Gale Warning 572, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
604 PM AKDT Sun Oct 25 2020

.ANALYSIS AND UPPER LEVELS...
A broad and deep area of southerly flow between a trough over the
Central Bering Sea and a North Pacific continues to surge a
significant amount of moisture and cloudcover to the South
Mainland. Within this southerly flow, a rapidly developing storm
force low located south of Sand Point is tracking towards Kodiak
Island. This will be the primary weather maker for the South
Mainland and Gulf Coast into Monday as it rapidly moves into
Prince William Sound. Widespread rain, mixed at times with a wet
snow, is already being observed from this system from Bristol Bay
eastward into the Gulf Coast and Southcentral. For the Anchorage
area in particular, the expected precipitation type changed pretty
dramatically between the 12z and special 18z soundings today, as
the low level dryness and warm air aloft left over from yesterday
began to erode away. This allowed for periods of ice pellets
today, and even snow to start mixing in along higher elevations as
the warm nose begins to erode away.

Fortunately with all of the active weather to the east of the
Bering trough, the weather from the AK Pen westward is very
benign and persistent for this time of year. There is a rapidly
weakening front moving through the Western Aleutians, however
since this has fallen well below gale force it is not expected to
be impactful.

&&

.MODEL DISCUSSION...
Synoptic model agreement with the approaching storm force Gulf low
has improved significantly, as there is now good agreement with a
low track through Shelikof Strait into western Prince William
Sound. Since this system is moving so quickly (around 30 kt) under
a very strong 150 kt jet, there remains some slight timing
differences, but this is having little effect on the overall
trends and thinking of the forecast. As a result, forecast
challenges continue to be with precipitation type and snowfall
amounts as the front makes landfall, and the area of deformation
behind it. Some of the guidance (especially the GFS) is not
adequately capturing the warming/drying from cross-barrier flow
as the front makes landfall over the Anchorage Bowl/Mat Valley
this late this evening. As a result the onset of significant
accumulating snow was delayed until after the front makes landfall
early Monday morning where there is much better confidence.

&&

.AVIATION...
PANC...Rain (mixed at times with snow), slowly lowering ceilings
will continue into this evening. Once the IFR snow develops around
12Z tonight, it is expected to continue into Monday afternoon
where conditions will begin to improve from west to east. As a
result, even as snow ceases at the terminal, show showers along
the mountains to the east are likely to continue for most of the
day Monday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Sun
afternoon through Tue afternoon)...

The forecast remains largely on track for snowfall across
Southcentral overnight tonight through Monday.

A strong upper level wave currently developing south of the
Alaska Peninsula this afternoon will lift northward directly over
Southcentral. This feature, combined with a strong 150 knot
southerly jet streak at 300 mb, will steer ample moisture inland
over Southcentral through Monday. Meanwhile at the surface, a 990
mb low forms south of Kodiak Island this evening and strengthens
as it tracks northward, passing over Kodiak Island and steering
over the Kenai Peninsula into Prince William Sound Monday morning.

One major change to this forecast package was a delay in the
transition from rain to snow from the Anchorage area north to the
Mat-Su Valleys. The main influence for this was continued signals
from models showing persistent midlevel southeasterly flow through
12Z Monday (4AM Monday). Once the southeasterly flow shuts off
after the front passes through, there is high confidence that a
change to all snow will occur. Prior to that time, current
thinking is rain or a rain/snow mix will occur. Perhaps one
caveat to this worth mentioning is that the warmer southeasterly
flow layer is shallow, and it is still possible for precipitation
intensity to cool and further erode this layer - a trend that has
consistently been observed in the 18Z and 00Z atmospheric
soundings today.

For the Copper River Basin, there has not been much change and Winter
Weather Advisories are still in effect. If the low takes a more
westerly track than advertised, this could result in less snow for
the Copper River Basin (as well as the Anchorage and Mat-Su
Valley area) as this would result in warmer air and more rain.
That being said, models do remain consistent in the track of this
low as currently forecast.

As this low continues to move inland Monday afternoon,
precipitation will eventually taper from west to east across
Southcentral through Monday evening. Tuesday will bring generally
quieter conditions across Southcentral with westerly flow
expected across the Gulf waters.

&&

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

The scatterometer swath from this afternoon points out the
the position of the weak circulation south of Nunivak Island.
Meanwhile, a low-key upper low lies offshore from Hooper Bay and
its expected to stroll southeastward along the coastal areas
through Tuesday morning. The combination of an existing long wave
and the onset of cold air advection from the north, including
falling temperatures aloft will bring overnight temperatures in
the lower to mid 20s with the highs in the 30s and 40s. Snow will
spread over the lower Kuskokwim Valley, Kuskokwim Delta and
Bristol Bay tonight and Monday, with a chance of snow Monday
night. Over at AKPEN, scattered to numerous showers tonight will
diminish between Monday afternoon and Monday evening, with a
chance of snow there between Tuesday and Wednesday. Essentially,
the upper level ridge behind the west will provide clear to partly
cloudy skies as it builds eastward and across the region through
the forecast period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Wednesday)...

A strengthening low and its frontal boundary to the south of
AKPEN will generate gales and large wave heights south of AKPEN
tonight. This situation will keep moving across the Gulf which
should exit the area by early Monday morning. Farther west, a low-
key frontal system from the Northwest Pacific will maintain only
small craft conditions through at least Monday. The front could
transition into a trough later on but for now will monitor the
weather observations and satellite for future updates. Overall, it
will be a relatively quiet forecast period.

&&

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

Models are in better agreement for Wednesday with the low over the
northern Gulf weakening as it drifts slowly to the east. Models
diverge heading into Thursday afternoon with the low strengthening
and bringing the potential for increased winds and waves to the
Gulf. There is low confidence that another low will move across
the southern Gulf and towards the north Gulf coast Friday. If this
solution were to pan out, gale force winds will be possible
across the Gulf. Models are in better agreement that a low will
skirt across the Aleutian Chain Wednesday night through Friday.
For now, gale force winds seem to accompany this system, however,
storm force winds cannot be ruled out at this point.

&&

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

A series of Arctic troughs track south across the state mid-week
into the weekend, advecting colder air out of the Interior and
into the Southwest/Southcentral regions. By late next week, day
time temperatures will fall, and overnight temperatures will drop
significantly for the Southcentral and non-coastal areas of
Southwest Alaska. Greatest impacts should remain north of the
Alaska Range, with the higher elevations and eastern portions of
the Copper Basin possibly seeing some snow accumulation.

Late next week and into the weekend, an area of low pressure
passes just south of the Aleutians. The ECMWF has committed to
matching closely with the GFS across all parameters, therefore the
Canadian solution is the outlier here and can be ruled out at
this time. Gales along the Aleutian coasts are very likely, and
storms are also a possibility. Towards the end of the weekend,
another low approaches the Aleutians from the west and quickly
veers south. This system seems to travel a similar route as the
previous one, tracking just south of the Chain. There is still
quite a bit of model disagreement on location, strength, and speed
of this storm, but the path seems relatively consistent across
all solutions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 101, 111, 131, 141, 145.
MARINE...Storm Warning: 120 351.
Gale Warning: 119 130-132 137-139 141 150 352.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CK/KM