Small Craft Advisory 20, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 PM AKST Sat Jan 4 2020

.ANALYSIS AND UPPER LEVELS...
Arctic air is firmly entrenched across the southern mainland of
Alaska, with sub-zero temperatures this morning for all but the
Gulf coast. A low complex is exiting eastward across the southern
Gulf. A trailing upper level short-wave extends from a low center
over the Susitna Valley southward across the western Gulf. There
is a surface trough just ahead of the upper trough along which is
a narrow band of snow. As the upper trough progresses eastward,
moisture and snow is being pulled northward into Prince William
Sound. Tight surface pressure gradients between general low
pressure over the Gulf and higher pressure inland are leading
to gap winds along the coast; particularly for Resurrection Bay,
Passage Canal, Thompson Pass, Valdez Narrows, and Valdez Arm.
There are some localized areas of fog across the region, most
notably in Anchorage and vicinity.

For the Bering Sea region, a closed low and short-wave in the
vicinity of the Pribilof Islands are tracking eastward toward
the Southwest coast. These weak features are producing light
snow showers and locally gusty winds. Zonal upper level flow
along the Aleutians is driving a weak surface low eastward across
the central to eastern Aleutian Islands. Snow (mixed with rain
in some areas) is spreading north and east with the low center.

&&

.MODEL DISCUSSION...
Models are in better agreement with more aggressively spreading
snow northward from the Gulf across eastern Prince William Sound
and into the Copper River Basin. There are still some key
differences in QPF/Snow Amounts, as would be expected with banded
snow and the eventual interaction with mountains. With higher
confidence that it will snow and there will be some localized
areas of heavy snow, will generally increase snow amounts.

Models continue to struggle with track of surface low currently
along the Aleutians and interaction with the trough and low over
the eastern Bering Sea. There is a fairly big spread in solutions
from north to south as the low crosses Bristol Bay and moves into
the Gulf. Will continue to go with a middle ground solution, but
up snowfall probability and accumulation where models overlap and
where there is high confidence.

Forecast confidence is much higher with development of deep low
over the Northwest Pacific Monday and associated occluded front
crossing the Aleutians. Thus, will continue to fine-tune the
forecast using the latest guidance. Solutions diverge again with
depth and track of a triple point low Monday night/Tuesday in the
vicinity of the eastern Aleutians. Will largely stick close to
inherited forecast until get a better idea on how significant this
low will be.

&&

.AVIATION...
PANC...Areas of fog have set up in Anchorage and there is nothing
in store to get rid of it. The challenge will be in determining
when and for how long fog will sit over the terminal. Will rely
heavily on trends in observations across the city to aid in the
forecast process. Sufficed to say, confidence in the visibility
at any specific time is low, especially beyond the first few hours
of the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A low moving into Prince William Sound enhanced by an upper level
trough will bring snow overnight into Sunday morning to the
Valdez/Cordova area. As such, the original Wind Chill Advisory for
Thompson Pass has been cancelled and a Winter Weather Advisory
for blowing snow has been issued in its place. However, despite
the additional cloud cover, wind chills will remain very cold,
approaching -35 in Thompson Pass. Light snow will continue
through much of Sunday. Uncertainty exists regarding a low
approaching the same area on Monday, however another round of snow
appears likely. Temperatures will remain well below average for
the foreseeable future with patchy fog again possible across
Southcentral Sunday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cloud cover moving in over the Kuskokwim Delta overnight will
help keep temperatures warmer than previous nights, but will still
see see cold temperatures. Meanwhile, cloud cover will also
return to the Alaska Peninsula and Bristol Bay ahead of a low that
will cross over these areas by Sunday afternoon. This low will
bring an increased chance of snowfall to the Alaska Peninsula
starting tonight through Sunday evening when the low moves into
the Gulf. There is still a bit of uncertainty with the exact track
of the low, but areas across interior Bristol Bay could possibly
see light snow accumulations through Sunday evening. After the low
moves out of the area, chances for snow diminish heading into the
beginning of the week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A weak low over the central Aleutians, centered between Adak and
Dutch Harbor this afternoon, will quickly track eastward over the
Alaska Peninsula through Sunday afternoon. Increased winds and
chances for snow will accompany this feature over the next day. A
second weak low near the Pribilofs will keep chances for snow
showers over much of the central and eastern Bering until it
merges with the first low over the Alaska Peninsula on Sunday.

The next big system moves over the western Aleutians and Bering
Sunday night into Monday morning. This front will bring a brief
period of storm force winds Sunday afternoon, weakening to gales
as the front continues to move eastward over the central Aleutians
through Monday afternoon.

&&

.MARINE (Days 3 through 5)...
The main impacts Tuesday through Thursday will be across the
Bering as an upper level low sends several fronts northward. The
first front pushing north across the western Bering on Monday is
followed by the next system Tuesday morning. This second system
will be a gale force system that tracks slowly north, with the low
center crossing to the Bering just west of Shemya Wednesday
evening. There is still high uncertainty for the following storm
developing over the North Pacific on Thursday which could bring a
gale to storm force front to the western half of the Aleutians by
Thursday afternoon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Monday night through Wednesday afternoon is expected stay mainly
dry and cold for the southern mainland as a surface ridge remains
dominant across the interior. Although coastal areas could see
light snow at times with fronts diminishing just offshore. Meanwhile,
the active pattern continues across the Bering with a parent low
just south of the western Aleutian Chain late Monday, lifting to
the Northern Bering waters Thursday night. This system is followed
by yet another storm system moving toward the southern Bering
Thursday night. The extended forecast this afternoon will be
updated with a blend of the operational models and ensemble
solutions which are in fairly good agreement with storm momentum.
Winds are going to mainly be gale force with the next several
systems, with the next gale system for the Gulf expected next
Thursday and Friday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 177 178.
 Heavy Freezing Spray 121 127 129 130 139 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...KH