Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 140141
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKST Thu Feb 13 2020

.ANALYSIS AND UPPER LEVELS...
An upper level ridge is over most of mainland Alaska this
afternoon with an upper level low over the central Bering Sea. The
strong surface low near the western Aleutians is tracking
northward and will become vertically stacked with the upper level
low soon.
This low in the western Aleutians is quite impressive-looking on
satellite as it has occluded and has wrapped itself up into three
frontal bands.
Also notable on satellite is the inflection along the front well
southwest of Kodiak Island where the jet stream continues moving
southwest-to-northeast toward the Gulf and the lower-level features
along the surface front break to the north-northwest. This area
will induce a triple-point low to develop this evening which will
track across the southern Gulf of Alaska through tomorrow night.

&&

.MODEL DISCUSSION...
Models remain in good agreement with the large Bering Sea low into
the weekend, but then start to really struggle with the track,
structure, and strength of the next low that will be approaching
the western or central Aleutians by Saturday night. About the only
information that can be determined at this point is that there
will be a large low that impacts all of southern Alaska during the
first part of next week.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Offshore flow will continue through tomorrow as the upper level
ridge weakens and moves overhead of Southcentral. This coincides
with weakening pressure and thermal gradients which will diminish
gap winds across the north Gulf coast and Prince William Sound.
Colder and drier conditions will remain across Southcentral
tomorrow through Saturday under the influence of the upper ridge.

A vertically stacked occluded low over the western Aleutians has
started to weaken, with its associated frontal system elongating
and weakening across the eastern Bering and Alaska Peninsula. This
frontal system will approach Kodiak Island tonight, bringing
snowfall to the Island tonight through Friday morning. Zonal flow
aloft will drive the triple point eastward across the Gulf, with a
low forming and deepening on its journey across the Gulf. As the
low develops the remaining occluded front will quickly fall apart
as it lifts northward toward the Kenai Peninsula. Thus, expect
little or no precipitation from this front to reach the Kenai
Peninsula. However, an upper level short-wave will approach Homer
and the Kenai Peninsula on Friday. Have increased the probability
of snow from chance to likely Friday night into Saturday morning
due to the combination of this shortwave trough and an upslope
component. Any snow that does fall will be light, with little snow
accumulation.

Forecast confidence really breaks down as we head through the
remainder of the weekend, with an active storm pattern across the
Bering Sea and Aleutians. There is a chance a frontal system from
one of these lows could reach Kodiak Island and the Gulf as early
as Sunday, bringing another round precipitation. For the rest of
Southcentral, the upper ridge will exit and will get into a broad
area of weak cyclonic flow aloft. This will likely lead to
increasing clouds, but chances of precipitation will remain low.

&&

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

An upper level ridge situated over western Alaska will break down
as the frontal boundary associated with a Western Bering low
pressure system moves into Southwest Alaska. Much of southwest
Alaska will see snow chances increase over the next 48 hours as
this system moves through. Temperatures will also be on the
increase thanks to some onshore air replacing the colder offshore
flow that has dominated the region recently. This also looks to
be the beginning of a more active pattern as model guidance is
hinting at some bigger changes.

&&

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

A weakening gale force low remains in place in the Western Bering
this afternoon. The central Aleutians and westward will see gale
force winds and rain and snow showers continue through Friday
night then taper off early Saturday. A similar pattern develops
for the Pribilofs with slightly lighter winds and a rain and snow
mix Friday through Saturday.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

A high amplitude upper level pattern will be in place by Sunday.
This is favorable for a series of deep lows to move quickly across
the forecast area. On Sunday, the primary low will be located
back along the Aleutians. Expect lows to form out ahead of this
low across the North Pacific and track northward. These will
initially move into the Bering Sea, but each successive low will
form and track farther east, affecting the Southwest Alaska
coastal waters and the Gulf of Alaska.

Forecast confidence in the pattern and overall storm track is
high, while confidence in the track or strength of any individual
storm is low. Expect fairly widespread gales across the Bering
Sea for most of the Sunday through Tuesday period, with gales
likely over the Gulf Monday through Tuesday. There is also
potential for one or more of these systems to reach storm force.

&&

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

Beginning the extended period forecast on Saturday evening, there
will be a fast zonal flow pattern in place as a series of subtle
shortwave trough features in westerly flow will be impacting the
Southern Mainland and Bering Sea. Snow showers will mainly be
across Southwest Alaska and the Gulf given the blocking pattern
the Alaska Range induces on air moving from the west into
Southcentral. Arctic high pressure will remain over Southcentral,
so temps there will remain below average, but otherwise, expect
quiet weather through Sunday.

Thereafter, a significant storm is likely Sunday evening through
Tuesday as a strong Pacific trough digs and amplifies the flow
pattern. What is known now is a warm front will into Southwest
then Southcentral Sunday night into early Monday. Thereafter, the
global numerical guidance among the ECMWF, Canadian GDPS, and GFS
diverge rapidly with the trough orientation and the possibility of
a strong low moving inland. With that said, they all depict a
potentially major storm for much of the Mainland with impacts
ranging from heavy snow to strong winds and coastal rain (with
heavy higher elevation snow for the Gulf Coast. The details remain
uncertain, but the signal is very strong and confidence is growing
dramatically that there will be a storm, just exactly how it
unfolds remains unseen. Stay tuned.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 127 173-178 185 352 411-413.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SEB/CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB
MARINE/LONG TERM...SEB/JA



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