Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 191359
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Thu Jan 19 2017

.ANALYSIS AND UPPER LEVELS...

Upper Levels...An upper level trough is moving across Southcentral
this morning. The axis of the trough looks to be the lowest the
500 mb heights will get for the foreseeable future, with
Anchorage at 495 dm. Low 500 mb heights are a direct result of the
entire lower atmosphere being very cold. Cold air is denser than
warm air, thus cold air takes up less space than the same volume
of warm air. Thus, low 500 mb heights mean that one does not need
to go quite as high up in the atmosphere to reach that pressure,
which can only happen if the air is cold.

Further west, a shortwave ridge is building northward in response
to a somewhat warmer air mass moving into Southwest Alaska. Over
the Bering, a deep positively tilted longwave trough remains
entrenched from the Pribilofs westward, with the associated very
cold air keeping a very large cumulus field alive and well over to
nearly the Kamchatka Peninsula. The cumulus field results from the
cold air moving over the relatively much warmer water of the
Bering. The cold air then picks up heat and moisture from off the
sea surface, which then rises up and creates a large area of
cumulus clouds.

Southcentral Analysis...The coldest air in several years has
moved into nearly all of Southcentral. For Talkeetna, it has been
18 years since it`s been this cold! High pressure is keeping many
areas cloud-free, with the exception of west Anchorage, which is
contending with patchy dense fog. The 00Z sounding shows a bone
dry atmosphere except just near the surface, where winds are
nearly calm.

Southwest Analysis...It`s a similar story in Southwest Alaska away
from the coasts, with temperatures in the lower Kuskokwim Valley
nearing -40 in Sleetmute, and -10s across much of Bristol Bay.
Near the coast however, the warm frontal shield of clouds
associated with a Bering low is pushing inland, with light snow
falling along the Kuskokwim Delta.

Bering Sea Analysis...The primary feature out in the Bering is a
low tracking near the Pribilofs causing blowing snow with gusty
winds through the Pribilofs and the Alaska Peninsula. It`s
bringing somewhat milder air to the eastern Bering, but even on
the warm side of the storm, the precipitation type is snow. Much
stronger winds are on the west side of the low as cold air wraps
around the storm. The 0728Z ASCAT Scatterometer pass shows winds
nearing storm force southwest of the low. The aforementioned
cumulus field encompasses the western Bering.

&&

.MODEL DISCUSSION...

After much wavering in the consensus of the models over the past
few nights, the models are in good agreement tonight with the
track of the Bering low. The GFS has been the most consistent of
the forecast models taking the track of the low north with time.
The northward shift in the track of the low as it heads towards
the coast of southwest Alaska tonight was much smaller than the
northward shifts of previous nights, which greatly increases the
confidence in its eventual track. Forecast confidence is high with
a fairly quiet pattern and good model agreement.

&&

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

An upper level ridge will move across Southcentral Alaska today
and tonight. Very cold temperatures will continue through tonight.
Fog and low clouds will continue near Anchorage as well, with some
flurries this morning. Clouds will increase later tonight as an
upper level trough approaches from the west. Temperatures will
begin to warm on Friday and Friday night, and chances of snow
will return to the area as an upper level trough approaches from
the west.

&&

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

Snowfall will continue to move into the Kuskokwim Delta and the
western capes of the Bristol Bay zone this morning ahead of a
rapidly occluding frontal boundary in the Bering Sea. Blowing snow
remains the biggest concern for the next 24 hours as snow
intensity increases this morning and winds between 25 to 45 mph
become common across coastal locations of Kuskokwim Delta. The
advisory for snow and blowing snow has been extended through 15Z
Friday as a favorable low track looks to linger precipitation and
stronger winds. Based on the vertical temperature/moisture
profiles, the snow should be quite dry (in the 15:1 to 20:1 range)
which will easily be picked up and blown around. That being said,
some blowing snow conditions will be possible for the rest of the
Kuskokwim Delta but should generally remain greater than 1 mile.

Snow amounts also have trended a bit higher across Southwest
Alaska with the new low track. Amounts between 4 to 9 inches will
be seen across the Kuskokwim Delta with the greatest amounts from
Bethel westward. The Lower Kuskokwim Valley could pick up some
snow but most accumulations will be limited to locations near the
Alaska Range. There will be a brief "warm up" today as the low
moves inland but temperatures should rapidly cool off on Friday as
strong cold air advection on the backside of the low brings
another round of frigid temperatures to the region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Conditions across the Pribilof Islands are expected to deteriorate
by late morning as strong snow showers coupled with gusty winds in
the 45 to 55 mph range bring the return of blowing snow. The
advisory start time has been pushed up a couple of hours based on
latest satellite imagery and model data that show gale force winds
quickly approaching the Pribilof Islands. The low will continue to
shift eastward and move inland over Southwest Alaska late tonight
but cold air advection snow showers will continue across the
Bering Sea through the weekend. A strong area of low pressure is
expected to move south of the Western Aleutians on Saturday and
push a pretty potent warm front into the southern Bering Sea and
allow some snow to mix or change to rain over the western and
central Aleutian Islands.

&&

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

There remains good general agreement in the models that the
current cold snap over much of the Mainland will begin to break
down beginning this weekend as the arctic trough begins to lift
northward. As this occurs, there are significant differences in
the track of low dissipating near Bristol Bay late Friday. This is
causing low confidence with this system, especially with regards
to chances of snowfall from the Southwest Mainland into Cook
Inlet.

Once this system dissipates on Saturday, there is good confidence
that a trough digging over the Bering will shift to the east,
bringing the North Pacific low track back into the Gulf beginning Sunday
and continuing into early next week. As is typical in the long
term forecast, there is enough uncertainty in the exact impact of
this pattern change. There is however reasonable confidence that
temperatures throughout the South Mainland will rise to near or
even above normal for the first half of next week, with the
greatest precipitation chances along the Gulf Coast.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131 145.
         Winter Weather Advisory 155 195.
MARINE...Gale Warning 165 170 179 180 181 185 351 352 412 413 414.
         Heavy Freezing Spray Warning 121 126 127 128 129 130 139
            160 165 179 180 185 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...DEK


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