Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 241505
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
605 AM AKST Sat Feb 24 2018

.ANALYSIS AND UPPER LEVELS...
The active weather pattern continues as the next snow event has
blasted through Southwest AK and is now moving into Southcentral.
This is all being supported by a semi-meridional upper level
(30,000`) jet at a 130 kt. This strong jet is helping to mix down
some gusty westerly winds behind the snow over SW AK. It is also
ushering in some cold air and heavier showers across the Bering,
especially near the Pribilofs. Over Southcentral, snow has been
falling for a good part of the night from Talkeetna over to
Cordova. However, a moderately strong triple-point low is now
taking shape over the Northern Gulf of Alaska. The net effect of
this is pulling down some northerly flow from the interior
portion of the state at the surface. This is allowing some drier
air to slide underneath the snow and slow accumulation rates thus
far. Radar was more impressive earlier in the night, but light
snow will still continue to fall through the morning.

Over the Western Aleutians and Western Bering, the next system is
already approaching. Another gale-force front will slide into the
area from the North Pacific later today. And then this will become
the next weather-maker for the mainland late Sunday into Monday.

&&

.MODEL DISCUSSION...
Models remain in very good agreement through the weekend. This
results in moderate to high forecast confidence in the broader
synoptic features. Areas of uncertainty with this package are
exact snow amounts across Southcentral and how much low-level
moisture (fog/stratus) will redevelop overnight across the
southern mainland. Generally higher-resolution models were used
for fine tuning the forecasts this morning.

&&

.AVIATION...
PANC...Conditions are holding tight in the IFR range this morning
with light snow. This will continue through about 21z when we
expect the snow to taper off and cig/vis to improve. It is
possible that we could see conditions go VFR for a few hours this
afternoon, but then we expect stratus to redevelop late this
evening. Forecast confidence in the stratus is low to medium at
this point as offshore flow could bring enough drying to keep it
from forming.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Widespread snow will continue this morning over much of
Southcentral as an upper low moves overhead before beginning to
cut off over the Gulf by this afternoon. Accumulations are now
expected to be slightly lower after seeing the 12z Anchorage
sounding this morning indicated only very weak instability in the
snow growth zone (-5 to -20C), causing snow ratios to be much less
than initially expected. Most areas of the Anchorage Bowl and
Matanuska Valley should see an additional few inches from this
event, however this is about half of what was first thought for
these areas. The Gulf low will then move into the Eastern Gulf by
this afternoon, which should allow snowfall to gradually diminish
from west-east as gusty westerly winds develop in the Gulf. The
strongest of these winds are expected from the Barren Islands
southward under the upper level jet where widespread storm to
hurricane force gusts are expected to develop by this afternoon.
These winds will then diminish throughout the area by Sunday
morning as the Gulf low quickly makes landfall over the Southern
Panhandle ahead of another shortwave ridge.

The very active pattern in the Gulf and South Mainland will then
continue into early next week as yet another system quickly moves
into the Gulf for late Sunday-Monday. This low will take more of a
northeasterly track into the Gulf ahead of arctic trough digs
through the Mainland. Snow amounts from this system will be
heavily dependent on how far east the low tracks as it makes
landfall over the North Gulf Coast early Monday, but there is
increasingly good agreement that this system will bring another
round of widespread snowfall to Southcentral and the North Gulf
Coast, which could be heavy at times in Anchorage and the
Matanuska Valley for the Monday morning commute.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal system is currently exiting SW Alaska this morning which
will bring an end to widespread snow seen across the area
overnight. Cold air wrapping around the base of a trough over the
northern Bering will help to support scattered snow showers across
the Kuskokwim Delta and River Valley today, though accumulations
are expected to be minimal for most locations. Strong westerly
winds will also continue to ramp up today, with gusts expected to
reach up to 40-50 mph along the coast from Nunivak Island south to
Bristol Bay. The long fetch westerly winds will also lead to rough
surf along the coastline during the day.

A brief period of high pressure will build in tonight before the
next stronger system moves in rapidly from the west. A front will
bring another round of snow to the area beginning Sunday
afternoon tapering off during the day on Monday, with total
accumulations expected to be between 3-5 inches. A bigger concern
with this system will be another and more intense round of long
fetch westerly winds aimed at the southwest coast Sunday night
through Monday, which could potentially lead to storm surge issues
for low lying areas along the coast including Kuskokwim Bay and
the northern Bristol Bay coast. The current timing of the storm
brings the peak surge to the coast at low tide Monday morning,
which would only produce a moderate surge of 2-4 ft above the high
astronomical tide line (HAT). If the system slows down by about 6
hours, the peak surge would coincide with high tide Monday
afternoon bringing surge levels possibly up to 5-6 ft above the
HAT line, causing some serious coastal erosion and flooding
issues. Stay tuned for more updates over the weekend as the timing
and impacts of this system are locked down.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A surface low in the northern Bering is leading to a tightened
pressure gradient and widespread westerly gale force winds across
most of the Bering. Cold air advection will lead to scattered snow
showers today over the eastern Aleutians and AKPEN before the
next system comes barreling into the western Bering/Aleutians
tonight. A surface low with associated frontal system will sweep
across the region tonight and Sunday bringing an area of storm
force winds across the southern Bering and Aleutians. This will
cause a rapid rise in sea states with waves building up to 35 ft
Sunday night and Monday for the southern Bering between the
Pribilofs and Aleutian Islands.


&&

.LONG TERM FORECAST (Monday through Friday)...The active pattern
remains on tap through next week.

For the southern mainland and Kodiak Island we`re still on track
for a low to traverse the mainland beginning Sunday night
supported by a large upper trough. This system remains
sufficiently cold to keep all precipitation of the frozen variety
except for Kodiak Island where rain should briefly mix in. A
triple point low forms along the North Gulf Coast Monday then
dives southeast Tuesday and Wednesday. A residual low remains in
the Prince William Sound area through Monday night. Coupled with
the supporting upper trough, this could result in the snowfall
staying over south central Alaska through Monday night. The low
and associated trough that extends to the northwest will result in
a tight gradient and thus, gusty strong outflow wind out of the
Chugach Mountains ringing Prince William Sound Wednesday and
Thursday. The gradient relaxes Friday and conditions should settle
down over the southern mainland. Previously we were expecting
a shot of warmer advection into southwest Alaska Thursday night
and Friday. The latest model solutions are backing away from the
warm advection intruding into southwest Alaska at this point.

Over the Bering Sea, Aleutians and AKPEN we start the week with a
large strong low over the eastern Bering Sea that barrels into
the mainland Sunday night. Cold air wraps around the west quadrant
of this low coupled with the tighter pressure gradient, should
result in strong gusty west and northwest wind over the eastern
Bering Sea and AKPEN Monday. A weak flat ridge in the wake of
this low will rapidly yield to the next incoming storm entering
the western Bering Sea late Monday. This low rapidly traverses the
Bering Sea before dissipating as it approaches the Bristol Bay
region Tuesday night. A large low is then on tap to push into the
western Bering Sea Wednesday. Yesterday it appeared this low would
push a front to the Bering Sea Coast and strong warm advection
into southwest Alaska. Todays model runs have backed off on that
solution opting for a more westerly track that will have the front
stall then dissipate over the eastern Bering Sea Friday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 130, 131, 136, 137, 173, 175, 177, 178, 351, 352,
413.
Gales 120, 132, 138, 139, 141, 150, 155, 160, 165, 170, 172,
174, 176, 179, 180, 185, 411, 412, 414.
Heavy Freezing Spray 130.


&&

$$

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...SA


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