Brisk Wind Advisory 18, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKST Fri Jan 31 2020

.ANALYSIS AND UPPER LEVELS...

A broad upper-level trough extending over the Alaskan mainland has
a closed low center over the western Gulf of Alaska. A trough-axis
extends southwestward from the central Gulf into the north
Pacific. Upper-level ridging currently extends northwestward from
the eastern Aleutians to Gambell Island. A second upper-level low
is currently centered over the western Bering near 58N 175E. The
polar-jet stream currently extends southeastward across the
eastern Bering/Aleutians to 50N 148W and then northeastward into
the Yukon Territory. The subtropical-jet stream remains south of
45N across the north Pacific through 135W and then extends into
southern British Columbia.

At the surface, snow showers continue to move northwestward from
the northern Gulf into Prince William Sound and the southern
Chugach. Observations show the strongest surface winds associated
with steep pressure gradients across Kodiak Island.

&&

.MODEL DISCUSSION...

Models generally remain in good agreement with the overall
synoptic pattern through the weekend before beginning to diverge
on Monday. Due to this forecast confidence for days 1-2 remains
high. A blend of high resolution models was used in this forecast
package with respect to snowfall amounts over northern Prince
William Sound and the southern Chugach Mountains due to them
having a better handle on mesoscale features.

&&

.AVIATION...
PANC...Light north winds with occasional MVFR ceilings and
visibilities.

&&

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

A surface low centered in the northern Gulf is continuing to
bring scattered snow showers to the Prince William Sound area. A
series of upper level shortwaves will cause these to move farther
north through the day. Areas along the immediate coast
(especially near Cordova) could see locally heavy amounts, though
most other areas will see minimal accumulations. Snow will taper
off overnight into Saturday morning as the low departs to the
southeast and skies will clear as high pressure strengthens over
the interior. This will subsequently lead to increased northerly
winds in terrain gaps and out of bays and passes, most notably
over Shelikof Strait and the Barren Islands. Sunday morning lows
well below zero are likely across much of Southcentral. Clouds
will increase Sunday afternoon and snow will develop by evening as
a front tracks over the region. While uncertainty still exists,
it appears much of the flow will be east-west oriented and thus
significant accumulations would not be expected in Anchorage.
Highest totals would likely occur over the northern Susitna
Valley, southern Kenai Peninsula and Prince William Sound through
Monday.

&&

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

An upper-level trough currently over the region will push eastward
tonight, with a ridge of high pressure building in for Saturday.
This will also allow a moderating trend to commence at the surface
and aloft tomorrow. Then as we head into Sunday, a warm front
reaches the coast while stretching down through the peninsula by
late morning, continuing its inward push. This front will have
reached the far interior areas by Sunday night or Monday morning.

This front will be the next weather maker of concern for the
region, as accumulation snowfall and gusty winds will precede the
fronts arrival, with blowing snow and reduced visibilities likely.
This could create enough of an adverse condition where a winter
weather advisory for snow and blowing snow may need to be
considered with later forecasts. After the front pushes trough,
temperatures will continue to warm pretty dramatically, as 850
mb (4000 feet) temps climb from their -23 to -28 deg. C values top
around ~ -4 deg. C. by Sunday evening. This translates to a good
20 to 30 degree increase if afternoon highs, as values to near
seasonal averages, if not above.

&&

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

A rapidly deepening low will approach the western Aleutians
tonight, sending a warm front rapidly north and east across the
Bering Sea and Aleutians. Strong gusty southeast to east winds and
accumulating snowfall will likely precede this front, with
blowing snow a concern for the Pribilof Islands. Further south,
temperatures should moderate enough for this to be less of a
concern temporally, but this will be monitored as well for a
possible headline with later forecasts.

For the maritime community, widespread sustained gales will
accompany this front, with some sustained storm force winds likely
for the western Aleutians and Bering, closer to the area of low
pressure. Given a long fetch length, waves of 20 to 30 feet will
also be common for the western portions of the Bering, and
anywhere along or south of the Aleutian chain.

&&

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

Bering Sea and Aleutian Coastal Waters: A large strong low south
of the Aleutians will have a front/trough extending through the
central Bering Sea northeast to the Kuskokwim Delta coast. North
and west of this trough a tight pressure gradient will keep gale
force winds going across the western Bering Sea through Wednesday.
This storm will usher in warmer air that will help to reduce the
amount and extent of freezing spray in the Bering Sea waters.

Gulf of Alaska: A low rapidly moves through the Gulf of Alaska
Monday that will be followed by another low that rapidly moves
across the Gulf of Alaska from west to east late Tuesday and
Wednesday. Gale force wind is likely in the Gulf of Alaska with
the first low Monday. Storm force wind is possible along the North
Gulf Coast Monday north of the low. The second low will be
crossing the southern Gulf of Alaska, thus any gales associated
with it will likely remain south of 55N.

&&

.LONG TERM FORECAST (Days 3 through 7, Monday through Friday)...
The long term forecast begins with a large north Pacific low south
of the Aleutians pushing warm air into the eastern and central Bering
Sea. We also have a moderately strong low moving across the Gulf
of Alaska into the North Gulf Coast Monday. Here the models
diverge on how strong this low will be, thus confidence on how strong
the wind will be along the North Gulf Coast is not high. This
could result in storm force wind, or minimal gale force wind and
will bear watching through the weekend. The location and intensity
of the low impacting the Aleutians is in good agreement Monday. As
we continue through the week the synoptic level pattern is in
decent agreement and the overall theme is an upper level low
anchored over the northwest Arctic coast that supports a series of
lows that transit across the Gulf of Alaska and mainland Alaska.
The models do struggle with how the low moves trough the Bering
Sea Tuesday and beyond. The differences in the model solution
translate to lower confidence in any one model solution, thus we
will take a more blended approach and any changes to the forecast
beyond Tuesday will be minimal and trend to a blended solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms 130 174 176 177 178. Gales 120 121 125 129 131 132
136-138 150 155 165 170-173 175 179 180 185 351-352 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...SA