Gale Warning 311, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT Sat Apr 20 2019

.ANALYSIS AND UPPER LEVELS...
A trough that has been digging south through the eastern Bering
now stretches through Bristol Bay and across the Alaska Peninsula.
This trough is helping to pull down colder air from the north. To
the southeast a low is approaching the Gulf from the south
bringing in a strong push of warm air. Out west, high pressure has
built in over the western Aleutians.

&&

.MODEL DISCUSSION...
There are two main areas of model concern in the short term. The
first is inconsistency with a low moving across Kenai Peninsula
Sunday. As it tracks across the peninsula and then Cook Inlet the
models are struggling to handle its track resulting in challenges
with winds in the Southcentral Alaska area and snowfall for much
of the southern mainland. The other issue is with a low moving
into the eastern Gulf late Sunday. The models are continuing to
have significant problems with the placement of this system and
that will have an impact for winds across the Gulf and potential
snow for eastern portions of Southcentral Alaska.

&&

.AVIATION...
PANC...The main challenge is with the timing of a front moving
through the area tonight. There is reasonable agreement that it
will happen between 3-6Z tonight and will bring conditions down
to MVFR. MVFR conditions are still possible through tonight and
Sunday morning, but at this time VFR conditions are expected to be
most likely. Gusty southerly winds are expected to develop Sunday
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A low is strengthening over the southern Gulf of Alaska this
evening with warm air advection already spreading rain/snow along
the coast. Some snow is beginning to move over the Chugach
Mountains as well. As the evening progresses cross-barrier flow
will increase and largely end any snow chances for inland low
elevation areas with the exception being the Southern Kenai where
snow and blowing snow will continue into Sunday morning. The low
will eventually reach low end storm force along the N Gulf coast
with strong gales moving through Prince William Sound. Easterly
winds will also increase through Portage Valley/Turnagain
Arm/Anchorage Hillside and along the Chugach Mtns Sunday before
the low moves inland over Cook Inlet. A brief shot of gusty winds
will move through the Mat Valley and Anchorage with the passage of
the low; temps will likely spike up as that occurs Sunday
evening.

The forecast gets more complicated Sun night as another low
tracks across the eastern Gulf of AK and merges with the parent
low center over SW Alaska. Some moisture may move westward far
enough to support snow as far west as Anchorage. This will be
occurring during the cold air advection stage, but for now, this
forecast will keep the best threats along the mountains until
there is better consensus with the second low. The better
agreement is for snow to move into the Copper Basin and along the
gulf coast as rain transitions back to snow Sun night and Mon
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Tonight
through Monday)...
Winter is on the way back! Starting with tonight, a short-wave
trough over Bristol Bay and another short-wave headed toward
the Kuskokwim Delta will maintain considerable cloudiness across
Southwest Alaska. Expect isolated to scattered snow showers,
mainly along and near the coast. On Sunday, the Bristol Bay trough
will become negatively tilted, helping to pull a low over the
Gulf of Alaska (and associated moisture) westward into SW Alaska.
Snow will develop along and just west of the Alaska Range late
tonight, then spread westward across Interior Bristol Bay and the
Kuskokwim Valley during the day Sunday. Snow will then continue
westward to Bethel and the Kuskokwim Delta Sunday night through
Monday. The heaviest snowfall will be along a deformation band
which sets up across the Kuskokwim Valley. For the lowlands, the
highest snow accumulation is expected between Stoney River and
Aniak. The negatively tilted trough will come full circle,
rotating back westward across the Kuskokwim Valley Sunday night.
This will shift the best deformation (and heavier snow) north of
the Kuskokwim Mountains, which should keep snow amounts just below
winter weather advisory criteria.

Meanwhile, Arctic air will dive southward across the eastern
Bering Sea and into Southwest Alaska. This will lead to
strengthening winds and an increase in snow showers along the
coast. As steadier snow arrives along the Kuskokwim Delta coast
on Monday, there is potential for very low visibilities in snow
and blowing snow. There is still some uncertainty in where steady
snow will set up, so do not planning on issuing an advisory for
this quite yet.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Tonight
through Monday)...
A couple weak upper level short-waves dropping south across the
Bering Sea will focus showers along the eastern Aleutians and
southern Alaska Peninsula tonight through Sunday. Arctic air will
then dive south across the central to eastern Bering Sea Sunday
through Monday, along with a strong upper level short-wave. This
will result in strengthening winds region-wide and an increase in
snow showers and freezing spray. The lone exception to this will
be the western Aleutians (west of Adak), which will remain quiet
underneath the influence of an upper level ridge.

&&

.MARINE (Days 3 through 5)...
Tuesday and Wednesday across the Gulf and eastern half of the
Bering...A broad low pressure over the southern mainland keeps
gusty north to west flow through the middle of next week.
Thursday night into Friday a new surface low pushes toward the
western Bering bringing a front to the Pribilof Islands by Friday
afternoon. Gales may develop along the boundary of this system,
however there is still uncertainty on the timing on the track of
this system.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The forecast beginning on Tuesday continues with the most active
weather across the southern mainland and nearby waters as an upper
low centers near the Alaska Peninsula. Strong vorticity on the
backside of the trough swings around the base which helps to set
the upper low in motion with the it centering over the Gulf on
Thursday. The ridge over the western Bering kicks into eastward
motion with the next front advancing to the western Aleutians
Thursday morning. There is much higher uncertainty with how this
overall pattern evolves through the end of the week as models
greatly differ on solutions. Only conservative edits were made for
the extended forecast with the ECMWF and its ensembles favored
today as it maintains the best forecast consistency.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119.
 Gale Warning 125 131.
 Heavy Freezing Spray 180 181 185.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
202 AM AKDT Sun Apr 21 2019

AKZ152-221800-
Lower Kuskokwim Valley-
Including the cities of Aniak and Sleetmute
202 AM AKDT Sun Apr 21 2019

...SNOW AND BLOWING SNOW POSSIBLE IN THE LOWER KUSKOKWIM VALLEY
THIS AFTERNOON THROUGH MONDAY MORNING...

A strengthening low tracking into the Gulf today will move over
Cook Inlet by this afternoon and is expected to bring
accumulating snowfall to the Lower Kuskokwim Valley through
Monday morning. Approximately 2 to 5 inches are possible from Port
Alsworth to Aniak. As cold air advects southward ahead of the low
center gusty winds are also possible, potentially causing periods
of reduced visibility.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
204 AM AKDT Sun Apr 21 2019

AKZ155-230500-
Kuskokwim Delta-
Including the cities of Bethel, Hooper Bay, and Nunivak Island
204 AM AKDT Sun Apr 21 2019

...SNOW AND BLOWING SNOW POSSIBLE IN THE KUSKOKWIM DELTA MONDAY...

A low tracking northward over central Alaska is expected to bring
accumulating snowfall to the Kuskokwim Delta on Monday. A few
inches are possible in the Bethel area with localized higher
amounts possible along the coast. As cold air wraps in from the
north, gusty winds will develop leading to possible reduced
visibility at times. The heaviest snow is expected across
interior regions early Monday morning through midday, with coastal
areas seeing the heaviest snow in the afternoon. Winds will
gradually diminish Monday night into Tuesday.