Gale Warning 585, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
531 PM AKST Mon Nov 18 2019

.ANALYSIS AND UPPER LEVELS...

A weak trough that brought some light snow to portions of the
Anchorage Bowl and Matanuska Valley overnight into this morning,
has now pushed into the Copper Basin. Further west, a 963 mb low
was positioned south of Adak, with its associated warm front
having lifted into south Bering Sea east of there. Multiple vort
maxes are seen in the model and satellite data, rotating around
the low while also diving into the backside of the trough.

&&

.MODEL DISCUSSION...

In the short term, the numerical guidance has come in to much
better agreement with the overall positioning of the mid level
lows and their surface reflections, for the storm moving into the
Bristol Bay region. However, some differences still exist with
how quickly a push of tropical-influenced air pushes into the
southwest, and Southcentral. This is largely in part to the models
being historically too fast with dislodging colder air
entrenched across a region, aided by a snowpack. Further
complicating this is all models show several pieces of energy
diving into the parent trough, inducing multiple areas of
cyclogenesis (low pressure development).

These additional lows eventually get absorbed into the parent
system, as they rotated around each other in a Fujiwhara-like
fashion. Each impulse brings additional bursts of ascent and warm
advection. The tricky part is figuring out which low will bring
the brunt of the warmer air aloft inland, while helping it mix
down to the surface. Here lies the biggest disagreement with when
does this occur to allow some location to warm sufficiently for
rain to fall.

Otherwise, the overall synoptic continuity in the models remains
fairly good through the weekend, before some amplitude and timing
difference begin to develop.

&&

.AVIATION...

PANC...Light winds and VFR conditions will prevail through 12z.
Thereafter, as a front approaches the Gulf Coast, light snow will
develop and then quickly change to light freezing rain around 16z.
As the front draws closer, enough warm air will develop for a
change to light rain expected, accompanied by LLWS developing
after 23z.

&&

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

The next front moves to Kodiak Island this evening with a quick
track to the north coast by Tuesday morning. Gusty gale southeasterly
winds and rain will accompany this warm core system. The surface
front stalls along the coast as the associated surface low tracks
over the eastern Bering. Light precipitation is expected to
develop across much of the southern mainland on Tuesday as upper
level waves begin to pull the warm maritime air mass inland. The
cold air mass at the surface will be relatively shallow across the
Kenai, Anchorage, and southern Susitna Valley areas through
Tuesday afternoon, so precipitation are expected to be a wintry
mix. Elsewhere...snow will be the prevailing precipitation type.
Issued winter storm warnings for 141 and 145 for heavy snow with
the afternoon package.

Tuesday night, temperatures hold nearly steady through the
overnight hours as a warmer column of air advects inland. The
biggest challenge with this warmer air mass is how quickly it
mixes down to the surface and how far north it advances.
Generally...conditions will stay on the warmer side through
Wednesday afternoon and then begin to cool Wednesday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A deep low centered south of the central Aleutians will steer a
secondary low across the AKPEN northward towards Nunivak Island
late tonight into Tuesday. With northeast flow and cold
antecedent conditions at the surface over the Southwest Mainland,
as the warm front associated with the low passes over, expect
freezing rain in the Bristol Bay area. Surface conditions should
warm fairly quickly and transition precipitation to rain;
however, the heaviest precipitation is expected just behind the
warm front. So, with the expected precipitation type of freezing
rain during the period of heaviest precipitation, we felt an Ice
Storm Warning was warranted. Up to four tenths of an inch of ice
accumulation is expected in the Bristol Bay area, from Dillingham
northward.

Farther north, freezing rain is less of a concern...but heavy
snow at times will likely cause travel hazards. In particular, a
Blizzard Warning remains in effect for the Kuskokwim Delta/Coast
Tuesday morning to early afternoon. Expect 4 to 12 inches of
snow, with winds gusting 40 to 60 mph bringing visibility down to
a quarter mile or less at times. In the lower Kuskokwim Valley,
winds won't be quite strong enough to bring visibilities down to
blizzard conditions, thus the Winter Weather Advisory was
continued.

As the low continues to slowly push northward, broad south flow
aloft will persist. Temperatures over the Southwest Mainland will
have a warming trend of 10 to 20 degrees Tuesday afternoon over
this afternoon. However, with many areas still below freezing,
there will be concern for freezing rain in the Kuskokwim
Valley/Delta Tuesday evening under a shallow warm layer aloft.
Confidence in the warm layer isn't great, and this will be behind
the bulk of precipitation. So, at the moment the main concern is
snow during the day Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A deep low south of the central Aleutians and a secondary low
north of the central Aleutians will continue areas of gales in the
Bering and Aleutians tonight into tomorrow, including Bristol Bay.
With strong offshore flow and cold conditions inland, expect areas
of freezing spray along the Kuskokwim Delta Coast and north of
Nunivak Island. Another low will track south of the Aleutians
Wednesday and bring areas of gales with widespread small craft
advisory level winds/seas.

&&

.MARINE (Days 3 through 5)...

Beginning the period early Thursday morning, a weakening storm
force low in the eastern Bering will still be responsible for
gales in the Northern Bering as well as Coastal waters of the
Alaska Peninsula. There will be isolated areas of storm force
winds as a strengthening frontal wave tracks into the
southwestern Gulf early Thursday morning with widespread gales
across the Gulf of Alaska and coastal waters of southern Alaska.
Expecting storm force winds to impact Prince William Sound and
offshore waters of the southwestern Gulf Thursday afternoon
through Friday morning. Friday evening expect northerly gales
from the Bering Strait into the southern Bering as cold air pushes
south from Siberia. An area of gales will move across the
Southern Gulf through Saturday evening while high pressure builds
into the Central Bering. The next storm appears to be developing
off the Kamchatka Peninsula Saturday evening and looks to be on
track to arrive in the western Aleutians sometime Sunday morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...

By Thursday, a deep upper level trough containing numerous
smaller troughs and surface lows will stretch from the Arctic
Ocean, south along to west coast of Mainland Alaska, across the
Bering and Aleutians. A very strong west southwest to east
northeasterly oriented jet stream will stretch from south of the
Aleutians into the southern Gulf will provide good upper level
support for developing lows along its north side.

A vertically stacked low centered over Bristol Bay Thursday
morning will swing inland Thursday afternoon and evening with the
portion of the associated frontal system over Southwest Alaska
lifting north. The portion of the front stretching across the
Kenai Peninsula and into the western Gulf will swing northeast to
reach the northern Gulf coast by Thursday afternoon and evening
while a second frontal wave develops into a closed surface low
east of Kodiak Island. Another frontal system stretching from a
surface low tracking to the east through the central Aleutians to
a second low south of the eastern Aleutians will continue east to
merge into a single low near the eastern Aleutians Thursday
evening with an upper level trough and frontal system stretching
east then south of the western Alaska Peninsula.

The very active storm track will continue Friday through the
weekend. The surface low developing off Kodiak Island Thursday
afternoon will continue to strengthen as it tracks northeast just
offshore of the Kenai Peninsula coast Thursday night to reach
Prince William Sound Friday morning. The next incoming frontal
system will track to the east just south of the Alaska Peninsula
late Thursday night through Friday morning then move quickly
northeast across the Gulf through the day Friday. Another strong
frontal wave slightly further to the south will follow Friday
through Saturday. Colder upper level troughing will dig in over
the eastern Bering and Bristol Bay area Friday and Saturday.

Upper level ridging building over the western Bering and Aleutians
on Saturday will shift over the eastern Bering on Sunday as a
strong frontal system pushes into the western Aleutians. The
ridge will push east to reach the eastern Bering on Monday with
the front swinging into central Bering. The front will reach the
central Aleutians Monday morning and eastern Aleutians Monday
afternoon and evening. The cold upper level trough will become
well establish over Mainland Alaska Sunday through Monday with the
Bering frontal system slowing Monday night as it approaches the
Southwest Alaska coast then shearing apart with the bulk of the
remaining energy sliding south across the southern Gulf in the
form of another developing low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 101 121 141 145 152 155 161.
MARINE...Storms 412 181.
         Gales 119 120 125 130-132 136 138 139 150 155 160 165
               176-180 185 411 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...LB/JR