Storm Warning 226, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKST Sat Dec 19 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked area of low pressure over the southern Gulf
is slowly lifting northeast this morning. As it does, an area of
rain is moving into the northern Gulf. Radar returns shows a
steady rain falling over Middleton Island with rain and snow also
beginning to develop over the entrance to Prince William Sound.
To the west, a shortwave moving east from the AKPEN toward Kodiak
Island is interacting with an inverted trough extending from the
aforementioned low northwest over the Barren Islands and southern
Kenai Peninsula. Here, cloud cover is on the increase with returns
showing up on radar from Seldovia to Palmer. However, with drier
air in place at near the surface, thus far only Homer, Seldovia,
and Whittier have reported snow as of 5:00am. Farther south, light
snow is falling across Kodiak Island. Elsewhere across
Southcentral, water vapor imagery denotes the wave that brought a
period of very light snow from Anchorage to Talkeetna and
Glennallen yesterday afternoon is continuing to weaken as it
pushes farther over interior Alaska.

The coldest air is again situated in the Kuskokwim Valley, where
temperatures this morning are hovering around -10F. Temperatures
are warmer along the southwest coast where low stratus, patchy
fog, and isolated areas of light snow persist. Over the Bering,
the main weather feature is a 974 mb low just east of the
Pribilofs. This system is producing areas of rain and snow from
the Pribilofs to Cape Newenham and Bristol Bay.

&&

.MODEL DISCUSSION...

Models are coming into better agreement with what looks to be
another light snow event for the Kenai Peninsula north across the
Anchorage Bowl and Mat-Su Valleys today. There is still some
subtle differences in the position of the surface low as it tracks
to Prince William Sound, as well as the timing and strength of
the 700 mb and 500 mb troughs as both lift across Southcentral
through the day. Even with these differences, it does appear that
a deformation band will set up from the northern half of Kenai
Peninsula to perhaps as far north as Talkeetna. The differences in
the models relate to where the heaviest precipitation within the
band sets up, based on the position of the upper-level features,
and how long it lingers over the area. So although the sensible
weather is the same between models, snowfall amounts differ. Thus,
a blend between the NAM and GFS was preferred with a general 2 to
5 inches of snow expected. There is also some model disagreement
as to whether the light snow will shut off completely moving into
Saturday night and Sunday. The NAM and GFS both indicate a
secondary trough moving through with a southwesterly up inlet flow
for Sunday. This would likely allow light snow to persist through
the weekend, although intensity and amounts would be less than
what is expected today.

Farther west, there is good model consistency with the evolution
of the low currently east of the Pribilofs as it tracks toward
King Salmon by Sun morning. Models are also in good agreement with
deepening a North Pacific low as it moves toward Attu on Sunday
as it lifts a front over the Aleutian Chain. Guidance, however,
then breaks down as the low becomes vertically stacked with
multiple upper-level shortwaves lifting north in advance of the
main low. This translates to the various models spinning up
multiple deep lows at various points along the front at various
times through early next week. Basically, all models indicate the
development of a broad and complex low moving from south of the
Chain to the central Bering through the short term forecast
period. The exact position and strength of these features along
the front and in relation to the parent low, though, is near
impossible to pin down right now. These solutions will also have
an impact on the strength of the front as it approaches the
southwest coast Monday. The only consistent agreement between the
models is that the parent low will be quite strong (sub 960 mb),
as will any subsequent surface lows that spin up along the
associated front.

&&

.AVIATION...

PANC...VFR ceilings and visibility this morning will lower to
MVFR as snow develops over the terminal by mid-morning Saturday.
IFR ceilings and visibility are expected by noon and to continue
through the evening, especially in heavier bands of snow. 2 to 4
inches of snow is expected for the airport complex. The snow is
expected to end late Saturday night or early morning Sunday with
improved visibility; however, low stratus should persist with MVFR
ceilings.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Sunday Night)...

The next snowfall for the Southcentral region is already beginning
to take shape over some of the area. Radar imagery this morning
shows Gulf moisture streaming northwestward into the Kenai
Peninsula. This moisture is feeding a developing snow band,
already causing snow in Homer and Seldovia. The band is expanding
northward along the western edge of the Kenai Mountains. This is
likely going to be the snow band that impacts Anchorage and the
Mat-Su through the day today. While it is difficult to say exactly
where the heaviest snow associated with the snow band will set up,
it is progged by many of the models to set up over Anchorage and
the Mat-Su from the Parks Highway east. The forcing for this band,
an approaching upper level disturbance, will lift northeastward
through the day, which will keep the light snow going through the
day, but the entire area of snow will also gradually shift
northeastward with time. Thus, the snow ongoing in Homer and
Seldovia now will end first this morning, and the back edge of the
snow will gradually move northward through this evening. 2 to 4
inches remains the general consensus on how much snow any one area
can expect. If the current pattern of a solitary heavier band
persists through the day and the band is slow-moving or remains
stationary, there certainly is potential for localized areas to
get more. However, this would also mean areas outside of the snow
band would get less. This is because these snow bands generate
their own circulations, with lift and heavier snow within the
band resulting in subsidence and light/no snow in areas
immediately outside of the snow band.

Outside of Anchorage and the Mat-Su, a bit heavier amounts of
snow are expected in the Kenai Mountains today, as these areas are
closer to the Gulf moisture source. The entire area of snow shifts
northeastward tonight, so heavier/more persistent snow will move
into Prince William Sound, including Cordova and Valdez this
evening into tonight, along with the Copper River Basin overnight
tonight. The snow will taper off along the Wrangells Sunday
morning.

Another upper level disturbance is expected to cross Southcentral
Sunday evening. This feature will have considerably less moisture
to work with, but it should be strong enough to get flurries to
light snow started up again across the Cook Inlet region Sunday
afternoon into Sunday night. Accumulations will be minor, under an
inch for most areas.

Late Sunday night into midweek, the weather pattern becomes even
more active, shifting towards the warmer/windier. A strong Bering
low bringing a similarly strong front and possible atmospheric
river eastwards into Southcentral. Impacts could include the
potential for high winds through Turnagain Arm and the Gulf
waters, Chinook/downsloping into Anchorage at the Mat Valley,
heavy snow through the coastal mountains causing localized
blizzard conditions, heavy rain along the coast increasing
flooding concerns, rising snow levels, heavy snow along the
Alaska Range, increased avalanche concerns, and treacherous travel
conditions in many areas. Stay tuned for more on all those
hazards.

What can be said with high certainty is that stargazers hoping to
catch the conjunction of Jupiter and Saturn Monday evening should
plan not to be anywhere near Southcentral Alaska, as cloud cover
will be widespread here.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

This morning there is a 974 mb low over the Pribilof Islands that
will continue to track eastward towards Bristol Bay. This storm
will gradually weaken along the way and the pressure gradient
forces will steadily decrease. The associated frontal boundary
will approach the Southwest coast, bringing a round of light snow
showers to the coastal areas. Significant accumulations are not
expected with this system. There is a secondary area of low
pressure between St Matthew Island and Nunivak Island, light
showers will be possible in the vicinity of Nightmute to St
Matthew Island. Additionally, a new storm is brewing in the North
Pacific. The models have not yet converged on a precise track yet,
but they are all suggesting that the low will be deep and have a
fetch of deep moisture associated with it. The precipitation
shield from this new storm will reach SW AK on Monday.

&&

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

This morning there is a 974 mb low over the Pribilof Islands that
will continue to move eastward towards Bristol Bay. This storm
will gradually weaken along the way as the low fills and the
pressure gradient forces will steadily decrease. There is a secondary
area of low pressure between St Matthew Island and Nunivak Island,
light showers will be possible in the vicinity of Toksook Bay to
St Matthew Island. Additionally, there is a new storm churning in
the North Pacific. The models all project that this low will
undergo rapid intensification today and early Sunday as it moves
towards the western Aleutians. High confidence in gale- storm
force winds and a widespread precipitation shield. By Sunday
morning, the leading front of an incoming storm force low west of
Shemya enters the Western Bering.

&&

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

Gulf of Alaska:

There is good confidence on a quiet start to the period with high
pressure extending across much of the Gulf. By Monday night
southerly winds will be increasing across much of the western Gulf
of Alaska as a front approaches. The current forecast calls for
widespread Gales, but some areas of storm force winds appear
possible. On Tuesday we should see these southerlies subside.
Wednesday there is significant model disagreement on the
development of an area of low pressure in the northeastern Pacific
that would bring stronger easterlies back into the Western Gulf,
for now the forecast leans towards this development remaining too
far south and too weak to have any significant influence on
winds/waves in the western Gulf.

Bering Sea:

A broad complex low with multiple centers will be centered over
the Bering Monday into Tuesday. The strongest winds will be found
along the northern fringes of this broad low across the Northern
Bering, with good confidence on generally low end Gales. By
Wednesday the broad low looks to break up into multiple weak lows
across the Bering Sea. Confidence on the placement of these
individual lows is very low, but none of them look to produce more
than some very small areas of Gales.

&&

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

On Tuesday a deep trough will extend across much of the Bering,
with a ridge across the Gulf of Alaska through eastern Mainland
Alaska. A pronounced shortwave will be rounding the base of this
Bering trough with a low deepening across the western Gulf. This
low will have a great connection to some moist air from the
tropics and thus looks to set up widespread precipitation,
potentially heavy from the AKPEN through the Northern Gulf Coast.
With the precipitation will come a significantly warmer airmass
so a quick change to rain is possible for portions of the AKPEN
and Kenai Peninsula. Some very heavy snows look possible for all
higher elevation areas surrounding Prince William Sound. On
Wednesday this system should begin to weaken with only some
lingering precipitation for the Northern Gulf coast. For
Thursday/Friday another storm system looks to lift out of the
North Pacific into the eastern Gulf of Alaska with most of its
impacts remaining east of our AOR. Across the Bering a complex of
weak lows look to keep the pattern active across the central and
western Aleutians, but with none of these lows looking
particularly impactful/strong.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 172 174.
		 Gale Warning 119 120 165 170 173 175-178 351 352 411 413.
		 Heavy Freezing Spray Warning 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...BG

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
212 PM AKST SAT DEC 19 2020

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

0211 PM     SNOW             ANCHORAGE               61.10N 149.84W
12/19/2020  M6.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            6 INCHES OF ACCUMULATION SO FAR. MODERATE
            SNOW CONTINUES.

&&

GJERDINGEN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
246 PM AKST SAT DEC 19 2020

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

0245 PM     SNOW             3 ESE STERLING          60.49N 150.71W
12/19/2020  M7.0 INCH        KENAI PENINSULA    AK   PUBLIC

            7 INCHES SINCE 12 AM. LIGHT SNOW CONTINUING
            CURRENTLY.

&&

GJERDINGEN