Area Forecast Discussion
Issued by NWS Anchorage, AK

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768
FXAK68 PAFC 030206
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKST Mon Dec 2 2019

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low is centered over Prince William Sound
and the northern Gulf, with a plethora of weak upper level waves
rotating around Southcentral. Arctic air is filtering into
Southcentral from the north and west. Light snow is widespread
across the region, locally heavier ahead of short-waves. Strong
surface pressure gradients between the Gulf low and a surface
high over Bristol Bay combined with cold air advection is leading
to strong winds across the western Gulf and Kodiak Island, with
observations indicating Gale to Storm force winds. There are
likely Hurricane force wind gusts from Kamishak Bay to the Barren
Islands, areas which typically experience the strongest winds in
this pattern. The cold air advection is weakening with time, so
winds should begin to diminish by late this afternoon.

Out west, a weak vertically stacked low is drifting eastward
across the northern Bering Sea, with an upper level short-wave
trough and surface cold front extending southward down to the
central Aleutians. This is producing a showery regime across the
central Bering Sea. Further south, a strengthening low is passing
just south of the central Aleutians, with the strongest winds and
heaviest precipitation all south of the Aleutian chain. The
airmass feeding into the back side of this system is quite cold,
with 850 temperatures of -8 degrees or colder. Thus, precipitation
from Adak eastward to Atka is falling primarily as snow.

Meanwhile, high pressure is building over Southwest Alaska. Aside
from areas of fog, skies are clear and temperatures are seasonably
cold (in the single digits and teens).

&&

.MODEL DISCUSSION...
Models are on good agreement with large scale features over the
next few days and forecast confidence is generally above normal.
The primary forecast challenge for Southcentral Alaska will be for
aviation interests, with areas of stratus and fog likely as snow
diminishes tonight.

&&

.AVIATION...
PANC...Ceiling and visibility will both be quite the challenge,
with periods of snow expected through early Tuesday morning. In
between areas of snow and, once it tapers off, stratus is likely.
Will largely make use of satellite and radar trends for the
beginning of the TAF. Confidence will drop significantly beyond
the first 3 to 6 hours. For the most part would expect MVFR or IFR
conditions to prevail, with improving conditions through the day
Tuesday as drier air filters in from the north.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tuesday
night through Thursday)...

Broad low pressure continues to spin over southcentral AK,
remaining nearly stationary, before it is expected to drift south
through tomorrow. The low is also decaying, with the last of the
main snow event tapering off this evening. The last of the main
push of snow is moving south over the Kenai Peninsula, so the
winter weather advisory for the western Kenai has been left to
remain in effect until 6PM this evening, when it looks that last
push will begin to taper off for the Kenai. After the last push is
done, snow showers will linger overnight for many locations in
southcentral AK, though little accumulation is expected...perhaps
a half inch for most areas.

While this low slowly exits, cold air is already filtering in on
the backside of the low. This is evident with some strong cold air
advection creating gale and storm force winds (especially through
gaps and passes) along the coast in the western Gulf of Alaska.
Other areas like Thompson Pass are also seeing strong offshore
flow. These winds will taper into tomorrow, but expect cold air
to continue to move into all areas of southcentral AK and stay in
place for the rest of the week. Substantial cooling will take
place, though it will battle cloud cover as it does. Therefore,
did slow the cooling trend and kept temperatures warmer.
Honed in on this for the lower Susitna Valley, Anchorage Bowl,
and Copper River Basin (to a lesser degree here) as stratus looks
to set up, preventing temperatures from bottoming out. From a
meteorological standpoint, it will be interesting to see how this
plays out as the week progresses.

&&

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

Conditions across southwest Alaska remain fairly steady over the
next 48 hours. With light winds and ridging building in, patchy
fog will be a concern for southwest Alaska in mostly the coastal
areas. Tuesday looks to be quiet as high pressure remains in
place. Winds and precipitation chances pickup later Wednesday
evening ahead of the frontal boundary approaching in the Bering.
The front will then kick off day 3 with some precipitation and
gusty winds as it reaches the coast Wednesday night.

&&

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

A stacked low in the northern Bering tracks southward along a
substantial ridge in the north Pacific Monday evening. The showery
conditions in the cold air on the backside of this low will
persist through Monday night as a frontal boundary pushes south
on the northeast side of the ridge across the Pribilof Islands,
clearing the Aleutians by Tuesday night. Ridging keeps conditions
in the eastern Bering quiet through Tuesday. A storm force front
enters the western Aleutians Tuesday morning and decreases to
gale force by Tuesday night as it moves into the eastern Bering by
Wednesday night. A mix of rain and snow will occur along the
boundary.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

The primary focus for this time frame will be a developing low
over the western Bering. Most of the models keep the center of the
low over the western Bering with only slow movement southeastward
towards the central Aleutians. Gales are expected around this
low Friday and into Saturday. Storm force winds are possible
across the western and central Aleutians as well. The leading warm
front will move into the western Gulf on Friday, which may persist
into Saturday. Gales are expected with the front as well. As the
front impacts the north Gulf coast on Saturday, a storm force
barrier jet may form through the north Gulf coast and into Prince
William Sound.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

On Thursday, a front looks to move into Southwest Alaska. It may
be associated with a weak low, but the models are not in agreement
about that. Warmer air will be moving in behind the front, but
since the air mass ahead of the front will be quite cold, much of
the precipitation away from the immediate coast will fall as snow.
The front will quickly weaken as it moves inland. Meanwhile, a
couple lows will be consolidating across the western and central
Aleutians on Thursday. There is understandably significant model
disagreement on how this interaction will turn out, regarding
where the primary low ends up forming. A combination of a strong
Pacific jet well to the south of the Aleutians and the tapping of
a Siberian polar air mass will act to strengthen both lows. The
foreign models favor the north and western-most low, which would
keep most of the strongest winds and impacts over the Bering. The
GFS favors a much more south and eastward track of the center of
the low, which in turn shifts the impacts east over much more land
area. Beyond that, the model disagreement is too large to glean
any meaningful insight into the forecast.

For Southcentral, the long-term will begin with very cold air over
the Copper River Basin causing strong gap winds through Valdez,
the Matanuska Valley, and the Copper River Delta. While there are
some differences in timing, the models agree on a warm front
moving into the area on Friday. By this point it will be slow
moving, so it could result in a return to warmer temperatures, but
also unsettled/precipitation likely weather for the entire
weekend. There are large differences in how much precipitation and
what form it will be in, but it seems likely that there will be
coastal rain, and inland snow or mixed precipitation, along with
areas of strong winds. Thus, the cold snap will be just that,
short-lived.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 121.
MARINE...Gale 120 132 137 172-177 185.
         Storm 130 131 178.
FIRE WEATHER...NONE.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB/KH
MARINE/LONG TERM...JW



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