Heavy Freezing Spray Warning 76, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKST Mon Nov 28 2016

.ANALYSIS AND UPPER LEVELS...

The weather across the Anchorage area of responsibility this
morning remains active, with a shortwave trough and associated
frontal boundary over Southcentral and the northern Gulf moving
quickly northeast toward the Alcan border region and the Yukon.
This feature continues to bring snow to eastern Prince William
Sound north through much of the Copper River Basin. Meanwhile,
cold air spilling through the Alaska Range terrain gaps has helped
to produce a mesoscale surface low near Kenai that is helping to
produce areas of light snow from near Anchorage south across the
western Kenai Peninsula. Dry air at the surface is causing the
snow to largely evaporate in many areas before it reaches the
ground, however.

To the west over the Bering and Southwest Alaska region, low
pressure continues to spin just east of Saint George moving toward
the Alaska Peninsula where it will dissipate into a trough later
today. A front out ahead of this low continues to move northeast
across the Bristol Bay marine area toward the Bristol Bay zone,
but is showing signs of weakening as it does so. Infrared
satellite indicates mostly snow and rain shower activity around
the occluded low.

&&

.MODEL DISCUSSION...

The GEM Regional seems to be verifying best with QPF associated
with the meso-low over the Cook Inlet region, along with its
associated areas of convergence as cold advection spills into the
region through the Alaska Range. This would indicate low-level
moisture and at least some flurries could linger around the
region through at least this morning, and maybe even through the
afternoon and/or early evening.

Otherwise, models have come into better agreement with low
pressure poised to move north through the Gulf of Alaska and
inland in the vicinity of Yakutat and the southern Alcan border.
This has allowed us enough confidence with outflow winds on the
western periphery to issue a Blizzard Watch through Thompson Pass
for Tuesday, as there is plenty of transportable snow from the
last and ongoing storms for blowing snow conditions to appear. The
main question will be how much snow falls depending on the low
track.

Out west, the GFS and EC are in good agreement with another fast-
moving low traversing the Aleutian chain from west to east late
Monday through mid-week before eventually moving into the Gulf of
Alaska. Forecast confidence with the synoptic features is medium
to high through the next five days.

&&

.AVIATION...

PANC...VFR conditions and light winds are expected throughout the
period, though there is some disagreement this morning through the
afternoon with the degree of low-level moisture remaining. Given
the success of the NAM model the last several days with
forecasting low-level moisture, will carry a SCT015 stratus deck for
much of the morning and afternoon in the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A developing storm-force low over the Northeastern Pacific
intensifies as it moves to over the northern Panhandle Tuesday
night. Storm-force winds are expected to develop over the offshore
Gulf waters tonight and Tuesday with this system with gales over
some coastal marine locations. The strongest winds are expected om
the backside (west) of the low center track. Pressure gradients
will increase across much of Southcentral Alaska with the approach
of the low center giving rise to brisk to very strong gap flows
beginning early Tuesday morning and continuing into Tuesday night.
The strongest channeled winds are expected across the eastern
Kenai Pen and along the North Gulf Coast. Very strong winds
through Thompson Pass on Tuesday combined with transportable snow
could create blizzard conditions. Wind chills become a concern
Tuesday through Wednesday with gusty winds and cold air spilling
through the gaps along the Alaska Range. The approaching storm
will spread measurable snow to the North Gulf and the Copper River
Basin Tuesday. There is still some uncertainty how far west the
snow will push Tuesday/Tuesday night as models differ some on the
low track.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The front along the southwest AK coast will weaken today with
some lingering snow showers. As the low associated with the
front slowly dissipates over the eastern Bering, dry and gusty
offshore flow will develop throughout much of the southwest
mainland through mid week. With the cold temperatures currently
over the area, this may create nighttime wind chills as low as 30
below in interior locations, especially Tuesday night when the
coldest air will move into the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Cold air advection is strong enough to bring wet snow today to the
eastern Aleutians as the low slowly dissipates to the north. This
will also bring gale-force gusts to the Eastern Aleutians,
however with the warm waters in the southern Bering, areas of
blowing snow are not expected. Afterwards, an active but far less
impactful pattern will continue into mid-week as a gale and storm
force low moves into the western Aleutians beginning late tonight.
This low will then spread rain and gale-force winds throughout
the Aleutians through Wednesday, with another chance of rain/wet
snow for the Pribilofs.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Quite a bit has changed amongst both ensemble and deterministic
guidance since yesterday. As was thought yesterday, an area of low
pressure will skirt the Aleutian Islands Thursday and move into
the Gulf of Alaska on Friday. That is where the similarities end.
Yesterday, models progged this area of low pressure to move across
the northern gulf over Kodiak Island and near Prince William
Sound which would have brought a very decent shot of snowfall
across much of Southcentral Alaska. However, models now prog this
low further south in the Gulf of Alaska which could potentially
keep a good bit of the moisture further south instead of pushing
it inland. At the same time, a brief warm up was expected with the
track of the low pressure system but now it appears that this will
not be the case as the more southern track will likely keep a tap
of colder and drier arctic air over Southwest and Southcentral
Alaska.

The take-away message is that snow chances for interior portions
of Southcentral and Southwest Alaska continue to look quite
disappointing as the synoptic features are tracking just a little
too far north or a little too far south. Some of the coldest
temperatures are expected by next weekend with the potential for
single digit high temperatures over west Anchorage next Sunday.
There is still the potential for snow across Southwest and
Southcentral Alaska as models fluctuate so it's something worth
keeping an eye on.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Watch...131
 		 Wind Chill Advisory...145.
MARINE...Storms...178 351 352
 Gales...119 120 127 129 130-132 155 170 172-177 179 410-414
 Heavy Freezing Spray...180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
AVIATION...CC
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...MC