Area Forecast Discussion
Issued by NWS Anchorage, AK

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062
FXAK68 PAFC 120103
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKST Wed Dec 11 2019

.ANALYSIS AND UPPER LEVELS...
A low south of Kodiak Island continues to track to the northwest
today. The main front associated with this low is ahead of the
low and moving across the eastern Bering Sea. A secondary low
is stretched across the north Gulf coast bringing rain to that
region. The 250 mb jet stream is running south of this low and
will remain south of the region. This will keep generally mild
air over southern Alaska for a number of days.

&&

.MODEL DISCUSSION...
Model guidance remains in very good agreement through the
remainder of the week. The only item keeping them from being in
excellent agreement is that the NAM is a little bit too fast in
bringing in the front to the northern Gulf on Friday.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist.

&&

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

Low pressure is moving north across the Gulf of Alaska toward
the north Gulf Coast. Winds over the northern Gulf will remain
easterly between the 40 to 50 mph range before diminishing
Thursday morning. A weak pressure gradient has established a Mat
Valley wind with gusts expected up to 25 mph out of the northeast.
The Copper River Basin looks to stay on the cooler side with
patchy fog being a concern as offshore flow has weakened since
yesterday. This looks to limit drying out the basin or scouring
fog out of the area until at least until Thursday morning.

Small waves of energy are rotating around this system, although,
without upper level support they don`t look to provide the
precipitation rates we`ve seen the last few weeks. The strongest
wave of energy is possible late Thursday into Friday where the
Copper River Basin could see a few snow showers with minor
snowfall accumulations. Due to the warm air push with the low
coming towards the north Gulf, coastal locations from Kodiak
Island to western Prince William Sound precipitation looks to
remain in liquid form with higher elevations seeing snowfall.
Most locations on the leeward side of the mountains will remain
dry due to downsloping from cross-barrier flow (flow that is
perpendicular to the mountains instead of parallel) so expect
limited precipitation across most of Southcentral.

&&

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

A stationary low in the Gulf of Alaska will bring scattered
showers to Bristol Bay and coastal areas of the Kuskokwim Delta
through Friday. Across the Alaska Peninsula tonight, a cooler air
mass across the Bering will change precipitation over to snow west
of Nelson Lagoon. Gusty northerly flow persisting overnight will
result in areas of blowing snow along the western Peninsula. Winds
taper off early Thursday as the low center lifts northeast;
however, precipitation continues as low pressure remains dominant
across the Bering and Gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Two low pressure centers will keep active weather across the
Bering through Thursday with gusty northerly conditions across the
eastern waters tonight tapering down on Thursday as the low south
of the Alaska Peninsula lifts northeast. The next stronger front
moves up to the western Aleutians Thursday night. This system will
bring brief blowing snow conditions for Shemya as it crosses to
the southern Bering. Otherwise...this system will bring mixed
precipitation across the rest of the central and eastern Aleutians
along with gusty gale force winds through Friday.

&&

.MARINE (Days 3 through 5)...

For the Bering: Weather starts out quiet on Friday with a high
pressure ridge in place over the Bering Sea. On Saturday, a strong
low pressure system will skirt the southern side of the Central Aleutian
Islands as it moves west to east. It will bring gale force winds
to the area as it goes, exiting Sunday. Model agreement is good
with this system and we are confident in the forecast.

For the Gulf of Alaska: A low will be present in the Western Gulf
of Alaska on Friday, bring a large area of small craft advisory
winds with isolated gales to much of the gulf. It will also keep
sea states elevated. The low moves ashore over Southwest AK
Friday night, calming winds and waves for Saturday morning. Later
Saturday into Sunday another front enters from the southwest,
bringing gales and small craft advisories back to the gulf. We are
confident in this forecast as model guidance looks to be on
track.

&&

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

Models have come into better, though not perfect, agreement for
the long term. Expect a continued active pattern over the
Aleutians, while quieter weather will persist over Southcentral.
Saturday, a low in the western Gulf of AK will continue to swing a
series of weak fronts toward the North Gulf Coast, bringing
periods of precipitation into Sunday. Precipitation will be
concentrated on southeast facing slopes, mainly on the Kenai
Peninsula and the Alaska Range. Meanwhile, surface high pressure
over the Yukon and interior AK will strengthen, and conditions
will trend colder over inland areas of the Southern Mainland.

Out west, a strong low south of the central Aleutians will swing
an occluded front across the chain and generate a healthy round of
precipitation over the Aleutians, the AKPEN and likely into the
Southwest Mainland Saturday night into Sunday. Precipitation
chances look highest in the Bristol Bay area, with the Kuskokwim
Valley/Delta likely staying drier in offshore flow. As the
occluded front pushes toward the Southwest Mainland, relatively
cold air in place over the mainland will enhance the pressure
gradient, and strong south/southeast winds are possible from
Kamishak Bay to as far north as the Kilbuck Mountains. There is
indication that a triple point low will form south of the AKPEN
late Saturday, enhancing precipitation over Kodiak Island and
along the south/southeast facing slopes of the Alaska/Aleutian
Range. However, it`s hard to have much certainty this far out and
with run-to-run model inconsistencies.

As that secondary low pushes northeastward Sunday into Monday,
expect a swath of precipitation and strong winds in the northern
Gulf of Alaska. In addition, expect another round of precipitation
along the eastern Kenai Peninsula Sunday night into Monday...then
precipitation will likely spread toward Prince William Sound and
eastward along the coast Monday.

Another low will approach the western Aleutians Monday, and push
an occluded front across the chain through Tuesday. Since there`s
still some uncertainty with the progression of the low downstream,
it`s tough to discern how the upstream low will affect the
Southwest Mainland. Models differ on whether the front will
dissipate before reaching the mainland.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 130 131 139 155 165 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...BB/MM



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