Area Forecast Discussion
Issued by NWS Anchorage, AK

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730
FXAK68 PAFC 221241
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
341 AM AKST Mon Jan 22 2018

.ANALYSIS AND UPPER LEVELS...
The longwave trough over mainland Alaska and the Gulf has evolved
somewhat over the past 24 hours and is now anchored by two
distinct low centers, one over the North Slope and another over
the far southern Gulf. Southcentral AK is currently under the
influence of weak cyclonic flow between these two features,
yielding mostly cloudy skies and isolated to scattered snow
showers. These showers are a little more prevalent along the Gulf
coast owing to a disturbance transiting the northern Gulf.
Meanwhile, mostly clear skies prevail across the southwest
mainland in brisk northerly flow between the longwave trough and a
stout, persistent ridge over the Bering Sea. Gusty winds aided by
a meridional jet streak moving down the western periphery of the
trough are contributing to very low wind chill values,
particularly through the Kuskokwim Delta where a Wind Chill
Advisory is posted through this morning. The cold northerly flow
is also initiating widespread convective snow shower activity
along the north side of the Alaska Peninsula and Eastern
Aleutians, with more benign conditions prevailing across the rest
of the Bering beneath the ridge.

&&

.MODEL DISCUSSION...
Models continue to show good synoptic agreement in the short term
as the longwave trough pivots and takes on a little more of a
negative tilt, eventually swinging the trough axis through the
Cook Inlet region by Tuesday. There is still some minor
disagreement on the numerous smaller scale features rotating
through the Gulf in this timeframe, but there is a general
consensus on the potential snowfall developing from the Mat-Su
through Anchorage and the Kenai Peninsula late this afternoon
through Tuesday morning. Largely stayed the course with the
inherited forecast for the morning package given good run to run
consistency.

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist through
this afternoon. Snow will begin to spread into the terminal by
late afternoon (around 02z), with ceilings lowering to around 5000
ft. Snow will increase in intensity by late evening and persist
through Tuesday morning as MVFR conditions develop after 08z.

&&

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

The two main forecast impacts through Tuesday are outflow winds
over the western Gulf and snow potential from the Western Kenai
Peninsula across the Anchorage Bowl to over the Mat-Su Valleys.
Strong pressure and thermal gradients will continue along the
Aleutian range. As a result, strong gap flows with gales to storm
gusts are expected to continue across the Western Gulf Marine
areas through Tuesday night. Gap flows across Prince William Sound
however should diminish today through Tuesday, but could see an
increase again late Tuesday night as pressure gradients start to
build again with cold advection.

Scattered showers are expected across many Gulf areas, Cook Inlet,
Anchorage Bowl and Susitna Valley today. Things get a little more
interesting this evening and Tuesday as a long wave upper trough
sets up just west of Southcentral. While this occurs a fairly
defined wave moves west across the Gulf and feeds low level
moisture west of the Chugach. This results in a broad deformation
zone from the Kenai Peninsula to the Mat-Su Valleys this evening
which will provide for steady snowfall. Snow will likely continue
fairly steady into Tuesday afternoon before tapering off to
flurries or snow showers Tuesday night. At this time we are
thinking sub-advisory snowfall, but this setup bears watching for
any additional clues for changes in intensity and duration of
snowfall.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
With mostly clear conditions and arctic air continuing to filter
in over the southwest Mainland, temperatures will continue to drop
as winds continue to increase during the overnight hours through
this morning. The models are still not in great agreement with
regards to how low the wind chill values will reach today, which
has caused some uncertainty in the forecast. However, based on
observations during the overnight and early morning period, some
observations around the Kuskokwim Delta are approaching wind chill
values of -25 F to -35 F. Thus, the wind chill advisory remains
in effect through noon today as temperatures are expected to
continue dropping this morning. Further upstream into the
Kuskokwim Valley, temperatures will remain much colder than the
surrounding Bristol Bay and Kuskokwim Delta regions. However, wind
speeds are struggling to reach 15 mph in the valley, keeping wind
chill values above advisory criteria.

Otherwise, a persistent area of high pressure across the Bering
will continue to funnel colder air into the area in an offshore
flow regime, with no sensible change in the pattern seen through
the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An area of high pressure will remain anchored across central
portions of the Bering Sea/Aleutians over the next few days. This
creates a rather stagnant pattern with a decaying stalled front
across western portions of the chain and Bering, and cold showery
conditions over eastern portions of the Aleutians and Bering. This
will bring continued freezing spray concerns to these eastern
areas through the period. Another front will clip the western
Aleutians and Bering Wednesday, but meet the same fate as its
predecessor, being blocked by the Bering ridge and not making much
eastward progression past Shemya.

&&

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

Beginning the extended period forecast Tuesday night, there will
be a rather potent cold upper level vortex over the northwest part
of Alaska with associated troughing extending into the Gulf of
Alaska. Low level cyclonic flow will preferentially wrap around
the Alaska Range into the Bering Sea. Snow showers over parts of
Southcentral will quickly be ending overnight (although they may
linger longer over the Copper Basin) as strong offshore flow
increases in response to cold air advection. Temperatures will
continue to drop across all of Southern Alaska as 850 temperatures
plummet to -20 to -30C on Thursday as the upper vortex moves south
across the region. AS the upper low digs into the North Pacific,
secondary cyclogenesis will develop in the Gulf with increasing
chances of coastal precipitation (in the form of snow) Thursday
into Saturday. Some locations such as Kodiak Island may see the
potential for good accumulations should the setup develop as
current global models depict...stay tuned.

Otherwise, the pattern looks to remain dominated by the eventual
large trough over the North Pacific into the Gulf of Alaska, along
with high latitude blocking over Eastern Siberia, for the
remainder of the weekend into early next week. This will likely
result in dry and continued colder than average inland weather as
the general storm track remains south of the forecast area.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155
MARINE...Gale Warning 127 130 131 138 150 155 165 172 180
 Heavy Freezing Spray 130 131 138 139 160 165 180 414

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...AHSENMACHER



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