Gale Warning 778, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
932 PM AKST Mon Dec 30 2019

...Updated Aviation Discussion...

.ANALYSIS AND UPPER LEVELS...
A broad longwave trough lies over the state, with an upper level
low located just south of Norton Sound and several surface
features in a line from the Al-Can border down through the North
Pacific, just south of the AK-Pen. Meanwhile, a strong high
remains stationary over Russia. The tight gradient is forcing
strong northerly winds and cold air advection through the Bering
Strait and across the Bering Sea, as seen on satellite imagery
this morning. This cold air is contributing to wind chills around
-20 along the west coast, but with the coldest air yet to come, we
 are not at advisory levels yet.

Aloft, the main core of the jet stream has moved further to the
southeast, but a secondary weaker jet is moving into the base of
the longwave trough and providing support for the surface low
currently still developing south of the AK Pen. Further eastward,
a weakening low pressure system is moving northward over the
mainland. A lot of warm air has moved northward with this system,
and this morning's upper air sounding shows a distinctive warm
"nose" below 850 hPa with temperatures approaching 1C. The
southeasterly winds ahead of the low have eroded away any
remaining cold air over Southcentral, so this morning saw a major
warm-up and quick switch to rain as the warmer air and
southeasterly winds moved in. Temperatures have risen above
freezing mainly west of the Copper River Basin, but the precip has
stopped for the most part, giving a quick break before the next
system moves in.

&&

.MODEL DISCUSSION...
Models have generally been trending towards a more easterly track
for the low developing south of the AK Pen. Today's track takes
the low center directly up Cook Inlet and then dissipates it
over the Susitna Valley and Prince William Sound. This unlikely
end to the central low indicates there is probably still more
uncertainty. The models are bringing a significant amount of cold
air -- dropping the 850 hPa temperatures by 15C in some places --
which is a really good set up for snow around the area. Overall,
the models are in good agreement on the track, with the GFS
prolonging the precipitation over Cook Inlet and the NAM ending it
early. Timing was about the same for both models, but we chose
the longer lasting solution as this is more likely in these type
of cold air advection events.

&&

.AVIATION...
PANC...VFR conditions and light winds at the surface will persist
through the evening. Aloft, southeasterly winds will continue for
the next 24 hours, contributing to wind shear conditions until
the second round of warm southeasterly winds hits. Ceilings will
remain VFR with downsloping limiting precipitation until 02z.
After this time, a cold front will quickly lift toward the
aerodrome from Cook Inlet. This will bring an initial round of
southerly winds and rain around 02z. Rain is then expected to
changeover to snow with MVFR ceilings and visibility (maybe
dropping to IFR) by 6z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Monday
night through Thursday)...

Warm weather with a wintry mix of precipitation is expected to
continue through New Year's Eve. As a result, an SPS has been
issued for Southcentral AK for icy conditions. The first low
pressure system that gave us a warm-up with wind and rain this
morning has pushed through, allowing for a brief break in the
weather. Some showery activity is moving into the Southcentral
Coast on the backside of this low, keeping inclement weather in
place for most locations. Due to the break in the weather tonight,
temperatures will cool slightly overnight. They will cool enough
to be at or below freezing, which will exacerbate icy conditions.

The next low will move into Southcentral AK tomorrow, bringing
another round of wind and warming temperatures. Behind this low is
the next weather story, with very strong cold air advection
wrapping around the backside of the low as it moves through the
area. This is the strongest push of cold air we have seen in quite
some time. The cold air will create very strong winds through
gaps and passes in the Western Gulf. It will then push up Cook
Inlet, bringing snow with it. The pattern setting up is one that
has the potential to bring heavy snow to the Anchorage area.
Recognizing this pattern, we have increased the snowfall forecast
New Year's Eve into New Year's Day to 3-5 inches. There is still
too much uncertainty in guidance to go full tilt on heavy snow, so
we will analyze the situation overnight and tomorrow morning, with
a decision on heavy snow with tomorrow's afternoon forecast
package.

Celebrations for New Year's Eve and Day will be impacted by messy
weather that will make travel difficult. Please stay tuned to the
weather forecast and adjust plans accordingly to stay safe.

&&

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

Strong winds and reduced visibility from snow will continue
across areas along the Alaska Peninsula and interior Bristol Bay
through the night as a front continues to lift northward into the
Lower Kuskokwim Valley. The biggest change from yesterday to
today's forecast is the track of the low center responsible for
snow, which is trending much farther east today. Snow will start
over Dillingham this afternoon stretching northward to Sleetmute.
Through the night, the heaviest snowfall will shift slightly
eastward, keeping areas from King Salmon to Lime Village in the
sweet spot likely to see at least a half foot of snow or more by
tomorrow morning. This differs from yesterday's forecast, which
had Dillingham northward with the highest snowfall totals. Snow
should start to taper off Tuesday evening as the deformation band
across interior Southwest begins to lift northward and weaken. By
Wednesday morning, snow starts to clear out across the region,
leading to an overall cooling trend across interior locations.

The shift eastward in snowfall, and therefore cloud cover, will
allow colder temperatures across the Kuskokwim Delta tonight
through Wednesday. These temperatures accompanied by gusty
sustained winds, extending from coastal location to inland areas
near Bethel, will allow wind chill values to drop to 45 below zero
as we head into the new year.

&&

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

A low pressure system centered over the AKPEN will continue its
eastward track into the eastern Gulf by Tuesday afternoon. This
will keep strong northerly flow on the backside of the low,
bringing widespread snow showers and freezing spray to much of the
Bering. Winds across the eastern Bering will increase to gale
force level tonight and continue through Wednesday when the low
moves out of the region. Areas along the AKPEN can expect to see
strong gusts out of bays and passes as this northerly flow
persists through Tuesday evening.

&&

.MARINE (Days 3 through 5/Thursday through Saturday)...

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

Northerly gales and seas 15 to 20 feet will continue across the
central to eastern Aleutians into Friday morning as a front
associated with an area of low pressure in the northwestern Gulf
remains over the area. A front moves into the western Aleutians
Thursday morning which will bring southerly gales to the area and
seas around 15 feet.

...Gulf of Alaska...

Broad westerly to southwesterly small-crafts will persist across
the southern Gulf through Thursday night. Seas will range from 15
to 25 feet as the low south of Kodiak Island lifts north through
Friday morning.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The focus on Friday will be on a 995 mb low south of Kodiak
Island. Most of the models keep the low over the southern Gulf
with the primary impact to Mainland Alaska being strong outflow
gap winds. High pressure will remain in place in the central
Bering throughout the long term. This will keep cold air moving
over the warm waters of the eastern Bering which will continue
producing widespread snow shower activity across the eastern
Aleutians and the AKPEN. That same high pressure system will keep
the western Aleutians in an active pattern as a series of fronts
move across the area through the weekend and block those fronts
from moving into the eastern Bering.

For Southcentral, Saturday keeps cold temperatures and dry
conditions across the Mainland. Models show significant
disagreement into the late weekend/early work week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145.
         Winter Storm Warning 152 161.
         Winter Weather Advisory 181.
         Wind Chill Advisory 155.

MARINE...Storm Warning 130.
         Gale Warning 132 138 150 155 160 165 170-172 175-176
         179-181 185 352 414.
         Heavy Freezing Spray Warning 150 160 165 179-181 185
         414.

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...CK

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
137 AM AKST Tue Dec 31 2019

AKZ101-111-121-145-010600-
Anchorage-Matanuska Valley-Western Kenai Peninsula-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Talkeetna, Willow, and Cantwell
137 AM AKST Tue Dec 31 2019

...HEAVY SNOW EXPECTED TUESDAY AFTERNOON THROUGH TUESDAY NIGHT
ACROSS THE COOK INLET REGION...

A very strong cold front moving up Cook Inlet will bring back
winter in a hurry this evening and tonight as temperatures plummet
rapidly and snow begins to accumulate from the southern Kenai
Peninsula northward to the Matanuska Valley. In the afternoon,
strong southwest winds and extremely cold arctic air will impact
Kachemak Bay and the southern Kenai, and this will support areas
of blizzard conditions given the ocean-enhanced snow showers
forming along Cook Inlet. By late evening, expect heavy snow to be
impacting areas from Anchorage to the Matanuska Valley. The
combination of rapidly cooling temperatures and periods of heavy
snow will likely result in potentially hazardous travel conditions
as this colder air quickly moves into the Susitna Valley and
Copper Basin by Wednesday morning as temps fall throughout the day
across all of Southcentral.