Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211427
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKST Tue Feb 21 2017

.ANALYSIS AND UPPER LEVELS...
Satellite this morning clearly depicts that the stage is set for
a transition in our weather today. A weak surface low near the
Prince William Sound continues to wrap some snow showers into
coastal sites such as Cordova and Whittier. This low coupled with
some potent cold air advection is leading to gusty outflow winds
in the typical spots (Barren Islands, Seward, etc.) as well. But
the low is being ushered to the east courtesy of the 500mb trough
that cleared through the Cook Inlet earlier this morning. Clearing
skies across most inland areas have led to the coldest temperatures
in over a week. But warmer air and moderate to heavy precipitation
are on their way.

A front currently approaching Southwest AK will bring a change in
weather to southern AK. The front brought gusty winds and heavy
snow to the Alaska Peninsula up through the Pribilof Islands
already this morning. It is also ferrying enough warm air
northward to change the precipitation type to all rain at Dutch
Harbor. Winds are already starting to increase across the
southwest in response to the tightening pressure gradient with
colder air over the mainland. Snow is just starting to fall over
some of the Western Capes and will only continue to push inland
through the morning.

&&

.MODEL DISCUSSION...
There was one notable differences with the synoptic features in
the models this morning. The NAM was clearly slower in bringing
precipitation inland, especially over Cook Inlet tonight. For
this reason, it was discounted. The GFS was preferred. Otherwise,
the higher resolution models were relied upon to capture details
in precipitation type, strength of winds, and timing of
precipitation. Over the Gulf of AK, the stronger solutions were
used to capture some gale-force winds as a closed surface low
exits on Wed.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough over Southcentral will exit to the east
today with a few lingering showers over Prince William Sound and
the Copper Basin. Expect abundant sunshine behind the trough with
temperatures struggling to make it out of the teens for most
inland locations.

A potent upper level short-wave and low level frontal boundary
will then rapidly cross Southcentral tonight through Wednesday
bringing a quick shot of moderate to heavy snow to most locations.
With deep and strong southwest flow and the track of the strongest
part of the short-wave across the Cook Inlet-Susitna Valley
corridor, expect the heaviest snowfall from the Western Kenai to
Anchorage and the Mat-Su. One complicating factor will be a period
of strong but shallow southeast winds across the Kenai and Chugach
Mountains tonight through early Wednesday. This will initially
limit snowfall intensity across portions of Anchorage and the Mat
Valley. However, as a surface low tracks south of the Kenai
Peninsula early Wednesday, these winds will rapidly diminish and
shift to the southwest, favorable for upslope enhancement of snow
along the mountains. Prior to the shift there will likely be a
convergence zone away from the mountains which could produce
heavier snow from west Anchorage to the western Mat Valley. Also
of note, the track of the surface low will lead to just south of
the Kenai Peninsula will lead to some of the heaviest snow over
the southwest areas from Homer northward to Ninilchik.

Winter Weather Advisories have been issued from the Western Kenai
to Anchorage and the Mat Valley along with the Susitna Valley from
Talkeetna southward. Although not all areas will see advisory
accumulation (6 inches), the fact that some of the heaviest snow
will fall during the morning commute warrants issuance of the
advisories.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A rapidly changing forecast today as the well-below negative
temperatures will warm as a front brings moderate snowfall to most
areas. The front itself is pretty moist with dew points in the
30s behind it along the Aleutians. The strength of the front
itself will be aided by replenishing of the arctic air from the
north as warm air advects over top. That profile will bring the
highest snow amounts from Dillingham westward through roughly
Kipnuk with advisory-level snow extending inland as well. Sub-
advisory level snow is expected around the Bethel area, including
Aniak, and eastern portions of Bristol Bay.

Falling and blowing snow will reduce visibility as low as one-
half mile, mainly along the coast. The front will continue inland
overnight tonight, bringing the same intensity snow to the
Kuskokwim Valley. The cold antecedent conditions will lead to
light and fluffy snow that will build up quickly. The front exits
the southwest late tonight, while another weak front brings
another shot of very light snow to the area on Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
One front exits, another enters. It`s the circle of life. Blizzard
conditions will wind down early this morning for the Pribilofs as
the front moves northeast across the Bering. The next storm-force
front moves into the western/central Bering this afternoon,
bringing another slug of warm air. The warm front will change snow
to rain from south to north, followed by an west-east moving cold
front on Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A very amplified ridge setting up over much of Mainland Alaska on
Thursday will surge above normal temperatures and moisture to the
Gulf and the South Mainland through late this week. This will
initially bring above normal temperatures but otherwise quiet
weather to most areas for Thursday, but this will begin to become
more active as the remnants of a frontal band becomes
reinvigorated by a shortwave traversing the Bering Sea. This will
bring precipitation mostly in the form of rain the Southwest
Mainland on Friday, which will then spread into Southcentral late
Friday-Saturday. What is less certain this far out in the forecast
is determining whether surface temperatures will rise above
freezing, so the possibility of freezing rain will need to be
monitored in the coming days.

By Sunday, there is good agreement that this warm-up will once
again be short lived as the amplified ridge flattens out as it
shifts to the south. This will allow for a series of arctic
troughs to track back into the Mainland, forcing temperatures back
below freezing and chances of precipitation to snow in most areas.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 121 145 152 155 161.
MARINE...Heavy Freezing Spray Warning 121 129 130 131 139 185.
         Storm Warning 178.
  Gale Warning 119 120 131 132 155 165 170 172 173 174
                      175 176 177 179 180 185.

&&

$$

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


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