Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 221346 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
546 AM AKDT Mon Apr 22 2019

.ANALYSIS AND UPPER LEVELS...

A persistent mid-level low remains anchored near King Salmon, with
broad cyclonic flow encompassing much of the Bering Sea,
southwest Alaska, and the Aleutians. A weak surface reflection
(likely an MCV) has recently developed southeast of Dillingham,
which is easily seen in the Doppler Radar Imagery. Further east, a
deepening area of low pressure in the eastern Gulf continues to
lift north towards Yakutat, with an inverted trough extending to
the northwest towards Prince William Sound and into the Turnagain
Arm and Cook inlet.

A convergence band has set up in this area which is nicely seen
in the Kenai reflectivity and velocity data. This feature may be
getting a boost from an MCV (Mesoscale Convective Vortex - or
essentially a locally induced area of low pressure that can be
initiated by orographic effects and/or thermodynamics), which was
located to the southeast of Seward.

As a side note, this mornings upper air sounding out of Anchorage
showed some instability in the sounding, with fairly steep lapse
rates (the rate of cooling as an air parcel ascends, which is
often a sign of potential instability). Similarly steep lapse
rates were also observed in the sounding out of King Salmon.

&&

.MODEL DISCUSSION...

The numerical guidance is in great agreement at the surface and
aloft on the overall synoptic fields, leading to a higher than
average confidence in the forecast. The challenge has been more on
the mesoscale, as the NAM/NAMNEST have waffled on if precipitation
makes it into the Anchorage metro, and how far west into the
Susitna Valley it makes. Meanwhile, the GFS, Canadian Regional,
and to some extent the ARW, have been much more consistent showing
the deformation precipitation shield reaching the Cook Inlet north
of Kenai and into much of the Susitna Valley. Based on the rapidly
cooling cloud tops, increased convergence signatures in the
surface observations and radar imagery, we prefer the latter short
term solutions.

&&

.AVIATION...
PANC...Based on pattern recognition, we inserted MVFR ceilings
and visibilities for a few hours. The key will be how far west an
area of rain and snow makes it (the guidance disagrees with how
far west into the area it`ll progress). If it stays east, we`ll
see VFR conditions prevail. Additionally, southerly winds gusting
near 20 knots will also develop this evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Mon and
Tue)...

A very unusual synoptic pattern is set to unfold today. A
vigorous low is taking shape over SE AK and will track towards
Yakutat through the day. Meanwhile, a cold upper level trough is
digging further south over Southwest AK. In between these two
systems lies a challenging winter forecast. The moisture from the
SE low has slid back to the west through the night. Kenai and
Middleton radars are picking up on this early this morning as the
band fans to the west. The cold air advection and instability in
southwest flow is also readily apparent with the cumuliform
nature of the clouds/showers over the Kenai Peninsula and the 12z
Anchorage upper air sounding. So in effect, the entire system will
act like a large cold front sweeping from south to north across
the entire area today.

The sharp upper level trough axis is the key feature in all of
this. It will pivot through the day from a NW to SE alignment to
more of a W to E. As it pushes through later, it will lift of all
the moisture from the SE system. The net effect will be widespread
snow stretching across the Susitna Valley through the Copper River
Basin. Moderate S to SW flow will enhance snowfall along the
terrain of Hatcher Pass and the AK Range making for locally higher
accumulations. The trough will pivot right over top of Anchorage.
So the best chance for any accumulating snow seems to be along the
Hillside, Eagle River, and up through the Palmer area. A Special
Weather Statement has been issued to address some of the potential
impacts.

Weather will remain unsettled heading into Tue. The cold upper
level low will sit right over top of the area making for the
threat of snow showers for just about all locations. However, we
are not expecting any consolidated feature to bring widespread
persistent snow over the mainland. Models are now starting to
hint at a triple point low developing over the Northern Gulf. This
could bring a period of moderate snow to the Eastern Gulf
Coast/Cordova 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...Blizzard Warning 155.
MARINE...Gale Warning 155 170 171 172 179 180 181 185
         Heavy Freezing Spray 180 181 185
FIRE WEATHER...NONE.

&&

$$

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


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