Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220021
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKDT Sun Apr 21 2019

.ANALYSIS AND UPPER LEVELS...
A low that has been moving across the Gulf has now tracked over
the Kenai Peninsula as it pushes off to the northwest. The front
associated with this low is bringing precipitation to areas all
along the north Gulf coast and western portions of the Alaska
Range. Areas near the water are seeing primarily rain while higher
elevations are seeing mostly snow. Cold air advection is
following behind the low which has helped bring snow to areas
around Kachemak Bay. Out west, cold northerly flow dominates the
central and eastern Bering while ridging is building in over the
western Aleutians.

&&

.MODEL DISCUSSION...
The models are in decent agreement with the main synoptic
features into the mid term. This includes locking in on the
position of a low moving into the eastern Gulf tonight, which has
been a challenge for the models to get a handle on the last
several model runs. Even though guidance is doing better handling
the synoptic features there are still some struggles with a
deformation zone over southwest Alaska this evening into Monday,
in particular with snow amounts and wind over the area.

&&

.AVIATION...
PANC...There are two main challenges for this forecast with the
first being the onset and duration of winds this afternoon. At
this point it looks more likely that Turnagain Arm winds will not
bend over the airport, but rather southerly winds from a low
moving through the area will move in as the Turnagain Arm winds
diminish. However, if the Turnagain Arm winds did move in then
southeast winds would come in and be stronger than the forecast.
The other challenge is with snow Monday afternoon and there is
still fairly high confidence that it should not drop conditions
below VFR, but it is possible.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong low is moving inland around Cook Inlet, with the
associated low end storm force front already clearing the North
Gulf Coast. The strongest winds are now in PWS and will diminish
rapidly as the low moves inland. Southwest cold air advection will
impact Kachemak Bay through this evening before weakening. Cold
air advection will move up Cook Inlet, but with the modified
marine air, temps will not feel the effect of the advection until
Monday night. A second low moving inland around Yakutat will
spread more moisture northwest tonight through Monday, bringing
more rain and snow to the coast. Moisture will spread inland
through the Copper Basin and northern Susitna Valley, with chances
of snow dropping into Anchorage and the Mat Valley as well Monday
and Monday evening. However, accumulations will be light in lower
elevations with the best accumulations over higher
elevations/mountains. Snow shower threats will continue into
Tuesday, favoring the mountains, as upper troughing remains over
the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Tonight
through Tuesday)...
A sharp upper level short-wave and associated surface low in the
vicinity of Cook Inlet this afternoon will track westward across
interior Southwest Alaska tonight. Snow will persist all night
from interior Bristol Bay to the Kuskokwim Valley and spread
westward into the Kuskokwim Delta. Meanwhile, cold air advection
across the eastern Bering Sea combined with tightening pressure
gradients ahead of the surface low will lead to strengthening
winds along the coast, particularly along the Kuskokwim Delta
coast and Nunivak Island. All of this cold air and wind will
produce a steady stream of showers over the open waters along the
coast, so bordering villages can expect snow and blowing snow at
times. As the steady snow reaches Bethel and villages along the
lower Kuskokwim River overnight, expect blowing snow and reduced
visibilities. However, the worst conditions will develop along the
coast Monday morning as steady snow arrives and combines with
winds gusting 35 to 50 mph. A blizzard warning has been issued for
the coast and Nunivak Island from 7am to 7pm Monday. Snow will
move in from north to south along the coast, so expect blizzard
conditions to develop near Hooper Bay and Chevak first thing in
the morning, then spread to Toksook Bay and Mekoryuk by around
midday. As is typical in these scenarios, winds will accelerate
through Etolin Strait, so also expect some locally higher winds in
Toksook Bay and vicinity.

The upper level wave and surface low will head back south across
the Eastern Bering Sea Monday night, bringing an end to steady
snow over Southwest Alaska. An upper level low will persist over
Bristol Bay, maintaining considerable cloudiness and a few snow
showers across the region through Tuesday. The chilly (below
normal) temperatures will persist.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Tonight
through Tuesday)...
The big story for the Eastern half of the Bering Sea, Pribilof
Islands, Eastern Aleutians, and southern Alaska Peninsula for the
next couple days will be strengthening winds and increasing snow
showers. A series of strong upper level short-waves will dive
south across the region along with low level Arctic air. Unlike
most of the showers of recent days, this will result in heavier
showers with much more vertical extent. The combination of wind
and more frequent snow showers will lead to blowing snow, with
visibilities one half mile or less in heavier showers/stronger
winds. While do not expect any prolonged periods of low
visibility, there is potential for steadier snow ahead of the
upper wave and surface low moving out of Southwest Alaska during
the day Tuesday. Will keep an eye on the track of this low and
associated steadier precipitation in case a blowing snow advisory
or blizzard warning are warranted.

&&

.MARINE (Days 3 through 5)...
On Wednesday low pressure over the Southwest and the Bering begins
a southeastward shift toward the Gulf. Two low pressure centers
look to merge into one. However, there is uncertainty on this
solution as they may just meander about each other while
continuing to move east. Either way...expect directional wind
changes with gusty conditions on Thursday across the Gulf. To the
west...much of the Bering stays under the influence of a building
ridge Wednesday and Thursday with a front moving to the western
waters late Thursday. Southerly Gales are expected along the
frontal boundary as it moves slowly toward the central waters on
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Wednesday, the low over the Southwest begins to move on a slow
southeastward track as a strong wave of vorticity dives down the
backside of the trough. This system continues to advect a colder
air mass across the Gulf and Southcentral. Meanwhile ridging
builds across the Aleutian Chain which pulls a subtropical air
mass across the Bering and to the Southwest coast toward the end
of the week. This pattern shift will moderate temperatures to
warmer trends as the tap of cold air is cut off. The main
challenge for the end of next week is on the timing and track of
the next low pressure system expected to push a front into the
western Bering mid week. The forecast blends for the end of the
week were once again heavily weighted toward the 00Z ECMWF/ensembles
to maintain consistency. There is low confidence in the extended
forecast as the 12Z ECMWF came into closer agreement with this
mornings GFS run.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 179>181 185.
 Heavy Freezing Spray 160 180 181 185.
FIRE WEATHER...NONE.

&&

$$

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



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