Heavy Freezing Spray Warning 61, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKST Sun Feb 7 2021

.ANALYSIS AND UPPER LEVELS...

An amplified pattern remains in place with an upper level ridge
extending along an axis from the northern Gulf toward the
northwestern mainland. Meanwhile to the east, a strong upper level
low continues to gradually move southward over the Alcan border,
pushing up against the ridge. The interaction between these two
features will help to keep northerly outflow winds in place
across Southcentral. Farther west, an upper level low over
Kamchatka continues to bring several upper level waves across the
Bering. A front remains nearly stationary over the eastern
Aleutians with moist southerly flow continuing to bring rainfall
to the eastern Aleutians and southern Alaska Peninsula.

&&

.MODEL DISCUSSION...

Overall, the model solutions are in good agreement through the
short term. Several key differences arise by Tuesday in the upper
level pattern. The 12Z NAM was an outlier and did not advertise a
strong upper level Arctic low over the North Slope. Ultimately,
this solution keeps much stronger ridge in place over the
Gulf/southern Mainland. In contrast, the GFS/EC/Canadian solutions
all advertise a strong upper level low developing over the
northern half of the state, and as a result, shows the ridge
flattening out by late Tuesday. This discrepancy has created
uncertainty regarding the timing of a front arriving over the
Southwest coast late Tuesday into early Wednesday. Of note, the
18Z NAM did come into agreement with the GFS/EC/Canadian solution
and therefore offers some boost of confidence, though there is
still time for solutions to flip-flop. For Southcentral, model
solutions are struggling to resolve a feature arriving over the
northern Gulf coast/Prince William Sound area on Tuesday.

&&

.AVIATION...
PANC...LIFR conditions are likely with stratus and fog continuing
to impact the terminal today. Conditions may briefly improve at
times as the fog/stratus occasionally drifts over the Inlet
however northerly winds should increase around 15z and will help
to mitigate the fog altogether, leading to VFR conditions by
Monday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Sunday
night through Wednesday afternoon)...

A quiet pattern is expected in the short term due to a polar low
over the Yukon Territory and a ridge over western Alaska.
Northerly winds are expected to develop across the region tonight,
bringing clear skies and an end to the low stratus and fog over
the Cook Inlet by Monday morning. A northwesterly jet streak over
the Copper River Valley and eastern Gulf will help to amplify
cold outflow winds through areas like Thompson Pass and the
Copper River Delta. A weak surface low is also expected to develop
over the northern Gulf in the right entrance region of the jet
streak. This will aide the cold air advection from the polar
surface high over the interior. As such, a Wind Chill Advisory
has been issued for Thompson Pass with wind chills below -40F
expected. Winds should begin to subside on Tuesday as the polar
low over the Yukon shears apart due to the advancing ridge from
the west.

The pattern changes late Tuesday as the ridge axis moves over
Southcentral. This pattern is fairly complex due to large
displacements of the low-, mid-, and upper-level high pressure
centers. At this time, light snow is likely for the western
Prince William Sound and eastern Kenai Peninsula. The forecast is
uncertain from the Western Kenai to the Mat-Su Valleys due to a
combination of weak southeast flow in the low-levels and southwest
flow in the mid-levels. The forecast question is whether the
downslope component of the southeasterly flow will dry out the
atmosphere enough to mitigate precipitation or if the moisture
will overcome the downsloping to bring light snow to the area.

&&

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

Southwest Alaska stays quiet the next couple of days with high
pressure and cold air over interior AK filtering into Southwest
AK. This is creating light northeasterly flow for the region and
cold temperatures. This dome of air is also holding off a weak
front approaching the Southwest Coast. Models have come into
agreement that this front will stay off the coast, hanging up just
offshore. This adjustment is reflected in today's forecast.

Tuesday into Wednesday the next frontal system moves towards
Southwest AK, this time managing to work inland. It is anticipated
to bring light snow, with a few inches falling at most. Some
disagreement in models still exists with this system, so
adjustments to this forecast will need to made, though they should
be minor. This system will come in from a southerly direction, so
it is anticipated to bring warm air as well, increasing surface
temperatures and kicking out the cold air. Overall, high
confidence going forward for an increase in temps, and moderate
amount in how much snowfall we'll get.

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

A weak front is nearly stationary in the Eastern Bering,
sandwiched between the Pribilofs and Mainland AK. This front
dropped a substantial amount of rain on the Central Aleutians,
with Dutch Harbor recording around 3 inches. This front will
continue to remain nearly stationary, as high pressure to the east
prevents it from moving ashore. In fact, it even retrogrades a
bit tomorrow. This is a bit of a change from yesterday, as models
now agree it will not impact Southwest AK, so this was adjusted
in the forecast.

A another weak front enters into the Western Aleutians tomorrow
and also moves slowly east, but never makes it past the Western
Aleutians before decaying. The next system of significance is a
gale force front that moves north across the Aleutians Tuesday
into Wednesday, coming out of the North Pacific. This system is
quite elongated, so most of the Aleutian Chain will see weather
from this system. Since this system is coming from the south, it
will bring warm air with it, so expect any precipitation to fall
as rain. Overall, moderate to high confidence with this next
system and with the Bering Sea in general the next few days.

&&

.MARINE (Days 3 through 5: Wed through Fri)...

Bering Sea: Models are coming into better agreement with a deep
North Pacific low on Wednesday positioned south of the western
Aleutians. The associated front will bring widespread high end
gale force winds stretched across the entirety of the Bering Sea
through Thursday afternoon as the low center lifts towards Adak.
There is some uncertainty of where the strongest core of winds
will be located, but there is a chance to see sustained storm
force winds Wednesday and Thursday across the Bering, which when
then start to slightly weaken on Friday.

Gulf of Alaska: There is good agreement of a front over the
western Gulf on Wednesday, which will increase to gale force
strength and persists through Friday morning. By Friday afternoon,
winds will decrease to small craft advisory level and continue to
weaken through the night.

&&

.LONG TERM FORECAST (Days 4 through 7: Thu through Sun)...

A cool but relatively quiet pattern is in store for the Alaskan
mainland as surface and upper level high pressure extend across
the area. There is good ensemble agreement that higher pressure
will be over the Alaskan interior and lower pressure off shore in
the Bering and Gulf, thus the flow will be offshore and thus keep
the cool and dry airmass in place. Deterministic models show an
upper level wave moving northward across Southwest Thursday night
into Friday. There is low confidence in the forecast of this upper
level wave due to significant discrepancies between guidance with
the placement and orientation of this wave. The predominant storm
track will remain over the Bering/Aleutians as North Pacific storm
systems move northeastward along the aforementioned upper high and
into the Bering/Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131.
MARINE...Storm Warning 178.
		 Gale Warning 127 128 165 177 180 181 185 411.
	     Heavy Freezing Spray Warning 127 139 140 160 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...CJ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...AH/ED