Winter Weather Advisory 10, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 AM AKST Tue Feb 9 2021

.ANALYSIS AND UPPER LEVELS...

Upper level ridging holds strong across the Gulf of Alaska,
extending across the Southern Mainland and into the Eastern
Bering. An upper level Arctic low pushes across the North Slope,
eventually moving offshore of the Arctic Coast. This will flatten
the ridge as it passes, confining it to the Gulf and
Southcentral/Eastern portions of Alaska. At the same time, a
strong North Pacific low lingers South of the Aleutians. Multiple
fronts and waves rotate cyclonically around the low center, moving
across the Bering and into the Southern Gulf. The first front
develops a mesolow south of the AKPEN on Wednesday evening, which
is expected to travel northward up the Southwest Coast, further
flattening and pushing the ridge eastward. The second front may
mirror the first, developing another weak low within the shortwave,
though there is still quite a bit of model discrepancy allowing
for a decent amount of uncertainty.

The Arctic low is not well reflected in the lower levels due to
the strong high pressure center located off the Arctic coast.
Thus, much of the state will remain under high pressure in the
lower level. Unsettled weather for the Southcentral will likely
be confined to the coastal areas as small weak lows develops
along the North Gulf Coast. A weak trough may also develop as the
low tracks towards Cook Inlet, which may bring a brief band of
precipitation further inland to the Mat-Su Valley areas tonight
into tomorrow.

&&

.MODEL DISCUSSION...

Model agreement is good on the synoptic scale for the short term.
A weak surface low in the Northern Gulf is causing some grief for
the models today but they quickly recover as the low dissipates
later in the evening. Model agreement has neither increased nor
decreased for the secondary low developing near Unalaska Wednesday
evening. The GFS/EC seem to be the most similar for track and
center location, but vary in strength with the GFS being slightly
stronger. The Canadian is the biggest outlier by merging the
parent and secondary low and moving a shortwave across the
Aleutians. The NAM keeps the secondary low much further east than
the EC/GFS, tracking it over Adak.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist throughout the
day. A band of snow looks to move across Anchorage late tonight
and into tomorrow morning. Lowered visibilities and cloud ceilings
may promote occasional periods MVFR to IFR conditions.

&&

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

A weak Gulf low/inverted trough has developed over the northern
Gulf this morning. It is very slowly drifting westward. The low
has formed in the right entrance region of a 105 kt jet that
extends from Denali southeast down the length of the Panhandle to
the Pacific Northwest. This favorable upper level pattern will
allow the low to very slowly strengthen, though the northwesterly
upper level flow will ensure the associated precipitation shield
also only slowly develops north and westward today. Additional
moisture is being added to the Gulf from a stationary front and
low over the eastern Bering. This is causing upper level cloud
cover to move over the western half of Southcentral, which has for
all intents and purposes stopped the radiational cooling in the
affected areas.

By tonight, the upper level jet streak will weaken and shift
northward around the southern periphery of a strong upper level
low over the northern half of the state. This will draw the Gulf
moisture northward. However, upper level ridging will move over
the Gulf low, which will lead to the surface low weakening.
Upsloping will enhance the snowfall rates across the Susitna
Valley and through Hatcher Pass tonight. The easterly flow
associated with the weakening low will result in some level of
downsloping for Anchorage, the Matanuska Valley and western Kenai
Peninsula. Thus, around an inch of accumulation is expected for
these areas, most likely on the front side of the moisture shield
this evening. The snowfall will continue shifting westward through
the day Wednesday, persisting longest in the western Susitna
Valley and through the Kenai Mountains.

By Thursday, the upper level pattern will be much more amplified,
with the aforementioned ridge associated with a very strong polar
high along the Alaska/Yukon border, while a very slow moving, but
moisture laden front approaches Kodiak Island. Kodiak will be the
main focus for the most active weather across Southcentral
Thursday and Thursday night associated with that front. Meanwhile
polar high pressure will persist across the rest of the mainland.
With the center of the high pressure over the Yukon, the synoptic
flow will be southeasterly across all of Southcentral, which will
result in cloudier, and significantly warmer conditions by late
week. For Kodiak, the positioning of the front will make all the
difference in the weather. Unfortunately there remains significant
spread among the model guidance on where that front will be.
Nevertheless, significant, at times heavy precipitation is
expected over much of Kodiak Island into Friday. The form that
precipitation will take is also very uncertain. What appears most
likely is that the long southeasterly flow over the Gulf waters
which are sitting around 40 degrees should result in mostly rain
in the populated areas along the coast, with heavy snow expected
at the higher elevations. With the amount of cold air in the area
uncertain, it remains possible that that moisture could fall in
the form of snow down to sea level, which may result in
significant accumulations. Interests around Kodiak Island should
continue to monitor the latest forecasts.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tuesday
morning through Thursday)...

An upper ridge will keep quiet and cold conditions around for just
a bit longer along with offshore winds for the rest of the day
today. A high deck of clouds wrapping into the region around the
upper ridge, with a few very light snow flurries falling from
this deck in a couple spots this morning.

By tonight, a weak front lifting out of the eastern Bering will
bring a band of snow into the southern Y-K Delta and Kilbuck
Range as it lifts slowly north. Snowfall amounts with this band
will generally be light, with about 1 inch or less expected for
lower elevations between Koliganek and Togiak. By Wednesday, the
band of snow will move up into much of the Y-K Delta where it will
intensify enough to drop around 2 to 4 inches of new
accumulation by the end of the day on Wednesday, roughly along an
axis from Bethel to Toksook Bay. Winds will begin to increase as
the pressure gradient tightens between a stronger front moving
into the Bering/Aleutians and the arctic high remaining in place
over the northern interior and far northern Bering. This may lead
to periods of blowing snow across the Y-K Delta by Wednesday
afternoon, particularly near the coast.

The picture gets increasingly more complicated from Wednesday
evening onwards. Winds are expected to continue to ramp up going
into early Thursday as the front rams against the strong ridge
remaining in place farther northeast. A series of shortwave
troughs will begin to approach the AKPEN and eastern Bering as
stronger southeasterly flow becomes more established upstream of
a powerful low over the North Pacific. Additional rounds of
snowfall are possible as these waves move across, particularly for
the windward faces of the Kuskokwim and Aleutian Mountains with
the upslope enhancement. Strong southeasterly winds will continue
to drive blowing snow across parts of the Southwest until warmer
air near or above freezing is advected into the area late on
Thursday, and this may necessitate weather headlines in upcoming
forecast updates.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tuesday morning through Thursday)...

Active weather continues across the Bering with a rapidly
weakening front still stretching across the far Western
Bering/Aleutians this morning. By tonight, a deep low in the
North Pacific will bring another more expansive frontal system
into the region as it moves slowly towards the Southern Bering. A
strong Arctic high in place over the Bering Strait and Alaska
juxtaposed with the approaching frontal wave and strong low will
result in a very tight pressure gradient across the Bering Sea,
and widespread easterly gale to locally storm force winds are
expected through Wednesday into early Thursday, and this will
continue to drive heavy freezing spray north of the Pribilofs and
around St Matthew Island, which may have impacts on fishing
operations in this vicinity. Widespread rain and snow showers are
expected across the Bering and Aleutians along and ahead of the
front as small waves work westward near the boundary.

By Thursday, a new low may form at the front's triple point near
the Central Aleutians, potentially bringing another round of gale
to storm force winds. However, models are not in good agreement
in regards to the placement and timing of this next system.
Therefore, details for possible impacts for this storm remain
unclear for the time being.

&&

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

Bering Sea: Models are in good agreement with a deep North
Pacific low South of the Western Aleutians. Its front brings
widespread high end gale force winds covering the Bering Sea
through Sat. A new low center lifts towards Adak. There is some
uncertainty as to how significant this will affect the overall
wind fields as its front passes through the Bering. Wave heights
to 29 feet in the Western Bering through Fri, subsiding Sat. Waves
to 25 feet Pacific side of the Central Aleutians Sat.

Gulf of Alaska: Models are in good agreement with a strong front
over the Gulf. Widespread southeasterly gale force winds and waves
with high end gale force gusts move into the North Pacific
through Fri. Local areas of storm force winds through the Barren
Islands on Thu. Southeasterly small craft winds take over the Gulf
Fri, diminishing. Wave heights to 25 feet from the Barren Islands
southward, subsiding Sat.

&&

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

A cool and dry pattern is expected across Southcentral in the
extended period as the combination of upper level and surface high
pressure dominate the area. There is good ensemble agreement in
the forecast surface pressure showing higher pressure across the
Alaskan interior and lower pressure offshore in the Gulf and
Bering. This pressure pattern supports cold and dry offshore flow
across Southcentral. The cold air is expected to moderate more
across Southwest compared to Southcentral as an upper level wave
is expected to move northward across Bristol Bay and the Kuskokwim
Delta on Friday. This upper wave will result in southeasterly
850 mb winds which will ultimately advect warmer air into the
region. Precipitation type can be an issue in Friday's forecast
across Southwest, specifically in coastal locations along the
Kuskokwim Delta and Bristol Bay. With respect to the potential
tracks of storm systems, there is a high confidence that the
Bering/Aleutians have the greatest chance of storm systems moving
into the area due to the upper ridging in place across the Alaskan
mainland. The upper ridge across the Alaskan mainland will allow
for North Pacific storms to move northeastward and enter the
Bering/Aleutians. There is good ensemble agreement in the surface
pressure fields that the lowest pressure will remain over the
Bering/Aleutians for this period. For the end of the extended
period, temperatures are expected to become colder across
Southcentral and Southwest Alaska as there is a hint at a pattern
shift with a weakening of the upper level ridging over the
Alaskan mainland as well as the development of an upper trough
across the Alaskan mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory: 131
MARINE...Gale Warning: 127 130-132 136-139 150 155 160 165 170-172 174-179 180 321-352 411-414
		 Heavy Freezing Spray Warning: 127 128 139 160 180 412
FIRE WEATHER...NONE.

&&

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