Small Craft Advisory 221, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
352 PM AKST Fri Mar 6 2020

.ANALYSIS AND UPPER LEVELS...
The upper level ridge over mainland Alaska has weakened
considerably and this is evidenced by the large amount of cirrus
that is riding over the ridge into the state today. In addition,
weak lower-level lifting due to cold air in place over the north
Gulf coast has brought in some snow showers to that area which is
also evidence of the weakening ridge aloft. A strong upper level
low is southwest of Attu with a 250 mb jet near 150 kt that is
running from near this low to the western Aleutians to the
Pribilof Islands, and then southeast to over the Alaska Peninsula.
This pattern will transition into a more zonal flow aloft by
tomorrow.

&&

.MODEL DISCUSSION...
The models remain in good synoptic agreement through the weekend
and are decent into the middle of next week. One of the more
challenging aspects of the models has to do with the embedded
shortwaves in the developing southwest flow over Southwest and
Southcentral ALaska and how that translates into snowfall. The NAM
has been holding firm with keeping the snow generally with the
shortwaves while the GFS has been keeping snow falling even
between the waves suggesting the forcing has to do more with the
general upper level flow and weak lower-level features.

&&

.AVIATION...
PANC...Snow may arrive by the middle of the evening which is a
little earlier than previous TAFs suggested. This will lower
ceilings and visibility some but the snow looks to put conditions
more in the MVFR range through the night though occasional IFR
conditions are possible after midnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main challenges for the upcoming weekend are snow coverage
and amounts as two upper level storms track across the southern
mainland from the Bering. There is some uncertainty on snow
amounts, with the highest focus for edits today across the Susitna
Valley as a deformation band is expected to bring a prolonged
period of snow.

Widespread snow will develop tonight with the first storm moving
across from the Southwest. The associated front pushes from west
to east on Saturday. Snow accumulations will average in the range
of one to two inches as the upper level trough quickly pushes
across by the afternoon. Upper level disturbances through zonal
flow overhead keeps ongoing periods of light snow activity across
the region as the air mass remains saturated. A weak area of low
pressure at the surface centers over the southern Susitna on
Saturday and remains throughout the weekend. This will be the
focus for persistent snow development along along a south to north
band stretching from the northern Cook Inlet to the northern
Susitna Valley.

Saturday evening, gusty winds develop across the northern Gulf.
Winds also increase for east to west oriented gaps due to a
tightening pressure gradient along the coast due to the
approaching front. A weak boundary along the coast will support
light snow to develop before pushing inland. Downslope flow across
the mountains limits any precipitation from making it inland.

Periods of snow Saturday night through Sunday afternoon persist
as upper level waves quickly move through a broad trough aloft.
Isothermal processes involving a warmer maritime air mass
overrunning a cold pool at the surface supports continuous snow.
The one limiting factor will be subtle pressure changes near the
surface.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The weak low pressure system that moved in overnight is currently
moving through southwest Alaska and starting to make its exit
from coastal areas. Interior areas of southwest AK can expect to
see continued snowfall for the overnight hours. Snow showers will
linger through tomorrow ahead of the next system, which is
currently entering the Western Bering.

This next low pressure system will move into southwest AK Sunday.
It will be similar to the low currently in the area. So, expect
some more light snow as this system pushes through. It will make a
quicker exit to the east, with weather clearing behind it Monday.
A return to calm weather and cooler temperatures is expected.
Forecast confidence is high through the weekend as models have
done a good job of capturing the current weather pattern.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The eastern Bering and Aleutians currently have a break in the
weather with a shortwave ridge moving through the region. This
ridge is sandwiched between the low over southwest AK and the next
low pressure system that is currently entering the Western
Bering. Taking a look at this next low, we did increase wind
speeds with it to a weak gale force low as it enters the Bering.
However, it will lose strength as it progresses across the region.
It will be very similar to the low that just moved through, so
expect about the same weather. Forecast confidence is also high
due to the fact that models locked in on the last low and we have
a bit of rinse and repeat...so we know what is coming. After the
exit of this system Sunday, the Eastern Bering will return to
clearing and cooling northerly flow (on the backside of the low)
while the Western Bering will continue to see a chance of
precipitation.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...
Gulf of Alaska: A weak low over the Gulf on Monday morning will
intensify and shift to the northern Gulf by Tuesday as cold air
moves into the western Gulf. Northwesterly gales will develop
through the mountain gaps, specifically through the Kamishak Gap
and the Barren Islands, and the southern end of Kodiak Island
starting Monday night, and persist in those same areas through
Wednesday. By Wednesday the winds will be more northerly.

Bering Sea/Aleutians: Northerly gales will be possible between
the Aleutian Islands from Dutch Harbor to Atka Monday morning. The
front causing the gales will weaken, though gusty north to
northwest winds will persist from Dutch Harbor east through
Tuesday. A front south of a low near Kamchatka may produce
southwesterly gales Monday evening across the western Aleutians.
The front weakens overnight, but gales will be possible on
northwesterly winds over the open waters of the western Bering
through the day Tuesday. A much stronger front will produce gales
with storm force gusts over all of the western Bering and
Aleutians Wednesday afternoon and evening. The widespread gales
will spread into the central Bering Wednesday night.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...
A highly amplified pattern will be set up across Southern Alaska
on Tuesday morning, with a low over the northeastern Gulf near
Yakutat. A high will be over the eastern Bering centered near the
Pribilofs, and a low will be moving into the western Aleutians.
This pattern will favor continued cold air, strong gusty
northwesterly winds, and a drier pattern across Southern Alaska on
Tuesday morning. The northeastern Gulf low will persist through
Wednesday, keeping conditions unsettled along the north Gulf
coast. The high over the Bering moves into mainland Alaska on
Wednesday. Thus, the winds will gradually dissipate from west to
east through Thursday.

A strong front will bring another round of warm air, southerly
winds and rain to the western Aleutians on Wednesday afternoon.
While the southern end of the front will move slowly eastward
across the central Aleutians on Thursday, the northern side will
move east much more quickly. Snow, potentially mixed with rain
near the coast, will move into the Kuskokwim Delta Thursday
afternoon, and overspread the rest of Southwest Alaska Thursday
night. A large point of uncertainty for Friday will be how much of
that moisture makes it over the Alaska Range and into
Southcentral. Both the GFS and EC do show some snow moving into
the Cook Inlet region, including Anchorage, during the day, and
lightly persisting into Friday night as the upper level front
continues eastward.

Temperature-wise, cold, well below normal temperatures will
persist through Thursday across Southcentral while things warm up
to around freezing across Southwest Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory (Snow) 111 145.
MARINE...Gale Warning 178.
 Heavy Freezing Spray 127 139 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...JW