Heavy Freezing Spray Warning 26, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Tue Jan 26 2021

.ANALYSIS AND UPPER LEVELS...

A high amplitude ridge currently exists over the Bering Sea with a
negatively tilted trough over the western Bering. This trough is
allowing for rapid cyclogenesis of a strong surface low estimated
around 940 mb early this morning. ASCAT passes over the western
Aleutian Islands have measured a broad swath of storm force winds
with surface observations at Shemya reporting winds of 58 mph
and gusts up to 75 mph.

To the east, the combination of upper level ridging, surface high
pressure over the interior, and a polar low over the Yukon have
lead to clear and cold conditions across most of the Alaska
mainland. Fog will remain the primary forecast concern on Tuesday
for Southcentral, particularly along the Glen Highway between
Palmer and Anchorage, and the Seward Highway between Girdwood and
Cooper Landing. Surface observations indicate visibilities near
one and a quarter mile this morning in these areas.

&&

.MODEL DISCUSSION...Models remain in good agreement in the timing
and placement of synoptic features in the short term. A few minor
discrepancies exist as to when the front will reach Southwest
Alaska. However, winds associated with the front are expected to
remain on the lighter side so winter weather impacts due to
reduced visibility are not expected.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Patchy fog has
been observed in the vicinity via satellite but is not expected to
impact the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...

Dry and seasonably cold temperatures will persist over
Southcentral through Wednesday under the influence of a high
amplitude ridge upstream. Weak short-waves dropping southward
along the ALCAN will usher marginally colder air across the
Copper River Basin and eastern Prince William Sound today.
This will kick up winds in Valdez/Thompson Pass and through the
Copper River Delta. The remainder of the Gulf coast can expect
weak gap winds, driven by the offshore flow and pressure
gradients, but lacking cold air advection.

The frontal system associated with the Hurricane Force Low
currently along the western Aleutians will reach the Gulf
Wednesday evening, with light snow overspreading Kodiak Island.
Expect a triple point low to form in the vicinity of Kodiak, which
means most of the above freezing air will continue eastward
across the Gulf. Kodiak can expect a mix with rain Wednesday
night, largely do to shallow warm air moving in off the warmer
Gulf waters.

Southcentral, on the other hand, can expect all snow. The upper
level short-wave looks fairly strong as it first reaches the
western Gulf. However, as it crosses the western Gulf an upper
level low will form overhead of the low at the surface, causing
the larger trough to shear apart. As a result, the snow that
spreads inland Wednesday night through Thursday will be light
and will rapidly diminish Thursday afternoon as the now
vertically stacked low over the northern Gulf focuses
precipitation along the coast.

&&

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

Calm and clear conditions will dominate across Southwest through
Wednesday afternoon. A front associated with a low in the central
Bering will then make its way to the coast; however, it looks to
weaken quicker than anticipated. Due to this, confidence of
blowing snow has lessened but still expect an increase in winds
and snowfall to the area. After this, the front looks to move
inland, bringing winds and snowfall to the interior southwest into
Thursday.

&&

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

The front associated with the hurricane force low south of the
western Aleutians has arrived in Shemya with wind gusts observed
above 55 mph and a rain/snow mix falling across the island. Adak,
on the other hand, started as rain but has now transitioned to
snow as their winds continue to increase. This feature will
continue across the western Aleutians and Bering Sea through
morning with blowing snow a possibility.

Cold air will wrap into the low center today, increasing wind
speeds to hurricane force. This looks to remain limited to Kiska,
reducing impacts to populated areas. This afternoon, the front
will propagate across the Pribilof Islands, bringing the potential
for blowing snow with the initial push. Immediately behind,
temperatures should warm changing snow to rain for the Pribilofs
and eastern Aleutians through Wednesday.

&&

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

Gulf of Alaska: Good model agreement through Friday but more
spread in models on Saturday leading to good forecast confidence
early on and then lower confidence by Saturday. Thursday a front
with gale force winds and a low forming just east of Kodiak
Island will move into the northern Gulf. On Friday the low begins
to slow and stall across the northern Gulf with Gales decreasing
to small crafts. On Saturday this low will slowly turn south and
weaken with mainly small crafts during this period but some Gales
possible as northwesterly flow develops across the Barren Islands,
Resurrection Bay, and other areas prone to gusty north to
northwesterly flow, however some uncertainty on track of this low,
so lower forecast confidence for Friday.

Aleutians/Bering: Good model agreement through Friday but more
spread in models on Saturday leading to good forecast confidence
early on and then lower confidence by Saturday. A strong low
moves over the Bering Thursday. High end gale force winds with
storm force gusts spreading over the Bering and Aleutians
Thursday. Wave heights 25 feet to 30 feet subsiding on Friday.
Friday winds will decrease to Gales across the central and
eastern parts of the Bering and Aleutians and continue to weaken
across this area on Saturday. Another front will enter the western
Bering and western Aleutians on Saturday bringing another round
of at least Gale force winds and increasing waves.

&&

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

Models are in fairly good agreement through Sunday with the key
features but are struggling with the complex low over the Bering
on Monday, this leads to higher than normal confidence through
Sunday with lower confidence for Monday.

A zonal pattern will transition to a more amplified pattern on
Friday and Saturday as low pressure from the northwest Pacific
pumps warm air into a building ridge across the Bering. While low
pressure over the Gulf begins to move south on Saturday.

A departing low over the eastern Bering on Friday will bring an
end to snow showers by Saturday as a ridge moves across the
Bering. The low over the northwest Pacific will push a front into
the western Aleutians and western Bering Saturday and Sunday with
increasing winds and rain but not too impactful. On Monday the
front moves across the Bering and Aleutians with minimal impacts.

Across Southcentral low pressure over the Gulf on Friday and
Saturday will bring rain to coastal locations and snow to the
coastal mountains, with slight chances of light snow in the
valleys including Anchorage. Sunday and Monday a low over the
Arctic regions to the north will dip south and bring colder arctic
air to Southwest and Southcentral Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 155 170 172 179 414
		 Storm 173-176 185 411-413
		 Hurricane Force 177-178
		 Heavy Freezing Spray 160 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...RC