Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 241647 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
847 AM AKDT Sat Oct 24 2020

.ANALYSIS AND UPPER LEVELS...

There is a vertically stacked low near the Pribilof Islands with a
tightly wound occlusion over the central and eastern Bering. The
jet core is over Kodiak Island this morning. The remainder of the
frontal boundary is in the western Gulf and the moisture fetch
extends into the central Pacific. Again, the latest ASCAT
(Advanced Scatterometer) data has pinged into gale force winds
over the Barren Islands. The radars at King Salmon (PAKC), Kenai
(PAHG) and Middleton Island (PAIH) have been active overnight and
early this morning.

&&

.MODEL DISCUSSION...

Models remain in decent synoptic agreement through 48 hours.
The GFS is the most bullish with the next system out of the North
Pacific but there is good consensus with the track of the surface
low and the deep fetch of moisture associated with it. The
thermodynamics of the models related to precipitation type
continue to be the bane of this forecast package even as we move
deeper into the cold season.

&&

.AVIATION...

PANC...Mid deck clouds have moved into the Anchorage Bowl. At
PANC and PAMR overnight the cigs ranged from 7,000-10,000. The
expectation is that the cigs will lower more. There is a good
possibility that the ceilings may drop into the 3500 to 4500 ft
range at times but are not expected to go lower than that.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Monday night)...

A frontal system over the western Gulf will continue to drift
northward toward the Kenai Peninsula, with persistent gales
right ahead of the front from the Barren Islands to Augustine
Island and southern Cook Inlet. Meanwhile, a series of upper level
short-waves ejecting out of the Bering Sea trough will spread
precipitation inland across Southcentral. While the air mass over
Southcentral has slowly moistened from the top down over the past
24 hours, the low levels remain quite dry. Thus, a lot of the
precipitation will evaporate and not reach the ground. The bulk of
precipitation will be confined to the eastern Kenai Peninsula, and
the northern to western Susitna Valley, aided by upslope flow.
Warmer temperatures accompanying the front will lead to rain along
the coast, while inland areas will see primarily snow.

The strongest of the short-waves will help flatten the upper level
ridge over Southcentral and the Gulf. This will then force the
surface frontal system eastward tonight, setting up over the
northern to eastern Gulf by early Sunday. Thus, the strongest
winds and heaviest precipitation will shift to the north Gulf
coast and Prince William Sound.

Meanwhile, the upper levels will amplify further over the North
Pacific. A strengthening jet will drive warm and moist air
northward into the Gulf tonight, along with a new frontal boundary
and series of surface waves. The closed upper low and associated
trough axis over the Bering Sea will then progress eastward and
lead to development of one of these surface waves. There is still
some uncertainty in the exact track of this low, but expect it to
track to somewhere in Prince William Sound or the north Gulf coast
Sunday night. This storm system will produce heavy precipitation.
The challenge is figuring out where the heaviest precipitation
will fall. At this point in time, the most likely locations to see
heavy precipitation are eastern Prince William Sound and northward
across the Copper River Basin. There is potential for a foot or
more of snow in the Copper River Basin, but this will depend on
the storm track.

The most difficult part of the forecast for Sunday through Monday
may be in Anchorage and the Matanuska Valley, where the airmass
will be right on the edge between rain and snow. Precipitation
type will likely be determined by the precipitation intensity,
with mostly rain if it is light and mostly snow if it is heavier.
Right now, it looks like light rain will prevail Sunday/Sunday
night. On Monday, precipitation could pick up in intensity as the
upper low approaches, so that seems to be the best possibility of
switching to all snow. Stay tuned as we continue to work out the
details of the forecast for this major storm system.

&&

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

Widespread precipitation will continue to spread farther inland
throughout the day today as a low over the eastern Bering
progresses eastward. Most areas should see rainfall associated
with this system with temperatures above freezing. However, areas
across the Kuskokwim Valley could see light snow accumulations
today. Showers will continue to linger across Southwest Alaska
through Sunday before starting to taper off on Monday as the
aforementioned low system weakens.

&&

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

A mature low over the central Bering that has been dominating the
weather over the past several days is continuing to weaken this
morning. This low centered near the Pribilof Islands will keep
small craft advisory level winds and widespread showers in the
forecast today as the system moves into the eastern Bering for
Sunday and then dissipates. Quiet conditions and northerly winds
will set up over the Bering behind this system. The next front
approaches the western Aleutians by Sunday afternoon, but this
front will remain nearly stationary through the start of the week.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

The big focus early Monday will be a low sitting somewhere over the
northern Gulf. There is some uncertainty in track and strength of
this low, but most solutions indicate gale force winds over the
central Gulf. The low will head inland by Monday afternoon, with
westerly winds strengthening as cold air moves in behind the low.
Once again, gale force winds seem most likely, though if the low
ends up being a bit deeper there could be a small area of storm
force winds through the Barren Islands. Also expect gusty gap winds
through bays and passes along the north Gulf coast and Prince
William Sound. Winds will diminish Tuesday with quieter conditions
then settling in on Wednesday. Out over the Bering Sea and
Aleutians, do not expect any significant storms for the first half
of the week.

&&

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

Model agreement increases for the surface low approaching the
Northern Gulf on Monday. Multiple embedded shortwaves pass through
the Gulf towards the Southeast coast early next week, with
moderate to heavy precipitation expected for the Southcentral
coasts. This is expected to be rain except for the higher
elevations with will see some snowfall accumulation. Additionally,
there is better model agreement for the arctic trough pushing
south across the Mainland mid-late next week, bringing colder
temperatures to Southcentral and Southwest Alaska. Temperatures
are likely to be below normal for both regions by mid-week.

There are still significant model discrepancies for the
North Pacific surface low approaching the Aleutians. The system
seems to be increasingly impactful, with the only similarity being
that it stays just south of the Chain. The Canadian matches well
with the ECMWF on strength but places the low much further
northeast than both the ECMWF and GFS. GFS is much deeper than
the other two, but is more cooperative with the ECMWF`s path.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 120 121 130 131.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ALH
MARINE/LONG TERM...SEB/KM


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