Heavy Freezing Spray Warning 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 PM AKST Tue Feb 2 2021

.ANALYSIS AND UPPER LEVELS...A fast moving pattern is unfolding
across the southern half of AK. Cold air is holding on across
nearly all of the state with ambient temperatures reaching down
to nearly -35F across the Copper River Basin this morning. This
cold air is helping to keep a very tight gradient from the
interior of Southcentral down across the Gulf Coast. The result is
some gusty winds in many of the typical gaps such as the
Matanuska Valley and Valdez. Thompson Pass has seen the strongest
winds with gusts continuing between 50-60 mph this morning
resulting in wind chill values near -50F.

A potent front is racing from west to east bumping into this cold
air. This is creating a healthy amount of snow and some blowing
snow across much of Southwest AK and Kodiak Island. This front is
now over the Southern Cook Inlet and is tracking from southwest to
northeast. Further west, over the Bering Sea, a relatively quiet
period of weather is taking place. However, another potent storm
is quickly approaching the Western Aleutians with another round of
gusty winds and snow in store.

&&

.MODEL DISCUSSION...Models are in very good agreement leading to
high forecast confidence. The couple areas of uncertainty surround
the timing and amount of snow across Southcentral tonight into
Wed. We have generally trended this forecast towards a slightly
faster solution with the onset of snow into Anchorage later this
evening. Amounts still remain somewhat uncertain as a low
develops in the Western Gulf. This will reinforce some dry air
into the low levels which could cut down on accumulations.

&&

.AVIATION...
PANC...The exact timing of snow could vary by +/- 3 hours.
However, once it begins, expect conditions to deteriorate into the
MVFR range fairly quickly. As the snow winds down Wed morning, low
stratus will persist across the area with the chance for some fog
to potentially reduce visibility late in the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through Day
3...Tuesday Afternoon through Friday)...

Strong winds will continue across Thompson Pass tonight before
decreasing so have extended wind chill advisory until midnight.

A shortwave trough is moving towards the area from the southwest
supporting a weak front currently moving through the Kenai
Peninsula. This front will bring a couple of inches of snow from
the Kenai Peninsula and northward towards Anchorage and the MATSU
valleys this evening and tonight. Generally 1 to 3 inches of snow
expected in most populated areas. This wave will push the snow
into the Copper River Basin tomorrow morning with only light
accumulations. The snow will end across the Kenai Peninsula and
Anchorage by tomorrow morning, with some lingering snow showers
possible through the afternoon as another shortwave skirts across
the Anchorage and MATSU valleys. Most of the area will clear out
Wednesday night. Fog and low clouds are possible along the Cook
Inlet near Kenai, Anchorage and the MAT valley Wednesday night
through Thursday morning.

Thursday the upper level ridge builds in from the west which
will cause outflow winds to increase across areas prone to
stronger winds with north and westerly flow, but no significant
impacts from these winds.

Friday another frontal system will move into the Gulf and spread
precipitation northward towards Southcentral. But some uncertainty
among models on evolution of this storm as the ridge may slow the
movement, so forecast confidence is lower for Friday.

&&

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

Snowfall continues across much of Southwest Alaska this evening as
a front pushes farther inland. The Winter Weather Advisories will
expire this evening, with an additional few inches of snowfall
expected with this system as it moves farther east overnight
tonight into early Wednesday morning. Snow showers will linger
across the region through Wednesday afternoon as the front begins
to fall apart and exit the area.

Upper level ridging begins to build in behind the exiting front,
which will bring clearing conditions across the Southwest Mainland
for Thursday into early Friday. Models begin to diverge throughout
the day on Friday with the next North Pacific low feature moving
towards the forecast area. There is much uncertainty in the track
of this low, the timing, and whether or not the associated front
will impact the Southwest. Thus, the chance for precipitation will
remain in the forecast for the end of the week until there is
better model agreement with this feature.

&&

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

Quieter conditions have settled in over the central and eastern
Bering and Aleutians this afternoon, while the next system begins
to enter the western portion of the domain. A frontal system
extending from a Kamchatka low will bring max gale force winds,
along with storm force wind gusts, to the western waters this
evening. As the front progresses eastward, it will slightly
weaken, keeping winds at gale force strength until it reaches the
eastern Bering by Thursday afternoon.

Behind this system, models are having a difficult time handling a
compact low over the central/western Bering on Thursday. There is
also uncertainty in a North Pacific low over the eastern Bering
moving into the region on Friday, which models have been
struggling with the past few runs. This system could bring a small
area of gale force winds to the eastern waters. Farther west,
models are in much better agreement with a storm force low
entering the western Aleutians and Bering by Friday afternoon.

&&

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

Gulf of Alaska: A low lifts along the Alaska Peninsula to form an
elongated trough along the North Gulf Coast. Model tracks and
development vary, but confidence is good through the forecast
period. Widespread high end gale force winds and waves move across
the Gulf Fri night through Sun, with a lingering area of small
craft winds from the Barren Island into the Gulf Sun. Winds will
be easterly along the North Gulf Coast, and westerly over the
Gulf. Wave heights build to 18 feet Sat over the Gulf, subsiding
Sun.

Bering Sea: A weakening low tracks northward over the Eastern
Bering through Sat. Widespread small craft winds and waves
dissipate Sat. Model tracks and development are mixed, so
confidence is low. A well developed low and front enters the
Western Bering late Fri. Widespread high end gale force winds with
the front move into the Central Aleutians and Bering for Sun.
Wave heights to 20 feet on the Pacific side of the Alaska
Peninsula subsiding Sun. Waves to 21 feet Pacific side of
Aleutians, 15 feet Bering side Sat and Sun.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

An upper level ridging pattern will extend across the
Bering/Aleutians and Alaskan mainland on Saturday and Sunday. This
will help bring warmer temperatures across Southcentral and
Southwest as well as the Bering/Aleutians for this weekend.
Uncertainty increases on Monday as differences in ensemble
solutions arise during this time. The GFS and ECMWF ensembles show
a deep upper trough building in from the northeast while the
Canadian ensemble keeps this upper trough far to the north as well
as keeps this trough much weaker. The discrepancies are from the
ensemble East Pacific Oscillation forecast where all ensembles
show the general trend of the negative East Pacific Oscillation
weakening early next week but show differences in how much
weakening will occur. For now, a blend of the GFS/ECMWF ensemble
is the preferred solution as all models support a strengthening
negative West Pacific Oscillation which will help amplify the
ridge across the Bering/Aleutians. This amplifying ridge is likely
to assist in driving an upper trough across the Alaskan mainland
during early next week which will help decrease temperatures
across Southcentral and Southwest Alaska. Overall, this pattern
will support quiet weather across the Alaskan mainland. Over the
Bering/Aleutians there is a better chance of storm activity,
particularly in the western Bering/Aleutians as the upper level
pattern will support storms to move northeastward along the
Kamchatka Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 155 161 and 131.
MARINE...Gales 173 175-178 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MK/ED