Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 230133
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...
An upper level trough stretches from Point Barrow through Bristol
Bay with ridging over the central Bering Sea. West of this trough
is the dry northerly flow with precipitation chances east of this
trough. Cold advection is evident at 850 mb with temperatures at
this level near -6C along the north Gulf coast and dropping to
-15C along the Alaska Range, with temperatures -20C or colder west of
 the Alaska Range. A wave along the north Gulf coast will move
 inland tonight and interact with the slowly moving 500 mb trough
 causing a deformation zone to develop overnight in Southcentral
 Alaska. Strong cold advection is also bringing gusty north-to-
 northwest winds from the Eastern Aleutians through Kodiak Island.


&&

.MODEL DISCUSSION...
Models are in good synoptic agreement with similar solutions for
the major features through most of the week. What is worth
watching is the way some subtle differences pop up from run-to-run.
The models are trying to locate the area where the deformation
zone will develop over Southcentral Alaska overnight which will
correlate with the heaviest snowfall. Right now the NAM is the
farthest east of the solutions with the GFS just west of it and
GEM the farthest west. The 500 mb trough is also a key player in
Southcentral snowfall and models have slowed its passage through
the Anchorage and Mat-Su areas until Tuesday evening which can
allow the snow to linger until it passes east of the area.

&&

.AVIATION...
PANC...Ceilings will begin to lower this evening as the snowfall
moves closer to the area but should remain in the 3500 to 5000 ft
range until around midnight when the snowfall will intensify. This
will likely bring in IFR visibilities and MVFR to IFR ceilings
though tomorrow morning. Lingering snowfall will persist until
late tomorrow afternoon, but overall ceilings and visibilities
should be better tomorrow afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
From a synoptic view, there is a polar vortex north of the Brooks
Range that will dig south and a decaying low over the Gulf of
Alaska that will eject a shortwave north towards Prince William
Sound. This setup will result in a batch of fresh powdery snow for
the Anchorage Bowl with a snow ratio of 20:1. The current
assessment is that the snow will begin tonight and ramp up early
in the early morning, impacting the commute. Expect 2 to 4 inches
of new snow to blanket the Anchorage Metro area and the Mat-Su
Valleys. The deformation zone that sets up over Prince William
Sound will linger until a shortwave from the polar vortex clips
the Kenai Peninsula, effectively putting an end to the moisture
supply and shifting the dynamics eastward. The temperatures aloft
will radically decrease, with some of the coldest air of the
winter settling in across the region on Wednesday.

The other sensible weather highlight this forecast period is the
outflow winds. There is a strong thermal gradient along the
Aleutian Range. At 925 mb north of the Aleutian Range the
temperature ranges from -20C to -30C and south of the Aleutian
Range the temperatures range from -2C to -12C. The models hang
onto this gradient most of the forecast period. This pattern will
result in rough seas and strong gap flows with gale to storm
gusts for the Western Gulf marine areas through Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tuesday and
Wednesday)...
Quiet weather ongoing across Southwest Alaska will be punctuated
by the only real weather hazard...frigid cold. An upper level low
currently centered just north of the Brooks Range of northern
Alaska will slowly drift southward over the next couple days.
The upper level low center signifies the core of the coldest air
across Alaska. Thus as it moves southward, temperatures will
continue to drop. So with as cold as it has been across Southwest
Alaska the past several days, even colder air is to come. Lows
across the lower Kuskokwim Valley will easily drop south of 30
below so long as winds remain calm. High pressure over the area
will continue to keep the winds light. Thus, despite the cooling
temperatures, wind chill concerns are not expected. However, in
any localized interior areas where the winds can climb, wind chill
concerns would be a real possibility.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The weather across the Bering Sea will be a bit more active than
mainland Alaska. A large ridge of high pressure over the central
Bering remains nearly stationary. It is acting as a block on any
significant weather moving into the Bering from the west. A front
attempts to intrude into the Bering on Wednesday, but is only
expected to get as far as Attu and Shemya before shearing out and
dissipating. This is thanks to the giant area of high pressure
over the central Bering. Underneath the high itself, temperatures
are much warmer than those being experienced on the mainland, but
still cold enough that there are some precipitation type concerns
from any showers moving over the area. Stratus remains dominant
over much of the central Bering under the high pressure dome. The
front moving into the western Aleutians will bring somewhat warmer
air to that area on Wednesday, so the predominant precipitation
type at that point will be rain.

For the eastern Bering on the cold side of the high, continued
outflow from Southwest Alaska will keep conditions cold and
showery. In this pattern, upslope snow will continue to be a
constant threat along the Alaska Peninsula, where Cold Bay
experienced snow and blowing snow through much of the day. Strong
winds will also continue into the day Tuesday, especially through
bays and passes. Otherwise, the sea ice edge continues to expand
out from the coast in these very favorable conditions for growth.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Wednesday night, the main
story will still be the very cold upper vortex stretching from NW
Alaska to the Gulf of AK. The coldest air mass of this winter will
be moving south into Southern Alaska as 850 hpa temperatures drop
to the -20C to -30 C range. Broad cyclonic flow will extend from
the Bering Sea to a developing low in the Gulf of Alaska as the
cold upper low digs southward. Most inland areas will be quite
cold Thursday morning with little to no relief during the day as
cold air advection increases. There is the chance some locations
Thursday actually get colder as the day progresses. Snow along the
eastern periphery of the upper low will spread from the Copper
Basin and North Gulf Coast to the Kenai coast and Kodiak Island as
the low retrogrades west on Friday. The low will eventually dig
south through the weekend which means most coastal locations will
dry out although Kodiak will remain under the threat for
accumulating snow as bands of precip wrap around the upper low
through the weekend.

By Monday, high latitude blocking will have established itself
over Eastern Siberia, keeping all of southern Alaska under the
influence of arctic high pressure and continued below average
temperatures. This pattern will keep the storm track over the far
western Bering Sea.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray 130 138 150 160 165 180 181.
         Gale Warning 127 130 131 138 150 155 165 172.

&&

$$

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



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