Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 260135
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKST Fri Nov 25 2016

.ANALYSIS AND UPPER LEVELS...
Satellite imagery shows a rapidly intensifying cyclone just south
of the Aleutian chain tracking northeastward. A strong jet streak
(estimated 125kt or more from 500 mb on up) and strong low level
baroclinicity and convergence support high winds, and moderate to
heavy precipitation. Also of note, a very strong potential vorticity
anomaly is evident in water vapor imagery (very dry air) just
south of the low center. This is favorable for thunderstorms as it
moves over top of low level warm and moist air. Sure enough, the
GLD lightning data is showing some strikes right along the leading
edge of the pv anomaly. As this storm continues toward the Bering
Sea conditions are rapidly deteriorating along the Central to
Eastern Aleutians.

Looking downstream, a high amplitude trough is digging
southeastward across the Gulf of Alaska with the main surface low
center tracking eastward across the central Gulf. A secondary
surface low has developed over the northern Gulf near the primary
upper level trough axis. Precipitation is focused right out ahead
of this across much of Prince William Sound. Areas of light snow
or flurries extend across the rest of Southcentral with weaker,
less focused lift ahead of a secondary trough lingering back over
Southwest Alaska and the eastern Bering. Mid and upper levels have
dried out from Southwest Alaska into the western Gulf and Cook
Inlet. This has led to development of fairly widespread stratus
and fog, which will continue to be a problem as we head through
tonight.

&&

.MODEL DISCUSSION...
Models are now in very good agreement with handling of the deep
storm system near the Aleutians. The trend has been toward a
slightly more southern track as the low tracks across the eastern
Bering and into the Gulf. More significantly, there is better
agreement in a more southerly position of the warm occlusion
front. The ramifications of this are the focus of forecast
adjustments. Forecast confidence with this system is generally
pretty high.

Aside from this storm, one of the biggest forecast challenges in
the short-term will be extent of fog and stratus across Southwest
and Southcentral this afternoon through tonight. Forecast
confidence for any specific location, including aviation terminal
forecasts, is low.

&&

.AVIATION...
PANC...The taf is very challenging for this afternoon through
tonight. An upper level trough will swing through, drying out
the mid to upper levels while weak flow in the low levels will
maintain moist conditions. This provides ideal conditions for
fog to form somewhere in the vicinity. For now have added VCFG,
but will monitor nearby obs and web cams for fog formation. If
fog does form, expect conditions to clear out by mid-day Saturday
as low level winds pick up ahead of the next storm system.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The remnants of a weak low is parked in the northern Gulf. This
is producing a showery regime along the northern coastal
communities with some moisture spreading inland this evening.
By Saturday afternoon a storm force low is positioned over Kodiak
Island, and produces gale force northeasterly winds through southern
Cook Inlet. Meanwhile, the associated weather front tracks through
the northern Gulf. Therefore, the pressure gradients strengthen
combined with abundant moisture streaming into the area will
produce blowing snow and snow issues through the Turnagain Pass
region late Saturday afternoon through early Sunday morning. Look
for the Eastern Turnagain Arm, and Portage region temperatures to
be slightly warmer which results in a wet snowfall scenario. By
Sunday morning the storm force low will be located in the
northern Gulf resulting in storm force westerly winds in the lower
Gulf. The Anchorage Bowl will have an inverted trough extending
into the region aiding in producing some accumulating snowfall
through Sunday. While on the backside of this system colder air
filters into the region producing gusty westerly winds through the
Barren Islands extending southward into Kodiak Island.



&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Areas of fog will continue in lower lying areas of Bristol Bay,
which may even be dense at times as weak ridging moves overhead.
This fog will then begin to dissipate late this evening as the
front from the a deepening Bering low begins to make landfall.
Confidence is increasing that this front will bring blizzard
conditions to interior and western Bristol Bay on Saturday as
gusty northeasterly winds develop along with heavy snowfall on top
of a few inches of new transportable snow. Further to the north,
confidence is slightly less along the Kuskokwim Delta coast as the
models have trended slightly to the south with the front, limiting
the available transportable snow. As a result the existing
Blizzard Warning was trimmed back to just include the Kuskokwim
Bay, as areas further north and east should not have enough
transportable snow to create blizzard conditions. Starting late
Saturday night, winds and widespread snow will begin to diminish
as weak ridging moves into the region for Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The weather will continue to focus on the deepening low currently
moving into the Bering. Confidence is good that this system will
bring high winds from Atka eastward into the AK pen, with the
strongest winds likely occurring over the eastern Aleutians.
Confidence is slightly less for the warning over the central
Aleutians as the strongest winds will only briefly move through
the eastern edge of the zone, with good agreement that the
strongest winds will occur away from Adak and Atka. This low will
deepen to hurricane force as it moves through the Aleutians,
creating seas around 40 ft on the Pacific side of the islands.
Winds throughout the area will then weaken late Saturday night as
the low crosses the AK pen into the Gulf and slowly dissipates.

A second much weaker low will then move from the central
Aleutians into the Bering for Sunday, however impacts from this are
expected to be much less the the current system.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Models have trended south with the low crossing the Gulf of Alaska
on Sunday but all still develop the remnant low over Prince
William Sound Sunday evening after the main low tracks into the
eastern Gulf then inland across Southeast Alaska. The combination
of the presence of this Prince William Sound surface low and the
cold air advection from the upper trough over southwest Alaska
pushing into southcentral Alaska increases confidence in the
potential for a snow event over southcentral Alaska Sunday night.
Current expected QPF amounts combined with a favorable snow growth
temperature and moisture profile have the potential to bring
around 4 to 6 inches of snow to the Anchorage Bowl and Hillside as
well as the Matanuska Valley from Sunday evening through Monday
morning. The snow will shift east over the Copper River Basin with
several inches possible there through the day Monday as the low
also shifts east.

Out west, a surface low over the Bering Sunday night will weaken
as it tracks east toward Bristol Bay Monday then dissipate as it
crosses the Alaska Peninsula Monday Night. A stronger incoming
frontal system will lift from the North Pacific up over the
Aleutians Monday Night. The parent low will track from west to
east just south of the Aleutians Tuesday and Wednesday as the
front progresses across the Bering. South of the Gulf, another
low will rapidly spin up Monday and Tuesday as it tracks toward
the eastern Gulf and then swings inland across the Southeast
Alaska Panhandle Tuesday Night. For Southcentral Alaska the
biggest impacts of this system will be to substantially increase
cold northwesterly outflow gap winds Tuesday and Tuesday night.

For the second half of the week, a cold air mass will persist
over much of mainland Alaska supported by continued upper level
troughing and periodically being reinforced by arctic troughs
swinging in from the northwest. A series of frontal systems
tracking in from the west across the Aleutians and southern Bering
as well as frontal systems lifting up through the Gulf will
additional chances for snow to the area through the end of the
week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 185 187.
         Blizzard Warning 155 161.
         Blowing Snow Advisory 125.
MARINE...Hurricane 155 170 172 173 174 176.
         Storm 150 165 171 175 177.
         Gale 119 120 125 130 131 132 139 160 178 179 180 181.
         Heavy Freezing Spray 180 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...JR



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