Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220236
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 PM AKST Sat Jan 21 2017

.ANALYSIS AND UPPER LEVELS...
An upper level trough over Southwest Alaska is slowly swinging
into the western Gulf today with an embedded vort-max headed
northward near Shelikof Strait. Out ahead of the upper trough a
weak low is situated just south of the Kenai Peninsula with an
inverted trough extending northward across the Kenai Pen to
Anchorage and portions of the Mat-Su. Low level convergence along
this axis combined with low-level instability moving northward
from the Gulf has been producing heavy snow showers across the
Interior and Eastern Kenai Peninsula with reports of 1 to 3 feet
of snow down the Seward Highway corridor. With colder air near the
surface on the west side of the Kenai and Chugach Mountains,
isentropic lift has been combining with shallower instability and
low level convergence to produce steady moderate to heavy snow
across Anchorage and areas to the north. As the upper trough
continues to swing into the Gulf drier mid-level air is advecting
northward to the Kenai Pen and the heavier snow is starting to
diminish.

The weather out over the Bering and Aleutians is quiet at the
moment, though a deepening storm force low over the North Pacific
is headed right for the Aleutians. With an arctic air mass in
place across the region this means both high winds and blowing
snow (or blizzard) will be an issue as this storm moves through
the region later tonight through Monday.

&&

.MODEL DISCUSSION...
Models continue to struggle with the snow over Southcentral, but
they all show a trend of snow becoming lighter this afternoon and
evening as forcing mechanisms weaken and drier air moves in with
the upper trough. Out west, models are all in good agreement with
track and strength of the deep pacific low. The biggest forecast
challenge is determining how quickly surface temperatures will
warm as winds strengthen, and thus, how long-lived blowing snow
or blizzard conditions will be. Thus, overall forecast confidence
is high while confidence in temperatures and visibilities in
blowing snow is lower.

&&

.AVIATION...
PANC...Light snow will linger into the evening hours, maintaining
MVFR conditions. With nothing to clear out the low level moisture
it will be favorable for fog to form. For now have just added VCFG
until see how things develop. If fog does not form or become
widespread, there may be a persistent MVFR ceiling. In either case,
expect improving conditions through the day Sunday as an upper
trough swings through.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Snow showers along Prince William Sound will be longer lived,
persisting into Sunday Evening as the upper level trough swings
through the area. Much of Southcentral will see a brief break from
snow Sunday night into Monday morning, with the exception of
Kodiak Island who will see increasing snow and winds as a front
moves over the area Monday morning. This front will track
northward and over the Kenai Peninsula and Northern Gulf coast by
Monday afternoon, bringing another round of snow and winds to the
Prince William Sound region.

There is potential for some patchy fog to develop during the
overnight period tonight and overnight Sunday, with weak low
level flow and lingering low level moisture remaining over the
Cook Inlet region and Mat/Su Valleys.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Frigid temperatures will persist across the southwest mainland
through Sunday night as arctic air remains in place over the
region on the backside of the departing trough lifting across the
mainland. Temperatures will begin to moderate on Monday as the
front over the Bering approaches the coast, however precipitation
type will still safely remain all snow as precipitation begins
along the Bristol Bay coastline by Monday morning, with some
potential for blowing snow along the western capes.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The North Pacific low currently located approximately 500 miles
south of Attu will undergo rapid intensification over the next
24-36 hours beneath a coupled jet structure as it moves just
south of Adak, swinging a powerful front over the southern Bering
tonight into Sunday. This will bring widespread stormy conditions
to the entire Aleutian chain through Monday morning as heavy
precipitation and storm to hurricane force winds accompany the
front. The front will first impact the Central Aleutians later
this evening with winds approaching high wind criteria and a brief
period of blowing snow before rain begins to mix in and improve
visibilities. A Winter Storm Warning has been issued to account
for these multiple impacts. Farther west, blizzard conditions will
impact Shemya from late this evening into Sunday afternoon as
strong northerly winds develop on the backside of the low. A swath
of hurricane force winds will be present over the waters between
Shemya and Adak through Sunday afternoon, producing hazardous seas
over the Western Bering. The front moves east tonight into
Sunday, with blizzard conditions spreading over the Eastern
Aleutians by Sunday morning and then to the Pribilof Islands on
Sunday afternoon. Conditions will improve by late afternoon at
Dutch Harbor as rain begins to mix in and winds subside, but
blizzard conditions will persist at Saint Paul through Monday
afternoon before the front finally begins to dissipate. There is
still less confidence as to how quickly the snow will transition
to rain across the Alaska Peninsula on Sunday afternoon, and the
Winter Storm Watch has therefore been left in place with the
afternoon package to continue to evaluate the potential impacts.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The longwave trough located over the Bering extending across the
Aleutian Chain into the North Pacific Ocean is the main synoptic
feature this upcoming week. As this feature moves eastward it
becomes negatively tilted as it moves over the Gulf by Wednesday.
This coincides with the strong meridional south to north jet
streak tracking into the Southeast Panhandle by Tuesday. This
pattern remains intact through next weekend. Therefore, look for a
warming trend with the southerly flow influencing the Gulf, and
extending through the Southcentral Alaska region. Look for the
temperatures to rise above freezing by the midweek time-frame
through the weekend. This results in abundant moisture streaming
into the Southcentral region resulting in a wintry mix along the
coastal communities. While the interior sets up with a better
scenario for moderate snowfall in favored mountain regions.
Meanwhile, the models continue to struggling with the complex low
in the North Pacific Ocean as it tracks eastward along the Eastern
Aleutians by Tuesday. As this system moves farther eastward into
the Gulf by the weekend the models have discrepancies with the
strength and timing with this feature. Therefore, the forecast
confidence is below average. Otherwise, the Bering will be under
northerly flow resulting in a showery regime heading into the
weekend. While the jet stream remains below the Aleutian Chain, it
keeps systems just south of the Chain before tracking into the
Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 185 191 195.
Winter Storm Warning 187.
Winter Storm Watch 181.
MARINE...Gale 150 160 180-185.
Storm 155 165 170-174 176 179.
Hurricane Force Wind 175 177 178.
Heavy Freezing Spray 121 127 130 138 150 160 165 179 180 185.

&&

$$

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



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