Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 042347
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Sun Dec 4 2016

.SYNOPSIS...Colder air aloft will infiltrate the region tonight with
unstable showery conditions. A surface trough will increase rain and
snow showers Monday with some wet snow accumulation on hilltops.
Fraser outflow develops late Monday afternoon and evening ushering
in colder air and producing snow showers at sea-level. The typical
convergence zone corridor from north King and Snohomish county could
see several inches of localized snowfall. Colder and drier weather
prevails Tuesday and Wednesday with lows in the 20s and highs in the
30s. A system Wednesday night and Thursday could produce some
lowland snow before changing over to rain. Rain and wind continues
Friday into the weekend.

&&

.SHORT TERM...IR satellite imagery shows cold topped cu associated
with the upper low along the central B.C. coast streaming southeast
toward the Pacific Northwest. Lightning strikes were picked up
earlier this afternoon near Vancouver Island as the -38C cold pool
at 500 mb and some dynamic lift with the low is triggering some
heavy showers. Shower activity will increase later tonight and
Monday morning with snow levels falling to near 500 feet. Heavier
showers or any convergence zone activity from Puget Sound northward
could drag the snow level to the surface at times. Wet snow is
likely to be seen across most of the lowlands but mixing between
rain and snow should prevent much in the way of accumulation at sea-
level. Above a few hundred feet, a couple inches could fall but
accumulation will probably be mostly on grassy surfaces and rooftops
as ground/road temperatures remain mild. Areas with persistent
showers, especially if a convergence zone develops, could have
slushy snow on roadways. Otherwise, not much impact is expected from
this initial batch of showers.

Models trended stronger with the surface high over B.C. later Monday
and Monday night as the main surface low tracks southward along the
Washington coast. The latest GFS/ECWMF show a weak closed surface
low centered near Astoria by late Monday evening with BLI-WLY
gradient peaking near -16 mb indicating Fraser outflow should be in
full swing. Meso models hint at northeast winds and lower dewpoints
filtering into the extreme north interior late Monday afternoon,
then spreading southward into greater Puget Sound. If outflow is as
strong as some models show and there is enough residual low level
moisture in place, a modified arctic/Canadian front could form and
drive south through metro corridor during the evening hours or
possibly later overnight. The WRFGFS4k has consistently showed a
convergence snow band near the King/Snohomish county sometime Monday
evening with a maximum of around 6 inches. It is not possible to
know how accurate the timing, strength, and location of this
convergence zone band is given all the variables. But model trends
and the basic pattern supports this idea. Decided to put up a winter
storm watch for the typical convergence zone prone corridor for
Monday night but kept it limited to north Seattle to Everett and
eastward to the Cascades for up to 4 inches. This can be converted
to a warning/advisory or dropped at some point as we get closer to
Monday night and it becomes more clear either way. It will cool down
Monday night and many areas will see light snow showers before
clearing out Tuesday.

A period of dry and colder weather is on tap Tuesday and Wednesday
with highs mostly in the upper 30s and lows in the 20s to around 30.
Black ice will be a concern during the commutes Monday night through
Wednesday night. If any snow does fall Monday/Monday night, melting
and refreezing will increase the danger of slick roadways.

.LONG TERM...Global models seem to be trending toward a more
negative tilt front approaching the region Wednesday night with
dry/cold air firmly in place. Most models indicate several inches of
snow could fall across the interior lowlands before a transition to
warmer air bring plain old rainfall. Forecasts should be monitored
closely over the next few days as significant travel impacts are
possible Thursday morning.

Models vary considerably on the evolution of the system by Thursday
night and Friday. The GFS shows a deep low tracking near or north of
the area which could be a windy pattern for the region Friday. Heavy
mountain rain or snow is also possible. Long range models continue
to look quite active with a series of systems expected to affect the
region into the weekend.  Mercer

&&

.AVIATION...Moderate west to northwest flow aloft. Post frontal
onshore flow will rapidly diminish this evening resulting in a
decrease in showers and convergence zones. The air mass is somewhat
moist and unstable. Moisture will increase from the west tonight
into Monday morning and snow levels will drop to around 500 feet
with the onset of precipitation.

Conditions are mainly VFR across the region. A convergence zone
extends from around central Whidbey Island southeastward while
another broad area of convergence is seen over Skagit county.
Conditions will be locally MVFR within these convergence zones.

Conditions will deteriorate from the west on the coast later this
evening then in the interior about 12Z early Monday. Rain or mixed
precipitation can be expected at the lower elevation terminals with
snow at higher terminals like KPAE and KSEA during the morning
hours. KPAE could remain predominately snow Monday afternoon with
total snow accumulations 1-3 inches. Slushy accumulations of an inch
or so is possible at KSEA.

Fraser outflow will develop during the late afternoon hours around
KBLI with N to NE winds increasing to 15 to 25 mph. Albrecht

KSEA..South to southeast winds 5 to 8 knots. VFR conditions this
evening. Snow showers with mainly MVFR conditions will begin around
the terminal about 12Z Monday and 1-2 inch slushy accumulations are
possible Monday morning by 18Z. Albrecht

&&

.MARINE...Post frontal onshore flow will decrease rapidly this
evening. Small craft advisories were stretched out until 7 pm PST
this evening for the strait and Northern Inland Waters and the
advisory was expanded to include Admiralty inlet until 7 PM.

Seas will remain above 10 feet through Monday. A small craft
advisory for hazardous seas remains in effect through late tonight
at the west entrance to the strait while the small craft advisory
will remain in effect for the coastal waters for a combination of
seas or winds at times through monday.

A weak low will move southward along the west coast of Vancouver
Island tonight. As the low continues south to the Washington coast
on Monday, a cold dome of high pressure will move south through
British Columbia. As a result, the flow across Western Washington
will turn northerly by late Monday. Cold Fraser River outflow winds
of 20 to 30 kt will likely develop across the northern inland waters
and Strait of Juan de Fuca Monday afternoon and continue through
Tuesday. A gale watch was issued for the Northern Inland Waters for
the potential of gales late Monday afternoon through Tuesday.

Northeasterly flow will diminish Tuesday afternoon as it gradually
turns more easterly. Then on Wednesday night or Thursday, strong
easterly flow will develop with gales possible over the coastal
waters and through the strait. The GFS has been stronger and faster
with the incoming frontal system than the ECMWF model. The forecast
is a blend between the two at this point. Albrecht

&&

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

$$

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...Winter Weather Advisory from 4 AM to noon PST Monday for
     most interior lowlands.

PZ...Small Craft Advisory for rough bar until 6 PM PST Monday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 PM PST Monday 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 7 PM PST this evening for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Gale Watch from Monday afternoon through Tuesday afternoon for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory for hazardous seas until 3 AM PST Monday
     for 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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