Area Forecast Discussion
Issued by NWS Anchorage, AK

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444
FXAK68 PAFC 101743 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKST Mon Dec 10 2018

.ANALYSIS AND UPPER LEVELS...
A large upper level trough extends across the eastern half of the
Bering Sea and western mainland with two distinct upper lows
embedded within the trough, one over the Bering Strait and the other
over the Pribilofs. A 965 mb surface low over the Pribilofs is
swinging a front north across the southwest mainland and into the
Gulf of Alaska and the North Gulf Coast. Radar imagery shows a wide
swath of precipitation moving into the Gulf Coast and the Kenai
Peninsula. Antecedent cold air is allowing precipitation to fall
mostly as snow, even along coastal areas this morning. Strong cross
barrier flow is keeping inland locations across Southcentral mostly
dry. Radar over the Southwest is also showing snow spreading north
and east across the AKPEN, Bristol Bay Coast, and Kuskokwim Delta as
the front moves inland.

Out over the Bering and Aleutians, strong northerly flow is
drawing cold air south from Siberia and moving it over relatively
warm sea surface temperatures. This is leading to widespread snow
showers seen over most of the region, including communities along
the Aleutian Islands.

&&

.MODEL DISCUSSION...
Models are in decent synoptic agreement with the first front
moving across the mainland this morning and through today. As the
upper trough moves east and settles in over the mainland, models
depict multiple low centers and waves rotating around the upper
trough. There is little agreement in the exact timing and
placement of the individual features, so confidence is lower on
specific forecast details including snow location and timing,
especially for locations not along the coast.

&&

.AVIATION...
PANC...VFR conditions and light winds will continue through the
day today as downsloping should keep the Anchorage Bowl dry,
though some isolated snow showers may move through early this
morning. As we head into the evening and the cross barrier flow
begins to slacken, there is a chance that a band of snow could
move north up the Cook Inlet. Model agreement at this point is
fairly low so only included VCSH until forecast confidence
increases.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Tuesday night)...
The colder air which moved across the Gulf and Southcentral
yesterday was the beginning of what appears to be a prolonged
shift to a more "typical" Alaska weather regime for this time
of year. Over the next couple of days an upper level low and
trough over the eastern Bering will move into the Gulf, with
a series of fast moving upper level waves and surface fronts
and lows tracking into the Gulf, then rotating northward toward
Southcentral. While it is nearly impossible to nail down all
of the details of each individual weather feature with any
confidence, there are some specifics of the forecast which
can be communicated confidently based on the pattern:

1. Most of the region (Southcentral Alaska, Kodiak Island, and
the Gulf of Alaska coastal waters) will likely see at least some
light snow accumulation between now and Tuesday night.

2. Strong low level cold advection will ensue across the western
Gulf, Kodiak Island, and southern Cook Inlet/Kachemak Bay on
Tuesday, resulting in very strong winds on the order of 40 to 65
mph, depending on location. Expect some "lake effect" type snow as
Arctic air moves over the relatively warm waters. In particular,
this is favorable for snow showers and blowing snow in Homer and
vicinity. Surface temperatures at sea level will be near freezing
as the winds arrive during the day Tuesday, which is not conducive
to really low visibilities in blowing snow. However, if steady
showers develop, higher elevations have the potential for periods
of white-out conditions. Will continue to evaluate this potential
as we get a better handle on what the extent of snowfall will be.

3. The aforementioned Arctic air will sweep across all of
Southcentral Tuesday night and Wednesday. Prior to this, towns
along the Gulf coast and Prince William Sound will see periods of
rain and snow. Once the cold air moves in all precipitation will
change to snow.

4. Most model solutions track the upper low center to the vicinity
of the Kenai Peninsula on Tuesday then lift it northward across
Southcentral Tuesday night. Meanwhile, they all indicate some type
of weak surface low center settling over western Prince William
Sound/eastern Kenai Peninsula. With the cold air moving in aloft,
the atmosphere will become quiet unstable. This pattern is
favorable for bands of heavy snow showers to set up. If the
general positions of both the upper and lower level lows is
accurate, this would be particularly favorable for bands of snow
to track from western Prince William Sound to Anchorage and the
Matanuska Valley sometime Tuesday through Tuesday night. Remember,
confidence in track of these features is low, but the potential is
there.

5. As indicated at the top of this section of the discussion, this
pattern looks to remain in place beyond the next couple days. With
cold air in place and the storm track remaining to the south,
there will be more chances of snow as we head through the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Wednesday)...
An active weather day has begun across portions of Southwest
Alaska this morning as snow associated with a developing low in
Bristol Bay is moving into coastal communities west of King
Salmon. The low center is expected to stall out along the coast of
the Western Capes west of Dillingham today. Meanwhile a front
with an upper level shortwave is expected to swing around the low,
bringing a brief period of snow that could be moderate or briefly
heavy at times through much of Bristol Bay today. Fortunately,
the area of snow is expected to be fairly quick-moving, which
should keep accumulations below advisory criteria. However, the
snow could come down heavily at times as the front moves north
and dissipates when it reaches the Lower Kuskokwim Valley.

The moisture and precipitation north of the low center is
expected to track northward into the Kuskokwim Delta, where it
will develop into a deformation zone today. Lift with the
deformation zone will be ahead of an approaching upper level low
drifting south out of the Bering Strait today. The Bristol Bay low
will effectively help draw the Bering Strait low south and east
today, causing the troughing associated with the Bering Strait
low to become more negatively tilted with time. This in turn will
increase the lift with the deformation zone, helping it to
maintain itself for much of the day and through most of the
overnight tonight as well. As with all deformation zones, this one
is likely to move east and west through the day, rather than stay
in one place, thus any one location is likely to see periods of
snow, with breaks in between bands. However, since most of the
snow is expected to be confined to a north-south corridor
encompassing Bethel and the surrounding area, it`s likely most of
that area will see storm total accumulations exceeding 6 inches,
with isolated amounts to 10 inches.

The deformation zone and a strong pressure gradient is expected to
keep the winds on the gusty side through much of the Kuskokwim
Delta today. Reinforcing cold air moving into the Kuskokwim Delta
will help to keep temperatures from moving much from their
current readings in the single digits along the coast (where the
cold air has already reached) to around 20 inland. These well-
below-freezing readings will act to keep the snow in a powdery
form, which will easily be blown around in the increasingly strong
winds tonight. With all of the above expected, a Winter Weather
Advisory is now in effect for the Kuskokwim Delta through tonight.

The center of the Bristol Bay low will move across the Alaska
Peninsula over King Salmon tonight and re-emerge in the Gulf by
Tuesday morning. Meanwhile the negatively tilted upper level
trough will continue building southeastward through the day
Tuesday. Those north and east of the trough, primarily north of
Iliamna and across the Lower Kuskokwim Valley are likely to see
snow shower activity continue for much of the day Tuesday, which
may amount to an additional 1 to 3 inches of accumulation. The
upper low itself may move over Southwest Alaska by Wednesday,
which will cause some of the coldest temperatures of the season
for Bristol Bay and the Lower Kuskokwim Valley as many areas drop
below zero. Onshore winds through the Kuskokwim Delta may keep
clouds over that area, so the bitterly cold temperatures are not
expected there.


&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Wednesday)...
The main action through this time period will be continued cold
air streaming south out of Russia, leading to widespread bands of
ocean-effect snow showers right through into Wednesday. The low
in Bristol Bay today and the influx of cold air behind it are
expected to increase the strong NW winds significantly over the
course of today from Kuskokwim Bay southward into the Alaska
Peninsula by tonight. The winds will only further intensify
tonight, causing widespread gales across the eastern Bering, with
storm force level winds expected over Bristol Bay and south of the
Alaska Peninsula in the waters south and west of Kodiak Island
late tonight. The winds will diminish during the day Tuesday as
the low moves further east into the Gulf. High pressure will move
into the western Bering tonight, allowing the winds to relax
there, but weak areas of low pressure will form along and south
of the Aleutians along the greatest temperature gradients on
Wednesday, leading to a mix of rain and snow. The lows will be
relatively weak however, so winds are not expected to be an issue.

The only other expected hazard in the Bering will be the strong
winds moving over the cold waters of the northern Bering causing
widespread heavy freezing spray conditions around Saint Matthew
Island for the next couple days. With no warm-ups in the forecast
at any point in the next week, freezing spray will continue to be
a concern for the foreseeable future for much of the Bering.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The general trends for the long range forecast is for the most
active weather to continue across area waters with temperatures
trending colder Wednesday night through the weekend. A long wave
trough Wednesday night is projected to have a parent low just
south of the Bering Strait and another low over the southeastern
Gulf of Alaska. Models are generally in good agreement with the
closed center over the Bering Strait remaining the dominant
system. They are struggling with the northward progression and
organization of the Gulf and system and the progressive wave
pattern over the Bering. The main changes for the long range
forecast were to decrease precipitation chances across the
southern mainland and to cool temperatures through the second
half of the week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Heavy Freezing Spray 185
Storms 130 132 150 160
Gales 119 120 131 136 137 138 139 141 155 165 172 180 351 352 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH



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