Area Forecast Discussion
Issued by NWS Anchorage, AK

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378
FXAK68 PAFC 240049
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 PM AKDT Tue May 23 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper low complex continues over the northern and central
Bering Sea. A trough on the eastern spoke of complex extends
across Southcentral and is exiting to the north. Cold air driving
south around the backside of the Bering complex is digging out the
base of the trough between 160-170E. A strong southwesterly
120-140 kt jet extends from the north pacific, across the central
Aleutians, to over Bristol Bay.

A weakening occluded front brought rain to the Southwest Alaska.
A surface wave, under the strong southwesterly jet, is forming
along the trailing front boundary extending along the Aleutians.
Rain and gusty winds were evident along the frontal boundary.
Cloud breaks were observed over much of Southcentral Alaska with
drier air on the backside of the retreating upper trough. Though,
higher clouds were starting to encroach from the southwest.

&&

.MODEL DISCUSSION...
Numerical forecast guidance are in very good agreement with the
timing of synoptic features. Generally will be using a little
GFS/NAM in combination with hand edits.

&&

.AVIATION...
PANC...Gusty southeast winds are expected across the airport
complex beginning late tonight and will continue through Wednesday
evening. Conditions are expected to drop to MVFR in rain
beginning Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Show the present forecast weather pattern to any meteorologist,
and you will be hard pressed to find any that would guess this is
the tail end of May. An upper level ridge over the area currently
is already being shoved to the east to be replaced by a incredibly
cold, deep upper level trough. Upper level troughs exist because
they are characterized by a cold air mass. Whenever an air mass is
rapidly replaced by one that has significantly different
characteristics, there`s usually precipitation involved. That will
certainly be the case this time as well.

The key to precipitation amounts in this forecast across
Southcentral has largely to do with one specific factor, wind
direction. The winds at all levels of the atmosphere will align
tonight in a southwesterly direction. For much of the state,
especially Southcentral, a southwesterly wind is the one direction
where there is no blocking by the mountains for most areas. Thus,
a steady stream of moisture moving northeastward straight up the
Cook Inlet will develop tonight. It will not take long beyond that
for precipitation to develop area-wide, as the cold air to the
west acts to lift the moisture out ahead of it, causing it to
condense and then fall as precipitation. This precipitation will
start by sunrise Wednesday and continue through most of the day.
For most low-elevation locations, including the Anchorage Bowl, it
will be rain. However, as progressively colder air moves in,
elevated areas will see a changeover to a rain/snow mix, or
perhaps all snow. Accumulations anywhere populated should be light
due to temps staying at or above freezing and the over 18 hours of
daylight working to keep ground temperatures warm. Valley rains
and mountain snows will continue into Wednesday night,
particularly in the northern Susitna Valley, the Kenai Mountains,
and points further east along Prince William Sound.

Shower activity will continue along the mountains Thursday, but
outside of a stray shower most valley locations should dry out
with peeks of sunshine as well. The cold air mass across western
Alaska will continue to make its way into Southcentral. As the
coldest air will be most effective moving in at higher altitudes,
the developing temperature profile of relatively warm air at low
elevations underneath cooling air at high elevations will make the
atmosphere increasingly unstable, especially by Thursday
afternoon. Plenty of remnant moisture from Wednesday`s rainfall,
an increasingly unstable air mass, and another upper level wave
moving towards the area from Southwest Alaska will combine to
produce a chance of thunderstorms over the western Susitna Valley
and the Alaska Range Thursday afternoon. Meanwhile elsewhere tall,
billowing cumulus over many of the area mountains may cause
occasionally heavy precipitation in those areas. The cold air
aloft will also lead to snow levels falling to as low as 1,500
feet Thursday night, so it is very possible that on the upper
Hillside and for anyone travelling through Southcentral`s many
mountain passes will see some snow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Widespread rains and cooler temperatures will spread throughout
the Southwest Mainland this evening as a frontal system moves
through the area. Behind this front, very cold air aloft will
provide enough instability to produce thunderstorms to inland
areas during the afternoon/evening hours on Wednesday and
Thursday. During the overnight hours cold air will descend far
enough into the low elevations of the Kuskokwim Valley and Delta
to bring light accumulating snowfall to many areas for Thursday
morning, which may also stretch far enough west to bring
accumulating snow into the Bethel area. Cloudy skies and cool
temperatures will then remain throughout the area for the
remainder of the work week as an upper level disturbances passes
overhead

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Very cold air associated with an upper level trough moving over
the area will bring showers and westerly winds throughout the
area into Thursday. With 850 mb temperatures falling below -6C,
this may even bring a brief round of snow to the Pribilof Islands
tonight, however temperatures will be much to warm for any
accumulations. Further to the west, a weak wave will skirt the
Western-Eastern Aleutians beginning Wednesday evening, bringing
mostly small craft winds and rain to the area through Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The very cold upper level low over the state will continue to be
the major story into the weekend. Snow levels will gradually rise
but the cold temperatures aloft will keep conditions conducive for
periods of showers over the majority of the region. A cutoff low
will develop from the main upper level low on Sunday and remain
over Kodiak Island as the main low finally lifts back into the
Arctic.

After this the uncertainty really increases as model spread
dramatically increases for the operational runs. The model
ensembles are indicating the next surface low will scoot just a
little south of the Aleutians but that solution is far from
certain.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 150 155 172.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...EZ



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