Wind Chill Advisory 6, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 AM AKST Sun Dec 29 2019

.ANALYSIS AND UPPER LEVELS...
A region of drastically different air masses continues to bisect
the Bering/Western Alaska Range and North Pacific this morning as
a powerful atmospheric river and warm front is currently moving
northeastward from the Central Pacific toward the Southern
Mainland. The current leading edge of the warm front has reached
50N with the base of this surge of warm air tapping a
latitudinal deep 20N. Meanwhile, the remnants of a once deep and
unusually strong polar vortex continues to support bitterly cold
air which is pouring into the Bering Sea from interior Alaska and
Southwest Alaska. 850 hpa temps over Southwest are as low as -35C
which, to be honest, is getting into the
-3.5 sigma territory. This deep and cold arctic trough with polar
 origins is centered over the Kusko Delta, with a lobe of this
 trough moving north up Cook Inlet. Finally, a weakening closed
 circulation in Prince William Sound is moving inland across Prince
 William Sound this morning, which is spreading snow showers
 inland from the Kenai Peninsula inland to the Copper Basin. This
 boundary between warm and moist Pacific air and bitterly cold
 arctic/polar air will set the stage for more active weather
 across the whole Mainland the next few days.

&&

.MODEL DISCUSSION...

The numerical models are doing a great job of setting the overall
large scale synoptic pattern for the upcoming wild weather in
store for Southern Alaska, but they are doing an absolutely
terrible job with the finer scale details which will have large
and important sensible weather differences after today. The
biggest differences for Southcentral revolve around how much warm
air moves inland as a series of fast moving impulses move up Cook
Inlet through Tuesday. Timing and placement make all the
difference in terms of how much the arctic air is eroded, which
also impacts what type of precipitation will fall. The scenarios
range from possible freezing rain, to rain, to a rain/snow mix
Monday and Monday night. For now a middle ground approach will be
taken which will support a slower warm-up and most of the "warm"
air (likely above freezing for many areas) coming Tuesday.

For Southwest Alaska, the GFS and ECMWF are much more aggressive
developing a more powerful warm front and associated deformation
zone aloft across the Kusko Delta on Monday night and Tuesday.
With very cold air and strong northerly winds still at the
surface, this pattern as progged by these models would support a
significant blizzard. For now, the current forecast is taking a
middle ground, but should things shift west, this could be a
significant concern for parts of Southwest Alaska.

&&

.AVIATION...
PANC...Active weather will impact the TAF for the next day and a
half as snow moves into the terminal late this morning, then followed
by increasing low level wind shear as the snow ends by the
afternoon. Expect brief IFR conditions in snow this morning before
it ends. Southerly winds move into the terminal late Sunday night
into Monday morning along with light rain. Conditions should
remain VFR by then, although CIGS will likely drop below 5000
feet.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong weather system currently southwest of Kodiak Island will
rapidly move northeast. Strong winds will spread across the Gulf
today. Winds will increase through Turnagain Arm and the Upper
Hillside of Anchorage this afternoon and continue through tonight,
with Gusts to 70 mph possible. Considerable precipitation is
expected with this system over the eastern Kenai Peninsula.
Precipitation will begin as all snow, but change to rain for
coastal areas sometime later today or tonight. Some blowing snow
is possible over all of the Kenai, especially this afternoon and
evening. For areas around Cook Inlet, the precipitation type will
be a challenge late tonight and Monday morning. The air aloft
will warm quite considerably, but at the surface may remain below
freezing for a while before some southerly winds kick in and mix
out the colder air. Therefore a mix of snow, rain, and freezing
rain is expected.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Bone chilling temperatures are expected to continue across the
region as arctic air holds in place. Overnight -29F was observed
at Sleetmute and
-22F was observed at Bethel. The models keep that arctic low in
 the vicinity of Nunivak and then it shifts to the eastern Bering
 and then comes back over Western Alaska on Monday. All the while,
 energy from the North Pacific will under cut this arctic feature
 enhancing the chances for precipitation and airmass
 modification. Late Monday and Tuesday the models bring a full
 blown intact frontal system into into southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The unsettled pattern will continue for the next few days as
wintery weather impacts the Bering and Aleutian Chain. On
satellite imagery there is an occluded front spanning much of the
Aleutians. Gale force winds will continue today. As the frontal
system moves eastward, northerly flow will set up and persist
through Tuesday. A dome of high pressure will approach the Western
Aleutians Monday but looking upstream there is already another
cyclone churning up over the Sea Of Okhotsk.

&&

.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

...Bering Sea and Aleutian Coastal Waters...

Northerly gales and seas 15 to 25 feet will continue across the
eastern half of the Bering and Aleutians on Tuesday as low
pressure south of the AKPEN moves toward Kodiak Island. For the
western Bering, winds and seas will relax as a ridge builds over
the region. The gales diminish in coverage on Wednesday as the
low enters the Gulf and the ridge expands eastward over the
central Bering. By late Wednesday into Thursday, a band of
southeasterly gales will develop ahead of a front will moving over
the Western Bering and Aleutians.

...Gulf of Alaska...

Broad southerly small-craft level winds, with perhaps some
embedded gales, will persist across the Gulf on Tuesday as a front
lifts toward the Southcentral coast. An area of westerly gales
will expand across the western and southern Gulf late Tuesday
into Wednesday as the low south of the AKPEN lifts toward a
position near Kodiak Island. These winds will continue into
Thursday as the low tracks in the northern Gulf. Sea will range
from 15 to 25 feet across the Gulf through late Thursday before
diminishing the low tracks toward the Alaska Panhandle.

&&

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

The active weather pattern that has been so persistent for well
over a week now will continue into the new year. By Tuesday, the
main weather feature will be a 960 to 970 mb low spinning up in
the north Pacific south of the eastern Aleutians with yet another
arctic trough digging south over the Bering. As has been the case,
model performance this far out has varied considerably. Thus,
confidence in the evolution of this surface low and its
interaction with the digging trough remains low. With that said,
it does appear that this system will bring accumulating snow and
strong northeasterly winds to portions of Southwest Alaska. The
main difference between the models is track and timing, and these
variables will impact where the heaviest bands of snow set up.
Right now, this could be anywhere from along the Alaska Range
north to Bethel.

For Southcentral, Tuesday will also be quite active as a series of
shortwaves ahead of the aforementioned low ushers in bands of
precipitation and much warmer air with a continued southerly
flow. It is hard to believe right now with so much cold air in
place, but this system looks to push temperatures above freezing
not just for the immediate coast, but also for areas farther
inland and around Cook Inlet. Obviously, any mention of
temperatures above freezing will mean a changeover in
precipitation from snow to a wintry mix or even all rain. The big
question remains how much of this precipitation makes over the
mountains and into places like Anchorage, as there will also be a
downslope component accompanying these shortwaves which will help
limit precipitation amounts. The warmer temperatures and mixed
precipitation will continue into Wednesday as the main low tracks
into the Gulf.

Conditions then turn colder and more showery for the second half
of the week as the low tracks south and a ridge develops over
interior Alaska. This set up will usher in colder air across the
Southern Mainland as northerly outflow winds pick up in intensity.
By the weekend, it appears the Southern Mainland will remain cold
and fairly dry. The active weather will stay over the Bering and
Aleutians where a series of surface fronts attached to a Kamchatka
Low pass over the region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory: 145.
MARINE...Storm Warning: 119.
Gale Warning: 120 125 130 131 132 138 139 150 155 165 170 171 172
173 174 175 176 177 178 179 181 185 351 352 411 412 413 414.
Heavy Freezing Spray Warning: 160 165 179 180 181 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...TM

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
446 PM AKST Sun Dec 29 2019

AKZ101-111-121-125-131-141-145-301500-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass, Valdez,
Thompson Pass, Glennallen, Eureka, McCarthy, Paxson, Slana,
Talkeetna, Willow, and Cantwell
446 PM AKST Sun Dec 29 2019

...BRIEF THAW UNDERWAY IN SOUTHCENTRAL TO FINISH UP 2019...

Warming temperatures and gusty southeasterly winds will affect
Southcentral as a pair of lows track west of the area over the
next two days. Impacts from this warm-up include slick roads and
melting snowpack.

Temperatures are already above freezing along the southern Kenai
Peninsula, Palmer, Turnagain Arm and at higher elevations along
the Anchorage Hillside. Above freezing temperatures will cover
much of the rest of Southcentral, including in the Susitna Valley
and Copper River basin Tuesday. Cold air will then return swiftly
New Year's night, with the possibility of another round of snow
along the Cook Inlet region. Please stay tuned to the forecast for
updates as these conditions unfold.