Flooding in Alaska, December 10 to 12, 2024

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
639 AM AKST Tue Dec 10 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Discussion:

The high winds of yesterday afternoon and evening for the Anchorage
Bowl, Hillside, and Eagle River finally started to decrease early
this morning as the shortwave pushed north and the pressure gradient
begins to relax. As a result of the decreasing winds, some of the
area temperatures have started to cool from the upper 40s and 50s
back down into the mid 30s to mid 40s as the deeper mixing and
strong downsloping ceases. Winds are expected to remain gusty
across the Mat Valley today as the gradient relaxes very little
there, but the Copper River Valley winds should settle down
through the morning hours. Early morning showers over the the Mat-
Su Valley will continue to push northward as the shortwave lifts
around the periphery of the ridge and into interior Alaska. Most
of Southcentral should see a relatively quiet day as we wait for
the next system to work its way up across the Gulf.

This next system is quite well defined on satellite with large dry
slot starting to wrap into the parent low, and front starting to
lift across the western Gulf. Precipitation will first spread into
Kodiak this afternoon with heavier precipitation looking to start
Tuesday night. Kodiak could see an additional few inches of rainfall
on top of the past several days totals. Seward also looks to see
decent rainfall of 0.75 inches to upwards of 1.5 inches.
Precipitation will continue to spread up the Gulf coast and into the
Prince William Sound by Wednesday morning. Whittier could see
several inches of rainfall from Wednesday through Friday.

- PP

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

As a low in the North Pacific continues to make its way towards
the eastern Aleutians and Alaska Peninsula today, its front will
move over Southwest and provide a round of precipitation for most
of the coastline and nearby interior locations. With strong
southeasterly flow prevailing over Southwest, places that
typically experience downsloping from this flow will receive
little to no precipitation from this wave today. Exceptions will
be along coastal areas that will have enough moisture from the
front moving over to still experience precipitation.

By Tuesday evening, the North Pacific low will get stretched from
southeast to northwest as it merges with the currently present
upper low in the northern Bering. This will result in a complex
low pressure system that will preside over the central Bering and
persist into the later stages of the near-term forecast period. As
the low repositions itself Tuesday night-Wednesday, the reduction
in widespread gales over the eastern Bering, eastern Aleutians,
and Alaska Peninsula will slowly erode and reduce to widespread
small craft force winds. Before that happens, strong gap winds
along the Aleutians, Alaska Peninsula, and Bristol Bay will occur
throughout the day today, with locations such as Cold Bay getting
gusts up to 50 mph in the morning and early afternoon hours.

Once the low repositions in the central Bering, the core of
strongest winds will be to the south and west of the low center in
the western Bering. Cold air from northern Asia will produce gale
force winds and showery conditions through Thursday, likely in
the form of snow. For Southwest, expect mostly dry and calmer
conditions with an occasional period or two of rain/snow showers
as several weak fronts pass over the area through Thursday.
Additionally, expect a gradual cool down by Wednesday night into
Thursday as cold air from the low shifts eastward towards the
mainland. Lastly, while models are not in complete agreement on
the timing/placement of it yet, a new North Pacific low looks to
be entering the western Bering/Aleutians by late Thursday and
could provide strong winds and widespread precipitation with it.
Continue to follow for more updates as the forecast develops.

-BS

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

By Friday, guidance suggests that a 500 mb negatively tilted
trough situates itself from the Bering Sea southeastward across
the Aleutians. This feature should provide adequate upper level
support for light precip in Southcentral AK with rain showers at
the coast and snow showers inland. Marginal temps here yield some
uncertainty with where the rain/snow line sets up and this will
have to be fine tuned with future guidance. Precip should wind
down by Saturday as ridging builds over the Gulf and the low
weakens. This trough flattens by late Saturday with another upper
trough in its wake producing another around of snow showers on
Sunday. By late weekend, 850 mb temp anomalies show temps falling
below climo as the Arctic airmass to our north begins to spill
southward. We are seeing trends of daytime highs for most falling
into the 20s for the last half of the period. Operational models
are hinting at another deep storm force to hurricane force frontal
low approaching the Aleutians toward the end of the period.

McCormick

&&

.AVIATION...

PANC...Gusty SE winds are expected to end this morning and be
replaced by light northerly flow. However, winds aloft will remain
southeasterly and wind shear could be an issue depending on what
level the strongest winds lie. As for the ceilings, most
precipitation will remain in the vicinity of the Anchorage
terminal as downsloping winds will prevent it from taking root.
Due to this mostly VFR conditions are expected in the area.
Though, it cannot be ruled out that a shower or two could sneak
over the terminal and briefly lower ceilings below 5000ft.

&&