Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 140201
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 PM AKST Thu Dec 13 2018

.ANALYSIS AND UPPER LEVELS...

An elongated trough that has encompassed the central Bering
and Gulf has started to become unorganized as it shears into a
a series of short waves. One shortwave is associated with a
northward migrating meso-low over Southcentral. While snow has
been persistent over the last few days, now that the shortwave is
shifting northward inland, there is a brief reprieve in snow this
afternoon. A front still remains active through the central
Bering. The frontal passage, isobaric packing ahead of the front
over the western Gulf, combined with strong cold air advection is
encouraging convective activity. This is manifesting as very cold
cloud tops, widespread cumulus clouds and lightning throughout
the Gulf. The other two shortwaves are coupled with a double-
barrel low system forming in the west. The first low is over the
central mainland. The second, and more impactful vertically
stacked low sits between mainland Alaska and the Pribilofs. This
low is wrapped in cold air and continuing the northerly flow over
the Bering. A broad band of increased vorticity is also present
over much of the eastern Bering and AK-PEN. The jet stream
continues to flow in a meridional pattern with a branch of higher
winds northwesterly oriented along the eastern Aleutians near Cold
Bay. This arm of the jet coincides with a front moving over the
central Aleutians which is bringing gusty conditions to the chain
and snow, and a few small lightning strikes through the eastern
Aleutians. This front is associated with a North Pacific low that
is skirting the western Aleutians.

&&

.MODEL DISCUSSION...

Models remain in good agreement through the period regarding the
track of the North Pacific low as it moves toward the Gulf
through the weekend. There is some slight discrepancy regarding
the strength of this low. The NAM and GFS are very similar
regarding the weather impacts while the EC and Canadian take a bit
weaker approach. On Sat afternoon, a front advances towards the
western Aleutians which guidance also is handling well.

&&

.AVIATION...

PANC...VFR conditions will persist through the evening with light
winds. Light snow should be coming near the terminal overnight [10
to 12z] but not expecting vis to drop below mid mvfr and clouds
should remain vfr. Snow will return Friday afternoon with better
chances of IFR cig and vis going into the early Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough that was over Southcentral today is moving
north of the area, leaving a break in snowfall this afternoon and
evening. Another shortwave will move across the area later
tonight. This is bringing chances of snow to Cook Inlet and the
Anchorage Bowl, as well as other portions of Southcentral. Amounts
are expected to be light with this fast moving system. Below
freezing temps will also keep the precipitation type as snow along
coastal areas including Kodiak Island.

The weather pattern that has been seen the past few days will
begin to change as a strong upper low south of the Aleutian Chain
moves into the Gulf Friday and Friday night. This low will push
a weather system over the Gulf northwestward across the area
Friday afternoon and evening, bringing some snow to the area.
Friday night and Saturday, Storm force winds associated with this
low will move into the Gulf from the south.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Thursday
night through Sunday)...
A compact low is pushing into King Salmon right now, which is well
depicted on satellite and RADAR. Meteorologically, it is an
interesting feature to watch though it is not having much of an
impact. Outside of this low, weather will be quiet until a
deformation band moves over the western AK range on Saturday.
Looking at eastern 152 and 161 picking up some snowfall with this.
The GFS is attempting to hone in on the band of precipitation.
While the GFS solution doesn`t look bad, held off on detailing
this band of precipitation for now, as there can be uncertainty
of where exactly these bands set up. So, will wait another model
run or two before making a call.

The coldest air yet this winter is still expected to arrive in
southwest AK this weekend. An arctic airmass in interior AK will
feed cold air southwest down the Kuskokwim River Valley. Due to
the nature of this cold air, it is not expected to moderate until
it reaches the coast. This means deep cold for many inland areas,
with minus 20F in the forecast for Bethel. When the deformation
band sets up Saturday,some temps may warm under it. Otherwise, a
cold weekend, though light winds will prevent windchill from being
an issue.


&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Thursday night through Sunday)...

The Bering Sea continues to see northerly flow. This pattern is
expected to continue through the weekend. The compact low that was
skirting along the north side of the Aleutians is pushing inland,
leaving weather pretty quiet in the Bering. Only snow showers and
freezing spray continue with the northerly flow.

The large low moving west to east along the south side of the
Aleutians is progressing as forecasted. We have been getting some
good scatterometer imagery today, showing storm force winds with
this low. However, they are on the southern side of the low and
will not enter into our forecast zones. Gales are well depicted on
the northern side and and still forecasted to reach into the
Aleutian Islands.


&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with the longwave trough extending
from Southwest Alaska to the Gulf Of Alaska. Meanwhile upstream
there is an upper level ridge that spans from Hawaii to the
Eastern Aleutians. As the forecast period evolves, another vortex
of brutally cold air will descend from Siberia and the longwave
pattern will become more highly amplified. The deterministic
models have depicted a series of lows impacting the Aleutians,
the Gulf of Alaska and the Prince William Sound. However, the
precise tracks of these lows remains uncertain. The models have
been struggling this winter locking onto a solution past 72 hrs
and that trend will continue for this forecast package.

PJS
&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm 352. Gales 119 120 130-132 138 129 150 155 172 174
176 177 None.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...BL/AH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PS



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