Area Forecast Discussion
Issued by NWS Anchorage, AK

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539
FXAK68 PAFC 110210
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 PM AKST Mon Dec 10 2018

.ANALYSIS AND UPPER LEVELS...
A large negatively tilted long wave trough encompasses the
eastern Bering, western mainland, and the Gulf. An upper level
vertically stacked low is embedded within the trough over Bristol
Bay this evening. This low is bringing in cold air along the
backside into the Bering and western mainland, while moisture is
being brought in along the leeward edge over the western coast.
The combination of cold air and moisture is bringing snow and some
blowing snow to the Kuskokwim Delta and Bristol Bay. Within the
flow of the low, one short wave is present over the central Bering
while another extends over the central Gulf. The shortwave in the
Gulf is associated with a surface low and front. The front is
pushing ahead of the low into Southcentral brining showers to the
northern Gulf coast. Northerly flow in Southcentral is creating
stronger winds within the northern Gulf and Cook Inlet.

&&

.MODEL DISCUSSION...
Models are in good agreement through Tuesday regarding the low
over Bristol Bay and the central Gulf. Wednesday morning, a North
Pacific low that was moving south of the Aleutians enters the Gulf
and there is some slight discrepancy regarding the location and
intensity. The GFS maintains a much stronger and more southerly
solution. The ECMWF and NAM are similar with putting the low
right on the southern edge of the Gulf at a similar strength.
Through Wednesday, the models continue to diverge and become less
consistent. Meanwhile in the west, various low pressure systems
are moving over the western Aleutians, none with a sense of
similarity between models.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through
tonight. Conditions should drop to MVFR in snow sometime late
tomorrow morning. Confidence in the timing of this is not high,
and it may start sooner or possibly not until Tuesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Tonight
through Wednesday)...
The broad upper level trough from the Bering Strait region south
to an upper low over the Bristol Bay region will persist as the
low in the base of the trough moves over Kodiak Island to the
North Gulf Coast tonight and Tuesday. The surface low will rotate
up into Prince William Sound bottoming out in the low 950 MB
range tonight with another surface low then forming southwest of
this low that will then rotate into Prince William Sound Tuesday
night. Since these lows are staying to the south and east of Cook
Inlet and the Susitna Valley, we will see the cold air remain in
place and in fact colder air should continue to advect into the
south central region through Tuesday night and Wednesday. Over
the Prince William Sound region expect enough warm advection
tonight to result in a mix of rain and snow. Colder air then moves
back into the Sound Tuesday afternoon and evening, thus expect
the precipitation to change over to all snow.

As we move into Wednesday a large intense low moves into the Gulf
of Alaska. The main low center moves toward the southern
Panhandle Wednesday night as a low forms to the northwest along
the north Gulf Coast. With the main low staying well south of the
southern mainland, cold air will continue to be pulled south
lowering the 850 MB temperatures into the negative teens over the
Matanuska and Susitna Valleys south across Cook Inlet and over the
Copper River Basin. The 850 MB temperatures remain cold over the
Sound as well, thus any precipitation will remain in the form of
snow.

Around the back side of these lows as they move into the Sound
will be strong low level cold advection with the typical strong
west and northwest flow, especially across Kodiak Island and lower
Cook Inlet.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Monday
night through Thursday)...
The low pressure system that has been approaching southwest Alaska
is currently sitting just off the coast in Bristol Bay. Its
frontal system has moved onshore and is currently working its way
through the region, bringing mostly snow as it goes. Windy
conditions with this system are still expected behind the low
tonight, so kept the winter weather advisory active through early
tomorrow morning.

Though the forecast is panning out largely as expected, one
difference is a pocket of relatively warmer (around freezing) air
that is hanging on right along the coast in the Bristol Bay area.
This is causing rain to mix into the snow at times. As such,
warmed temperatures this evening and overnight lows to reflect a
slower cooling rate on the coast. This also changed the precipitation
type from snow to a rain/snow mix through the evening hours. Used
a blend of the NAM/GFS going forward with the handling of the
remainder of this system. They are not quite in agreement, but
offer two reasonable solutions, so split the difference.

After the low exits to the east, a strong northwesterly push of
cold air will move behind the low. This will affect inland areas
the most, with temperatures forecast to drop below zero as the
week progresses. Temperatures will stay relatively warmer along
the coast due to the marine influence, but will still drop to
single digits overnight.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Monday
night through Thursday)...
The elongated menagerie of lows that was stretched east to west
through the Bering Sea has pushed east and is currently located
over the eastern Aleutians and southwest Alaska. The main low
center is currently wobbling offshore in Bristol Bay. Strong
northwest flow has come in behind the low center, with satellite
indicating widespread cold stratocumulus behind it. Snow showers
can be expected with this cloud cover.

A smaller, straggler upper level low is working east to west just
north of the Aleutian islands behind the main system. It is not
having any impact on the surface. This piece of energy will work
its way southeast across the eastern Aleutians tonight and exit
the area. Otherwise, expect continued NW flow for the next few
days. This will continue to drop temps and induce freezing spray.

&&

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

The long range forecast remains active across area waters as the
jet stream ushers storm systems along the Aleutian Chain to the
Gulf of Alaska. Inland locations across the southern mainland will
mainly be drier with cold air advecting the coolest temperatures
of the season for the end of the week. The next significant storm
system to monitor develops across the North Pacific Thursday night
bringing gusty easterly gales for the Aleutian Chain. This system
keeps a fast paced easterly track, but looks to move south enough
to keep significant impacts away from the Alaska Peninsula. Models
differ in timing and track on this system as it progresses
eastward, however they all lift it toward the Gulf Friday night.
The synoptic pattern amplifies over the weekend with a ridge to
the east and west of this storm system. This will likely slow the
northward progress of the front through the weekend. There is
high uncertainty in the forecast this weekend into early next week
as models struggle with the evolution of this storm solution.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory...155.
MARINE...Storms...130 132 150 160.
 Gales....119 120 131 136 137 138 139 141 155 165 170
  178 179 180.
 Heavy Freezing Spray...160.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...KH



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