Winter Weather Advisory 72, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
933 PM AKST Mon Dec 20 2021

.ANALYSIS AND UPPER LEVELS...

Southern Alaska is underneath the polar jet stream this afternoon.
A large upper level ridge is south of Sand Point, but is
flattened. Meanwhile a vigorous upper level shortwave is over the
Y-K delta. The area in between these features is supporting a 160
kt jet from the central and western Bering east into Southwest
Alaska. This feature and the associated upper level shortwave
will be reinvigorating the precipitation currently occurring over
Southcentral, helping it last into the evening.

The current conditions across southern Alaska feature widespread
fog and low stratus across much of Southwest Alaska, which is
persisting due to onshore flow from the Bering moving over the
cold ground. The rain and snowfall event over Southcentral has
dropped roughly an inch of new snow from the Susitna Valley
through to the Anchorage Hillside, with about a half inch of snow
at the forecast office. Southcentral is in a relative lull in the
snowfall until this evening, when the aforementioned shortwave and
jet streak move to aim at the region. More on that below.

A special 18Z Anchorage sounding was launched this morning showing
a fully saturated atmosphere from tropopause to surface. The snow
growth layer extends up from 14,000 ft up to 18,000 ft above sea
level. This leaves the 14,000 feet below that layer saturated with
supercooled water. Until the snow restarts, freezing drizzle is
common.

&&

.MODEL DISCUSSION...

The models are in good agreement on the large scale features
impacting southern Alaska through Wednesday afternoon. The ongoing
winter weather event remains a point of uncertainty, primarily how
heavily the snow increases this evening. Cooling temperatures in
the mid-levels from there will keep all areas as snow, with the
rain in Homer ending before any changeover.

As the weather quiets down tonight, confidence increases that the
cold air and gusty winds will return to all of the gaps, with
gales expected from Valdez through Seward, gusts to 45 kt near
Kodiak, and ground blizzard conditions are expected near Thompson
Pass as 70 mph gusts resuspend a foot of new snow.

&&

.AVIATION...
PANC...IFR to LIFR conditions are expected through around midnight,
as a renewed round of moderate snow occurs. The foggy conditions
are being supported by consistent influx of Gulf moisture into the
region. Thus, once a strong upper level wave moves in this
evening, the snow should become steady. The primary point of
uncertainty is how far west off the mountains that heavier snow
gets, and there is a chance that this heaviest snow stays east of
the airport around the Chugach. Behind the wave, expect IFR
ceilings to continue as visibility improves until the lower levels
can dry out Tuesday morning, which is when VFR conditions may
return. There is significant uncertainty on the timing of that
improvement.

&&

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

Precipitation continues this evening for much of Southcentral.
Upslope flow will lead to the highest snow totals along western
mountain faces for the Anchorage Hillside, Mat Valley, Kenai
Peninsula, and Copper River Basin. Warm air persists aloft, so the
potential for freezing rain and other forms of mixed precipitation
continues for the Kenai Peninsula. Winter Weather Advisories are
in effect for the areas mentioned above.

As the shortwave responsible for this precipitation moves
southward, a surface low forms in the Gulf of Alaska tonight. Robust
cold air advection developing behind the low will lead to
increased snowfall rates. Any falling precipitation should also
transition to snow before tapering off later tonight. With the
development of the Gulf low also comes a transition to colder,
clearer conditions and an offshore wind pattern for Tuesday. Gusty
outflow winds are expected through the usual areas, including
Kamishak Gap, Resurrection Bay, and the Copper River Delta. As
strong outflow winds are expected to resuspend newly fallen snow,
a Blizzard Warning is in effect for Thompson Pass for Tuesday.

Precipitation chances increase again on Wednesday and Thursday as
a Bering low pushes its front into Alaska. The front occludes as
it approaches Southcentral Wednesday night, so any precipitation
that falls is likely to remain snow for all but Kodiak Island and
the southernmost tip of the Kenai Peninsula. Additionally, snow
amounts are currently expected to remain low due to the front
weakening as it crosses the Alaska Range. Uncertainty remains with
the finer details of how much and where precipitation will fall.
Overall, the incoming system on Wednesday and Thursday looks to be
much less impactful than today's.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Thursday afternoon)...

Rain, drizzle and fog is currently persisting across much of
Southwest Alaska east of an upper level ridge. Fog and/or low
stratus will generally continue through Tuesday morning as a
result of residual moisture from the previous system and the
ridge axis moving overhead. A warm front will approach the region
by Tuesday evening. Snow will track west to east ahead of it.
Given the lack of forcing and relatively strong winds below the
snow growth layer, snow-to-liquid ratios look to be unimpressive
despite the initial colder air at the onset of the event. As
such, accumulations below advisory criteria (6 inches in 12 hours)
appear likely across the Kuskokwim Delta. Amounts will be
slightly higher over the Middle Kuskokwim Valley, however will be
over a slightly longer duration as the front slows. There is a
chance of very brief freezing rain near the frontal axis, although
with southwesterly flow aloft and generally southerly flow at the
surface, a quick transition from snow to rain appears like the
most probable solution. Precipitation will begin to taper off
Thursday as another upper ridge develops over the eastern Bering.

&&

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

A deep fetch of southerly moisture combined with a low is
expected to bring gusty southerly winds and moderate rain to the
Aleutians west of Unalaska through Tuesday morning. These
conditions will continue over the Pribilofs through the day. The
eastern Aleutians and Alaska Peninsula will remain generally quiet
near an upper level ridge. A second, stronger upper ridge will
build over the Bering by Wednesday and benign conditions will
continue across much of the region through Thursday, outside of a
weak front bringing rain to the western Aleutians.

&&

.MARINE (Days 3 through 5 - Thursday through Saturday)...

Bering...Forecast confidence is moderate with respect to the
Western Bering low center tracks through Saturday. A central
Kamchatka low moves to Northern Kamchatka late Thursday.
Widespread gusty winds with areas of gales spread over the
Aleutians and Bering, diminishing Friday. Seas to 20 feet over the
Western Aleutians and Bering, subsiding Friday. A North Pacific
low crosses the Western Aleutians and Bering Saturday. Widespread
gusty winds with areas of gales spread along the Aleutians
Saturday. Seas on the Pacific side to 18 feet.

Gulf of Alaska...Forecast confidence is moderate with respect to
the Alaska Peninsula Gap winds. Widespread gales with the the
strongest gales across the Barren Island into the Western Gulf,
diminishing late Friday. Gusty winds continue over the Gulf for
Saturday. Seas to 12 feet West of the Barren Islands Thursday,
subsiding. Seas building to 20 feet over the Western Gulf Friday,
subsiding Saturday.

&&

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

A large scale Omega Blocking pattern will continue to extend
across the North Pacific throughout the entirety of the extended
period. The ridge-axis associated with this Omega Block will
extend from the central North Pacific to over Southwest Alaska.
This will allow for warmer temperatures across Southwest Alaska.
Southcentral will be cooler with respect to temperature than
Southwest as Southcentral will be east of the ridge-axis and thus
allowing for northwesterly synoptic flow across the region.
Furthermore, strong subsidence east of the ridge-axis will allow
for mostly sunny skies by day and mostly clear skies and light
winds during the night. This will allow for cold nights across
Southcentral, especially in the Copper River Basin due to ideal
conditions for radiational cooling.

Broad southwesterly flow across Southwest Alaska will allow for
plumes of moisture to move across the region. The biggest forecast
challenge as precipitation moves across Southwest will be the
precipitation-type. Warm air advection across the region will
allow for a mix of precipitation-types. The best likelihood for
rain will be across the lower lying coastal areas and the best
chance for snow will be in the inland areas such as the lower
Kuskokwim Valley and in the Kuskokwim and Alaska Ranges.

There is good ensemble agreement the predominant low track will
be across the central to western Bering/Aleutians northeastward
into the Bering Strait and across the Seward Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 Thompson Pass.
         Winter Weather Advisory 101 111.
MARINE...Storm Warning 177 178.
         Gale Warning 119 120 121 126 127 128 129 130 131 132
                      170 173 174 175 176 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...KC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MK/ED

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
1237 AM AKST TUE DEC 21 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1000 PM     SNOW             BUTTE SOUTH OF KNIK RIV 61.45N 148.81W
12/20/2021  M7.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1100 PM     SNOW             WASILLA                 61.63N 149.49W
12/20/2021  M6.7 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

            STORM TOTAL.

1114 PM     SNOW             PALMER EAST OF MATANUSK 61.62N 149.06W
12/20/2021  M8.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1142 PM     SNOW             PALMER                  61.62N 149.21W
12/20/2021  M6.7 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

            STORM TOTAL.

1229 PM     SNOW             5 E ANCHORAGE           61.19N 149.76W
12/20/2021  M2.5 INCH        ANCHORAGE          AK   PUBLIC

0321 PM     FREEZING RAIN    ANCHORAGE (E NORTHERN L 61.19N 149.76W
12/20/2021  E0.01 INCH       ANCHORAGE          AK   PUBLIC

0915 PM     SNOW             UPPER HILLSIDE          61.12N 149.72W
12/20/2021  M4.5 INCH        ANCHORAGE          AK   PUBLIC

            18F, WINDS SLIGHT, TOTAL SNOW ACCUMULATION
            SINCE ABOUT 10:00 AM.

0932 PM     SNOW             POTTER VALLEY           61.07N 149.77W
12/20/2021  M3.5 INCH        ANCHORAGE          AK   PUBLIC

            OVERNIGHT AND THRU THE DAY. STILL SNOWING.

0959 PM     SNOW             EAGLE RIVER             61.30N 149.53W
12/20/2021  M7.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

            NEW SNOW SINCE 7AM: 3 INCHES BEFORE NOON.
            MIX OF SNOW AND SLEET THIS AFTERNOON. T:
            19F. CALM. LIGHT SNOW, SMALL DENDRITES.

1000 PM     SNOW             ABBOTT LOOP             61.14N 149.82W
12/20/2021  M4.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

1108 PM     SNOW             EAGLE RIVER             61.32N 149.54W
12/20/2021  M6.3 INCH        ANCHORAGE          AK   PUBLIC

1116 PM     SNOW             S. FORK EAGLE RIVER     61.23N 149.44W
12/20/2021  M6.8 INCH        ANCHORAGE          AK   PUBLIC

1145 PM     SNOW             8 SSE ANCHORAGE         61.11N 149.78W
12/20/2021  M4.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            TRACE OF FREEZING RAIN BY 7:30AM. 1.0 INCHES
            SNOW BY 12:30 PM AND ANOTHER 3 INCHES
            BETWEEN 5:30PM AND 10:30PM.

&&