Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 080309 AAA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
609 PM AKST Wed Dec 7 2016

.ANALYSIS AND UPPER LEVELS...
A high amplitude Ridge over the Bering sea is blocking the
approach of systems from the west while a cold upper level trough
remains nearly stationary over the eastern Interior and
Southcentral Alaska. At the surface there is high pressure over
the northern half of the state and through the central Bering Sea
with a broad complex low in the Gulf of Alaska. This is a typical
pattern for this time of year and is increasing the pressure
gradient in Southwest mainland Alaska as well as along the north
Gulf coast.

&&

.MODEL DISCUSSION...
Models are continuing to be in good agreement with a stable
synoptic pattern over the state. The one wild card remain in the
Gulf of Alaska with the complex low. While this low will persist
the details on the movement of the individual waves is causing a
large range of forecasted snowfall amounts along the north Gulf
Coast and especially Kodiak Island. Over Kodiak, the GFS and NAM
differ on predicted snow amounts with the NAM showing three to
four times the amounts from the GFS. Looking through the detail,
the lower GFS values seem more reasonable.

&&

.AVIATION...
PANC...Periods of light snow will occur into the evening with
some warm air over-running the cold air in place over the
Anchorage bowl. This pattern will allow ceilings and visibilities
to remain mainly VFR with only a chance of occasional marginal
conditions. Winds will continue to be from the north for the
foreseeable future.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A cold upper level low remains the focus for precipitation across
the Gulf of Alaska and Kodiak Island. 850-500mb lapse rates are
fairly steep, with the 00Z Kodiak sounding showing 7.9 degrees
C/km. Thus, the character of precipitation is showery. A weak
upper wave is headed westward across the Kenai Peninsula this
afternoon, but with dry low level northerly flow, don`t expect
anything more than flurries or very light snow from the Western
Kenai Peninsula up to Anchorage and the Mat-Su. There is also some
low to mid level isentropic lift, as marginally warmer air
overrides the deep arctic air sitting over interior Southcentral.
This will continue to produce some persistent light snow over
portions of the Copper River Basin.

The next feature of interest is an inverted surface trough sitting
over the Eastern Gulf. This will bring snow to the Copper River
Basin and Prince William Sound region on Thursday. A 20 to 30 kt
low level jet will advect warmer and moister air northward and
combine with strong low level convergence along the trough axis
to produce moderate snow in places. The cold air in place will
produce a low density snow, which is favorable for blowing snow
and reduced visibilities. With persistent strong winds through
Thompson Pass have issued a Blizzard Warning for this event.

Forecast confidence starts out high, but quickly drops to low
Thursday Night and Friday. The trough and associated forcing for
snow will weaken as it rotates westward, with a building ridge
rapidly taking its place. The question is how fast does it weaken
and how far west does the snow spread. For now have introduced a
chance of snow from the Western Kenai to Anchorage and the
Matanuska Valley. There is potential for light accumulation of
snow, though the persistent dry northerly flow will work against
this idea.

&&

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

The pattern for the southwest mainland will largely be
characterized by dry offshore flow through Thursday on the
backside of the trough centered over the western Gulf. This will
yield frigid temperatures beneath mostly clear skies, although
numerous disturbances embedded in the upper level flow will
continue to provide chances for periodic cloud cover over the
Bristol Bay region. Persistent gusty winds will lead to very cold
wind chill values across the Kuskokwim Delta, although these
conditions are not expected to meet advisory criteria at this
time.

&&

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

Cold, offshore north to northeasterly flow over the near-shore ice
will allow for continued sea-effect snow showers to continue over
the southeastern Bering over the next several periods, and also
allow for continued freezing spray issues. Easterly waves will
begin to undercut the ridging to the north and west over the
Bering region and allow for increasing precipitation chances for
the Alaska Peninsula Thursday night into Saturday.

Meanwhile, on the western side of the high pressure ridge, the
gale-force front across the western Bering and Western Aleutians
will continue to weaken as it slowly moves into the high pressure
downstream. As this front gets sheared apart, low pressure will
develop over the southern Bering and move across the Central
Aleutians late Thursday into Friday, bringing small-craft-
advisory-level winds. This front will dissipate by Friday as
easterly flow overtakes the region ahead of a stronger front that
will attempt to push north toward the Western Aleutians later
Friday. Models are beginning to diverge with how far the front
makes it to the Aleutians through the end of the week.

&&

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

High latitude blocking ridge will be the dominant mode across the
Alaska region for the beginning of the weekend and into the middle
of next week, as a stout upper high builds across northern
Alaska. Even though temperatures aloft will actually be warming in
response to this feature, temperatures at the surface will be
dominated by increasing inversions. Temperatures will once again
fall below climatological norms by the weekend as skies clear and
outflow winds persist into next week. While the numerical models
differ in the exact details for much of next week, the ensemble
and multi-model approach all depicts similar sensible weather
solutions as the ridging rebuilds across the eastern Bering Sea
and western Alaska. Offshore flow will remain dominant across
Southern Alaska with no precipitation events expected in the
foreseeable future for inland locales. In other words, continued
cold and dry conditions over much of the mainland, with the main
storm track over Siberia and the Western Aleutians and Western
Bering Sea.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard warning Thompson Pass.
MARINE...Gales 127 128 138 139 177 128 411. Heavy Freezing Spray
160 180 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...JA/TP


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