Area Forecast Discussion
Issued by NWS Anchorage, AK

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581
FXAK68 PAFC 180137
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKST Tue Jan 17 2017

.ANALYSIS AND UPPER LEVELS...

Cold air settled in across much of the mainland under a broad upper
trough. Soundings across the southern mainland showed significant
cooling in the upper and lower levels over the past day.
Satellite imagery showed partly to mostly cloudy conditions across
the mainland as well as much of the Gulf and Bering Sea. Isolated
snow showers associated with the upper trough and weakening
surface low over the Gulf produced scattered areas of snow
showers. At the surface, a ridge along with cold temps was evident
over western Alaska with a weakening surface low near the North
Gulf Coast. A pinch in the pressure gradient between the mainland
ridge and Gulf low produced gusty outflow winds across western
Prince William Sound and the western Gulf. A developing low center
was evident near the central Aleutians.

&&

.MODEL DISCUSSION...
Models overall are in general agreement on the positioning of the
surface pressure centers. However, significant differences exist
with the positioning of the warm front heading north toward the
Pribilof Islands Wednesday. This has lead to low confidence on
system evolution with this feature. Higher resolution models were
the choice in both domains for this forecast package.

&&

.AVIATION...
PANC...VFR conditions will persist into Wed.

&&

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

High pressure will filter into the area through Thu with the
coldest temperatures of the season so far. Low and high
temperatures will remain below zero, and only north Gulf coast
locations will be above zero in the teens. Another effect of the
interior high pressure will be moderate to strong winds through
channeled terrain, such as Broad Pass, Thompson Pass and the
Copper River Delta. Very cold temperatures in combination with
wind will result in winds chills to 45 below zero in Broad Pass,
mainly after midnight tonight and Wed night. Plenty of
transportable snow in Thompson Pass from recent snow storms, will
bring reduced visibility in blowing snow beginning midday Wed and
lasting through Thu night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
The cold and dry pattern that we have been in will slowly change
through the week. The first piece of energy that will bring signs
of the change is a polar low tracking towards Nunivak Island. This
system will peak in strength offshore, but it could bring some
gusty winds along the coast of the Kuskokwim Delta late tonight
and a dusting of snow to inland locations as we go into Wed. It
will then get caught up in a developing warm front over the Bering
and could help to produce some more light snow into Thu. The other
big story will be a trend towards slowly warming temperatures
through the end of the week. The system over the Bering will bring
some warmer air as flow switches to more south-southwesterly on
Thu.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed and
Thu)...
A weak surface low near Atka and Adak will continue to develop
tonight. Meanwhile, yet another polar low will swing down from the
Gulf of Anadyr accompanied by a push of arctic air. The trough and
cold air will provide the necessary energy to allow the low near
Atka and Atka to further strengthen as it tracks to the east. Exactly
where it deepens is the crux of the forecast challenge. Models
have shown wild variability with this system (likely due to the
complex nature of the polar lows that are factoring in). At this
point, it looks like the system will sling a warm front through
the Pribilof Islands Wed morning bringing some moderate snow and
gusty south winds. We do expect some blowing snow conditions with
this system as well, but with the warmer sea surface temperatures
and winds coming out of the south, we do not think it will reach
advisory levels at this time. We will be watching this system
closely though as a small variation in track could lead to much
heavier snowfall for the islands. The other impact will be some
snow and gusty southerly winds as the occlusion sweeps from west
to east across the chain. Models are still sorting out how quickly
the low center moves east. As it does so, it will usher in some
more snow and cold air with gusty northwest winds. This could
present another opportunity for blowing snow to reduce visibility
over the Pribilofs.

&&

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

Beginning the extended period forecast Thursday evening, a compact and
strong low over the Bering Sea will be moving into the Southwest
mainland while shortwave ridging ahead of it moves over
Southcentral. Bitter cold temperatures will begin to give way
toward moderating temps (although still cold) as clouds and
moisture associated with the Bering spread over Southern Alaska.
Snow will move into Southwest Alaska through Friday with the low
then stalling and weakening as it moves toward Southcentral. This
will introduce snow chances for Southcentral although no
significant precipitation will come out of this pattern given the
weakening nature of the low and poor moisture availability. By
Sunday and Monday will see temperatures moderating back close to
normal as the deep polar air mass retreats and a North Pacific low
moves toward the Gulf of Alaska. All global models are depicting a
similar pattern by midweek with a broad trough over the Bering
and southerly 500 hpa flow across the mainland. This should bring
temperatures to, if not above, normal for Southcentral with
precipitation largely favoring coastal areas.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 145. Blowing Snow Advisory 131.

MARINE...Gale 120 127 130 131 132 170 173-175 177-179.
Heavy Freezing Spray 121 126-130 139 141 150 160 165 180 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JA



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