Gale Warning 681, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
413 PM AKST Thu Dec 19 2019

.ANALYSIS AND UPPER LEVELS...
IR satellite imagery and surface observations continue to show an
expansive area arctic air encompassing the northern two thirds of
the state. While the crux of this airmass remains north of the
Alaska Range, some of the arctic air continues to filter into the
Kuskokwim Delta and Copper River Basin, where temperatures this
afternoon remain well below zero. A thin band of mid-level clouds
stretching from Paxson to Cooper River (with some light snow in
Eureka) marks the position of an upper-level trough, and is
progressing slowly eastward across Southcentral. This feature is
helping drive the arctic air farther south, with no noticeable
uptick in daytime temperatures across the northern and western
portions of the Susitna Valley, even with the low-angle, yet
abundant, wintertime sun. Northerly winds, while still light, are
also increasing behind this trough.

Farther south, a decaying surface low continues to spin over the
extreme southern Gulf. This feature remains nearly stationary as
it remains cut off from the main upper-level jet positioned over
the northern Pacific with a jet streak of 150kts entering the
Pacific Northwest. A series of shortwaves associated with this low
continue to produce scattered showers along the immediate
Prince William Sound coast as they move east to west across the
northern Gulf. Gusty northerly winds also continue to increase
this afternoon along the southcentral coast as both the pressure
and thermal gradients between the main Gulf low and the arctic
ridge strengthen.

Farther west, a mix of rain and snow showers, along with northerly
wind gusts, continues across the eastern Aleutians and southern
AKPEN as colder air dives south behind a shortwave. A gale-force
front is positioned between Adak and the Pribilofs and is tracking
east across the Bering. This front is associated with a deepening
Kamchatka low situated along the Russian coast. Its trailing cold
front is advancing toward Shemya.

&&

.MODEL DISCUSSION...
Models remain in decent agreement through Sunday. The ensemble
members of the GFS continue to tightly cluster through 18z Sunday
but shifts out of phase with the deterministic runs of the GFS and
EC by Monday. All guidance, however, indicate a shift in the main
trough from the western Bering to western Alaska by early next
week. This would bring a return of more active weather to
Southcentral by early next week. This lends to an above average
forecast confidence through the weekend with a diminishing level
confidence beyond that time.

&&

.AVIATION...

PANC...VFR conditions are likely to persist. There is the
potential for fog and low stratus to again form along the Knik Arm
by mid-morning Friday. If this does occur, a light northerly wind
may transport any fog / low stratus over the airport complex by
late Friday morning. Confidence in this is very low, however, and
is currently dealt with using "VCFG". Light winds are expected,
generally out of the north.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An Arctic trough will continue to move south across Southcentral
Alaska this evening. Colder temperatures accompany this trough,
along with clearing conditions as light snow tapers off tonight
over the Copper River Basin. This cold air advection pattern is
favorable for gusty conditions through mountain gaps and bays
along the coast. These winds will taper down for a short time on
Friday afternoon before quickly returning on Saturday when the
next cold air advection event swings into the region from western
Alaska, which will help reinforce the cold air that is already in
place. Strongest winds associated with this event will be over the
western Gulf. As the cold air settles in over the weekend, low
temperatures will be the coldest seen so far this season.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Cold and dry conditions will persist over Southwest overnight
tonight as an upper level ridge moves farther inland over the
mainland. Elevated northerly winds through the usual gaps and
passes along the Alaska Peninsula will eventually diminish this
evening. A front in the central Bering reaches the Southwest coast
this evening, though in a weaker state. Nunivak Island may see
some snowfall starting this evening and by Friday, areas across
the Kuskokwim Delta Coast may see some snowfall as well, though no
significant accumulations are expected. As the front continues to
move inland, westerly/northwesterly flow aloft will bring in
colder air, leading to generally colder temperatures across
Southwest for the weekend. By Sunday, the next system approaches,
bringing gusty southeasterly flow to the southern Alaska
Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A gale force front over the central Bering will continue to move
eastward this evening. Showers will persist over the Bering for
Friday ahead of an incoming storm force low. This low will arrive
over the western Aleutians late Friday into early Saturday
bringing southeasterly winds and rain as it advances eastward
through the day Saturday. The strongest winds will be on the
backside of the low with hurricane force gusts expected Saturday
afternoon over the Rat Islands and western Andreanof Islands,
including Adak. In addition to strong winds and high waves,
blowing snow and blizzard conditions are possible over the western
Aleutians Saturday evening. Please stay tuned to the forecast as
we continue to make updates as we have more information on where
exactly this strong area of low pressure is tracking.

&&

.MARINE (Days 3 through 5)...
A storm force low will track from the western Aleutians to the
southern Bering Sea Saturday into Sunday. Expect widespread storms
and gales over the Bering and Aleutians, with the strongest winds
on the south side of the low, i.e., near the Aleutians with 40
foot combined seas possible. Farther east, Arctic high pressure
inland will facilitate strong offshore flow along the North Gulf
Coast. Expect gusty winds over the Barren Islands and out of the
Copper River Delta, Resurrection Bay and in the Valdez
Narrows/Valdez Arm Sunday. Conditions will become more benign
Monday into Tuesday... the Bering low will weaken and move inland,
and several weaker lows will develop in the Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast on Sunday, the regional
upper air pattern will already be undergoing a rather dramatic
change as the mainland will be dominated by arctic troughing. This
is a huge change from the last several months (arguably, the
least year and a half) which has seen incredible and unprecedented
anomalous upper level ridging. Over the Bering Sea, there will be
a weakening storm force low with the front moving into the Gulf
of Alaska as a triple point low by Sunday afternoon and evening.
Besides below average temps over the entire Mainland, expect
increasing threats for accumulating snowfall as these warm fronts
move up along the North Gulf Coast, in conjunction with an arctic
trough over the western half of the Mainland. By Monday large
enough differences among the global models lends to increasing
disagreement with the placement of a mainland arctic trough,
which, as usual, dictates snowfall potential inland. The ECMWF is
most bullish developing an arctic trough farther south across
Southwest Alaska whereas the GFS and Canadian develop it farther
north, which effectively allows for more southeast cross-barrier
flow and warmer temps. The ECMWF would result in colder temps
lasting all week, along with much higher chances for accumulating
snow across large swaths of the Mainland. The GFS and Canadian
also support snow, but would be more localized and harder to
predict in their projected patterns. The takeaway: wintry weather
is here for the Christmas holiday week and the potential for
accumulating snowfall exists across the entire southern Mainland
region. Expect an active low track along the Aleutian Chain as
well as colder air shunts the storm track southward.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 121 127 130 131 138 150 176>178 181 185.
 Heavy Freezing Spray 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...AH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...MM/JA