Small Craft Advisory 279, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sun Nov 20 2016

.SYNOPSIS...A weak low pressure system will bring occasional
showers to Western Washington tonight as it moves across Vancouver
Island. Showers will give way to sun breaks Monday afternoon.
Western Washington is in store for wet and windy weather next week
with frontal systems arriving around Tuesday, Thanksgiving Day,
and Saturday. These fronts will bring snow to the mountains.

&&

.SHORT TERM...The northern portion of the split upper trough over
Western Washington will shift slowly eastward tonight. Winds will be
breezy tonight, except locally windy across the north interior close
to the filling 1000 mb surface low over central Vancouver Island.
Showers will generally decrease behind the 500 mb trough axis later
tonight. However, weak surface ridging with onshore flow will be
favorable for convergence zone activity. Southwest flow at 925 mb
would tend to keep convergence zone showers confined to areas north
of Everett to Bellingham and over the Cascades from Stevens Pass
northward through Monday. With snow levels near 4000 to 4500 feet, a
couple inches of snow at top of Stevens pass is possible and perhaps
3 to 5 inches at higher spots like Mount Baker. Partial afternoon
sunshine should develop across the remainder of the lowlands
including Puget Sound with highs close to 50.

A short wave ridge will traverse the area Monday evening as the next
system approaches. Models show the next trough elongating and
slowing as the frontal band pushes into the area through Tuesday.
The system should still hold together enough to bring a couple
inches of rain to the coast and Olympics. Snow levels will fall to
around 3000 to 3500 feet but precipitation amounts inland and over
the Cascades will be lighter, so perhaps 3 to 6 inches of snow could
fall at higher passes and ski resorts.

The cooler air mass will stay in place with showers across the area
Wednesday. Snow levels will remain relatively low, around 2500 to
3000 feet through the rest of the week. A stronger front with rain,
wind, and mountain snow will affect the area Wednesday night through
Thanksgiving. Additional snow is likely in the mountains, including
the passes, which could affect travel. Mercer

.LONG TERM...From previous discussion...The next front will move into
the forecast area on Thanksgiving Day, and it looks like it will be
followed quickly by a vigorous frontal system on Friday. Recent
model runs are showing surface development with the Friday system,
but there is lots of uncertainty in the details. For now, suffice it
to say that this will be a windy and wet period. The models have
become more inconsistent beyond Friday, but at least this will be a
cool showery period, and there's a good chance another front will
move through. The snow level will be roughly 2500 to 3000 ft
Thanksgiving Day through Sunday, so all the highway passes --
including Snoqualmie Pass -- should receive snowfall. Cascades and
Olympics snowfall totals of 1 to 3 ft for that four day period look
entirely reasonable. McDonnal

&&

.AVIATION...An upper level shortwave trough with WSW flow aloft will
move across W WA tonight through Monday morning. The air mass is
weakly unstable with areas of showers expected through 14Z. A
convergence zone is expected to develop over south Whidbey Island
later tonight.
VFR conditions predominate over the interior at 04Z. However a shift
to onshore flow overnight should allow CIGS to lower to MVFR levels
then continue through Monday morning.

A weak upper level ridge will move over W WA Monday afternoon,
allowing showers to begin diminishing after 14Z and CIGS to rise
back to VFR 030-050 after about 22Z.

KSEA...VFR conditions are expected to continue through 10Z with a
SCT-BKN045 layer over the local area along with scattered showers.
MVFR CIGS are expected to develop after 10Z and continue into early
Monday afternoon. Southerly wind 5-10 KT. Kam

&&

.MARINE...A 1000 MB low over northern Vancouver Island at 04Z/8
PM will fill as it moves east tonight, allowing pressure gradients
and winds over W WA to weaken. SCA winds are still expected on the
coast, through the Strait of Juan de Fuca, and over the North Inland
waters through tonight, but the winds should drop below SCA levels
by Monday morning. W flow is still expected to increase in the
strait but it will be borderline at best.

A weak surface ridge will move over the area on Monday for sub-SCA
winds.

The next frontal system is still developing this evening offshore W
of 140W. Models remain in good agreement that a vigorous occluded
front will develop offshore on Monday with gales developing over the
coastal waters late Monday afternoon and continuing Monday night. S-
SE gales are expected to develop over the north interior waters
Monday night and extend into Tuesday. Another occluded front will
arrive around Wednesday night or Thursday. Kam

&&

.HYDROLOGY...Flooding is not expected during the next 7 days,
with the possible exception of minor flooding on the Skokomish
River. Mercer

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory coast, strait, and north inland waters.
     Gale watch Coast, Entrances to the strait, North inland waters
     Monday night and Tuesday.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html