into the mid 20s, along with N/NE winds of 20
to 30 mph with gusts to 40 mph. Snow showers are expected this
evening across western Whatcom, western Skagit and the San Juans
then push south of the area after midnight tonight.

There is also a threat of heavy snow if a convergence zone develops
in the north/central sound tonight (including Seattle) and a
Winter Weather Advisory is in effect. A convergence zone may form
where the north outflow winds meet the moist S/SW flow through the
interior. But there is high uncertainty in the models: will the
convergence zone form, where and how long? A narrow band with
heavy accumulations is possible if the convergence zone forms or
stalls.

Dry but cool weather is expected from Tuesday afternoon through
Wednesday. By then we will see pretty solid offshore flow as the
strong ridge over interior B.C. sags south and southeast into MT.
Skies will clear for chilly lows Tuesday night - lows in the 20s.
Highs on Wednesday will only be in the low to mid 30s.

Another round of lowland snow is possible on Thursday and impacts
may be more widespread, including the Cascades and passes. This
is an overrunning event where moisture moves up from the south
while the low level air mass remains cold. The likely scenario is
snow in the morning, changing over to rain in the afternoon or
evening as warmer air surges inland. But there are still questions
on when the precip type will change, which directly affects the
snow totals and impacts. In addition to snow, strong east winds
are also possible, mainly through the Cascade gaps. Lots more to
look at as this system draws near. 33

.LONG TERM...The weather pattern through the long term remains
cool and showery as a series of systems clip through Western
Washington. But temperatures, at least through early next week,
look like they will remain above freezing. The mountains, however,
should see several inches of new snow through the period. 33

&&

.AVIATION...An upper level shortwave trough over Vancouver Island
22Z will move SE over W WA around 09Z then move over E WA around 15Z
Tuesday. Moist and somewhat unstable W flow aloft will continue
through 09Z then shift to a drier northerly flow as the trough axis
moves east of the area. A strong surface high is in the process of
building over B.C. which will influence surface winds especially
over NW WA tonight.

Satellite imagery and RADAR show a lot of showers moving across W WA
at 22Z. The greatest activity was along the coast and north and
south of Seattle. The Olympics are shadowing the central Puget Sound
region from the worst of the showers. The lower air mass remains
moist and MVFR conditions with local IFR conditions prevail. Surface
temperatures have warmed enough so that snow showers have ended for
now. MVFR and IFR conditions will continue tonight and cooling
temperatures will allow showers to change back to snow this evening.

The building surface ridge over B.C. will allow E-NE surface winds
to develop over the N interior tonight, mainly 15-25 KT and higher
gusts around and north of BLI and over the northern San Juans.

KSEA...MVFR conditions will continue tonight, lowering slowly to IFR
CIGS after 08Z and continuing through possibly 16Z or 18Z. It looks
like shower activity will be minimal due to shadowing from the
Olympics. A convergence zone north of Seattle is still possible but
appears to be becoming less likely. There is still a low chance of
snow showers tonight but accumulations, if any, will be less than an
inch. Surface winds will be S-SE 3-8 KT through 08Z then shift to E
and then finally become NE around 12Z. Drier northerly low level
flow should bring VFR conditions after about 20Z. Kam

&&

.MARINE...A weak 1016 mb surface low near over the N WA coast this
afternoon will drift slowly S along the N Oregon coast by early
Tuesday morning. A strong surface ridge will continue building over
B.C. tonight. These two evolving features will shift the area from
the current southerly flow to E-NE flow later tonight and on
Tuesday. The rising pressure over B.C. will lead to NE outflow winds
from the Fraser Valley across the North Inland waters.  A gale
warning remains in effect for this but only borderline gales are
likely. The swath of winds will affect mainly the waters around the
San Juan island and northward to Point Roberts, peaking late tonight
and early Tuesday morning. The NE pressure gradient will weaken
through Tuesday afternoon with winds easing down to SCA levels.

The easterly flow will bring SCA winds to the Central and West
Strait tonight and Tuesday as well as much of the coastal waters.

A strong Pacific frontal system will approach the area Wednesday,
probably producing east to southeast gales over much of the area
Wednesday night and Thursday. Models are in pretty good agreement
that strong easterly pressure gradients will develop ahead of the
front and associated surface low. However there are significant
differences in the track and strength of the surface low as it
nears the coast. Kam

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 6 AM PST Tuesday for Admiralty
     Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity-
     East Puget Sound Lowlands-Everett and Vicinity-Seattle and
     Vicinity.

     Winter Weather Advisory until midnight PST tonight for San Juan
     County-Western Skagit County-Western Whatcom County.

     Winter Storm Warning until 9 AM PST Tuesday for Eastern Strait
     of Juan de Fuca.

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

     Small Craft Advisory for hazardous seas until 10 PM PST this
     evening 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 from 10 PM this evening to 4 PM PST Tuesday
     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 until midnight PST tonight for Northern
     Inland Waters Including The San Juan Islands.

     Gale Warning from midnight tonight to 9 AM PST Tuesday for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 10 PM this evening to 3 PM PST Tuesday
     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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