Small Craft Advisory 27, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Mon Jan 6 2020

.ANALYSIS AND UPPER LEVELS...
The jet-stream continues to remain south of 50N over the Pacific
as a closed 500 mb low persists over the interior of mainland
Alaska. The 12z Fairbanks sounding was taken near the center of
this cold upper-level low, recording a 500mb height of 494 Dm. An
eastward-moving positively-tilted trough axis associated with this
upper-level low currently extends southwestward across Cook Inlet
and Kodiak Island. Current radar imagery shows snow shower
activity near Middleton Island and eastern Prince William Sound.
Radar is also showing a weak polar low over Hinchinbrook Island
from the cold airmass over the area. IR satellite shows a second
and more well defined polar low moving towards Bristol Bay from
the Pribilof Islands. Several 12z observed soundings show strong
temperature inversions across southcentral Alaska due to the lack
of surface winds combined with weak upper-level flow. The upper-
level flow is faster across the Gulf of Alaska where the 500 mb
height gradient is steeper. Water vapor imagery shows a
northeastward moving occluded front currently over the western
Aleutians with drier air southwest of the frontal boundary.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the overall synoptic
pattern across the region. Mesoscale models are also performing
well as the AK NAMNest, HiRes W-ARW, AK-HRRR and the Canadian
Regional model all picked up on the polar-low southeast of the
Pribilof Islands. The NAMNest wind forecast was favored in the
Copper River Delta due to this model currently having a good
handle on the mesoscale features in that area. The GFS, ECMWF and
Canadian model all have similar solutions with respect to the low
expected in the western Bering during the end of the week.

&&

.AVIATION...
PANC...Fog continues to be a challenge around the terminal and
likely will be in-and-out for the next few days. Going with some
improvement overnight in the latest TAF since the fog is
developing over Knik Arm and moving over the airport in light
north winds. Winds are expected to go calm this evening which
often allows some improvement as the fog in Knik Arm is less
likely to drift over the terminal. Not a guarantee and forecast
confidence is rather low.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A few residual snow showers continue to fall along the northern
Gulf coast this afternoon as a weakening area of low pressure
lifts to the northeast. Offshore flow across the southern mainland
will generally allow for clear to partly cloudy conditions
through Tuesday with the exception of some patchy fog over the
Anchorage Bowl and Cook Inlet/northern Kenai Peninsula area
through the morning hours. Midlevel northerly flow will continue
through early Wednesday morning, allowing for sub-zero
temperatures to persist through midweek. The cold core low
responsible for the colder temperatures does eventually move to
the southeast late Wednesday as an upper level ridge moves in
over Southcentral. This pressure gradient will allow winds to
increase across the northern Gulf, with gusty outflow conditions
expected for Seward, Thompson Pass, Valdez, and the Copper River
Delta. With colder temperatures still in place at this time, wind
chills will continue to be near advisory levels.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
An Arctic trough remains oriented over the mainland, with very
cold conditions continuing in the Kuskokwim Valley/Delta and
Bristol Bay. The weather should be similar tomorrow, though some
high clouds will likely start streaming in from the southwest
towards the end of the day. A warm front associated with a low
south of the Aleutians will lift northeastward, bringing a
transition to southerly flow and warmer, wetter weather Wednesday
into Thursday. With the exception of coastal areas along Bristol
Bay, temperatures will likely still be below zero Wednesday, but
should cross into the positives in the Kuskokwim Delta and Greater
Bristol Bay area Thursday. Snow along the front will likely reach
the southwest coast Wednesday night, then move inland Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A strong low roughly 500 miles southwest of Shemya will continue
to push a gale force front northeastward through the Aleutians and
Bering, bringing a wide swath of small craft advisory level winds
with embedded gales. In addition, expect snow ahead of and along
the front, switching over to rain in the trailing warm air mass.
An embedded triple point low is developing south of the central
Aleutians...stronger winds and heavier precipitation are expected
in its periphery. The front will dissipate as it heads northward,
but will likely be strong enough to bring some light snow to the
Pribilof Islands. Tuesday into Wednesday, another gale force front
will lift northward from the central and western Aleutians,
bringing another round of precipitation to the chain and southern
Bering. Of note, this system will bring another round of warmer
air, and the rain/snow line will lift farther north. Thursday
looks to be a transition day...the parent low will push north
into the Bering and weaken. A storm force low originating from the
coast of Japan will usher in another round of strong winds and
precipitation Thursday night.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...
Bering Sea: Models are consistent from yesterday indicating a
very active pattern for the Bering into the weekend. A broad ~960
mb low will approach the far western Aleutians Thursday evening
bringing a large area of generally southerly gales to areas east
of and including the Pribilofs. As the low deepens on Friday night
into Saturday, storm force winds are possible over the western
Bering, north of the Aleutians. If this verifies, waves up to 40
ft would be expected in that area. The low will slightly weaken
Saturday night, however strong westerly gales will likely to
continue.

Gulf of Alaska: A strengthening low will move west to east over
the southern Gulf on Thursday. There is disagreement among the
models regarding its exact track, however a more northerly
solution would lead to widespread easterly gales and higher seas
into Friday morning. A southerly track would likely keep winds
sub-gale. In any case, a ridge will quickly build in behind the
low by evening. Another, weaker low, will move into the Gulf by
Saturday. Winds look to remain below gales in non-channeled areas
at this time.

&&

.LONG TERM FORECAST (Days 3 through 7: Thu through Mon)...
Models continue to indicate a pattern shift later this week,
especially across western Alaska. An upper level low will move
west of Attu Thursday into Friday. The associated surface front
will bring heavy precipitation to the western and central
Aleutians along with the potential for storm force winds in the
western Bering. This low will amplify the North Pacific ridge out
ahead of it, which will lead to temperatures returning to near
freezing in the Bristol Bay area. This will coincide with the
frontal passage Friday evening and bring heavy precipitation to
all of Southwest Alaska. Southwesterly onshore flow will continue
into Sunday, keeping the warmer temperatures and precipitation
chances for the area through the weekend. Flow will switch to
offshore Monday under a ridge, slightly cooling temperatures.

Well below average temperatures will persist across Southcentral
through Thursday. As noted above, the upper low will deepen in the
Bering and therefore a moist southerly flow aloft will develop in
the western Gulf, bringing relatively warmer temperatures to the
region this weekend. Highs across Anchorage could reach the low
20s on Sunday. Given the lingering cold air in place to the north,
gusty winds and colder wind chills will remain likely in terrain
gaps, including Thompson Pass. Lastly, a weakening front will also
bring a chance of snow over the weekend across Southcentral,
though confidence remains low as to its exact timing or intensity.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121.
MARINE...Gale Warning 173 175>178.
 Heavy Freezing Spray 121 127 129 130 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED/EZ
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...AD