Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 240204
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKST Fri Feb 23 2018

.ANALYSIS AND UPPER LEVELS...
A progressive upper level wave pattern continues across the
southern tier of Alaska. An east-west oriented trough over the
Bering Sea is digging eastward toward the southern Alaska
mainland with the support of a zonal jet streak near the Aleutian
chain. At the surface, an occluded front is elongating across the
northern Bering and Southwest Alaska coast with a triple point and
trailing cold front headed eastward across the Alaska Peninsula
and into the western Gulf. Snow is spreading north and east across
Southwest Alaska, with rain in the warm sector along portions of
the Alaska Peninsula and Kodiak Island. Gusty winds along portions
of the Kuskokwim Delta coast are producing blowing snow and low
visibilities - and a Winter Weather Advisory remains in effect for
this area until 9 pm this evening.

Back behind the occluded and cold fronts a cold an unstable air
mass is leading to snow showers and gusty winds across the
southern Bering Sea and Aleutian Islands. The GLD lightning system
even picked up on a few isolated strikes. The next storm system is
beginning to take shape over the northwest Pacific as a short-wave
trough ejects out of Asia.

Out ahead of this storm system most of Southcentral is starting
out with a "bluebird" day, with just a few areas of low clouds
over interior valleys. Locally gusty offshore winds are quickly
diminishing as gradients slacken ahead of the approaching front
over Southwest Alaska.

&&

.MODEL DISCUSSION...
Models remain in excellent agreement with large scale features
and show decent run-to-run consistency. Thus, forecast confidence
is high and only minor adjustments will be made to the existing
forecast.

&&

.AVIATION...
PANC...Another round of snow will arrive around 09z tonight, with
ceilings/vis rapidly dropping. With a weaker overall system
compared to the last storm, expect snow to be lighter. However,
with a colder air mass in place this will lead to lower density
snow which will be prone to blowing around and reducing
visibilities as vehicles or planes move through it. Thus, still
expect a period of IFR conditions, most likely between 12z and
18z. Forecasts confidence drops significantly once snow ends, as
there could be a period of clearing or stratus and fog (with IFR
or LIFR) could settle in behind the snow during the day Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper trough and developing frontal wave crossing the Alaska
Peninsula this afternoon will swing over Kodiak Island this
evening and then spin up to a closed low just to the east of the
Barrens by around midnight. The surface low will rapidly deepen as
it continues east across the Gulf late tonight through Saturday.
With the rapid cyclogenesis and strong cold air advection
following the pattern looks good for storm force west to north
westerly wind along the back side of the low and very strong
westerly outflow winds across Kodiak Island, Shelikof Strait and
the Barrens late tonight through Saturday. Additionally, strong
northwesterly outflow winds will strengthen along the eastern side
of the Kenai Peninsula and western Prince William Sound Saturday
afternoon through Saturday night.

As the associated front lifts north into Southcentral Alaska
tonight through Saturday, it will bring another round of snow to
most areas with around 3 to 5 inches possible in the lowlands and
5 to 9 inches in mountains, with west to southwest facing slopes
favored for higher accumulations. In addition to the strong cold
air advection accompanying the front, another aspect of this
system very favorable for snow production is the deep dendritic
growth zone that the temperature and moisture profile support. An
environment profile like this will allow for very efficient snow
production, leading to higher than normal snowfall amounts for the
amount of liquid moisture available and a very light and fluffy
snow pack.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The frontal system currently tracking inland over the southwest
mainland will continue to bring snow and gusty winds to much of
the mainland through tonight, with some of the gustier areas
experiencing reduced visibilities due to blowing snow.
Temperatures will also continue to warm, but the majority of the
mainland should remain cold enough for the precipitation type to
remain as snow. Along the Kuskokwim Delta coast, gusty east winds
accompanied by moderate to heavy snowfall rates will persist into
this evening, leading to blowing snow and periods of decreased
visibility below one-half mile. Thus the Winter Weather Advisory
remains in effect, though the timing was bumped up 3 hours as the
system is arriving a little sooner than originally expected.

Winds behind the front will switch around to the west and begin
to increase as cold air advects in from the north tonight. Gusts
are expected to be as high as 40-50 mph along and near the western
coast line from Bristol Bay up to the Kuskokwim Delta. These
gusty west winds will continue through much of the weekend and
allow some rough surf to develop along the coastlines Friday night
through Saturday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front over the eastern Bering will finish pushing inland this
evening, leaving colder air and gusty westerly winds in its wake
over much of the Bering. As seen in recent satellite imagery, a much
more unstable/convective regime can be seen over the Bering which
should keep any fog from forming over much of the area. Further
west, the next system will start pushing into the western and
central Aleutians Saturday morning, and continue to spread north
and eastward through the weekend. This system will bring the next
round of stratiform precipitation and gusty winds to the area as
it progresses northeastward over the weekend.

&&

.LONG TERM FORECAST (Monday through Friday)...The active pattern
remains on tap through next week.

For the southern mainland and Kodiak Island we`re still on track
for a low to traverse the mainland beginning Sunday night
supported by a large upper trough. This system remains
sufficiently cold to keep all precipitation of the frozen variety
except for Kodiak Island where rain should briefly mix in. A
triple point low forms along the North Gulf Coast Monday then
dives southeast Tuesday and Wednesday. A residual low remains in
the Prince William Sound area through Monday night. Coupled with
the supporting upper trough, this could result in the snowfall
staying over south central Alaska through Monday night. The low
and associated trough that extends to the northwest will result in
a tight gradient and thus, gusty strong outflow wind out of the
Chugach Mountains ringing Prince William Sound Wednesday and
Thursday. The gradient relaxes Friday and conditions should settle
down over the southern mainland. Previously we were expecting
a shot of warmer advection into southwest Alaska Thursday night
and Friday. The latest model solutions are backing away from the
warm advection intruding into southwest Alaska at this point.

Over the Bering Sea, Aleutians and Alaska Peninsula we start the
week with a large strong low over the eastern Bering Sea that
barrels into the mainland Sunday night. Cold air wraps around the
west quadrant of this low coupled with the tighter pressure
gradient, should result in strong gusty west and northwest wind
over the eastern Bering Sea and Alaska Peninsula Monday. A weak
flat ridge in the wake of this low will rapidly yield to the next
incoming storm entering the western Bering Sea late Monday. This
low rapidly traverses the Bering Sea before dissipating as it
approaches the Bristol Bay region Tuesday night. A large low is
then on tap to push into the western Bering Sea Wednesday.
Yesterday it appeared this low would push a front to the Bering
Sea Coast and strong warm advection into southwest Alaska. Todays
model runs have backed off on that solution opting for a more
westerly track that will have the front stall then dissipate over
the eastern Bering Sea Friday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155. MARINE...Storms 130 131 136
137 351 352. Gales 120 132 138 139 141 150 155 160 165 170-181
185 411-414. Heavy freezing Spray 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...JER
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...SA



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