Gale Warning 627, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A flattening trough over Southcentral is lifting northeast out of
the area tonight, as a weak jet streak on the back side of the
trough helps support the flattening of the trough. Until the
trough axis crosses through the area, however, clouds and light
snow over portions of the Mat-Su, Prince William Sound and the
Copper River Basin continue this morning. Cold advection over Cook
Inlet has largely helped to clear skies over the southern Susitna
Valley and western Kenai Peninsula. "Lake-effect" clouds persist
over the southern Cook Inlet and along the axis of the Kenai
Mountains. Temperatures are in the teens across the Copper River
Basin, 20s for the Mat-Su, and low 30s for Anchorage, the Kenai
Peninsula and Prince William Sound areas.

Temperatures are much colder this morning across Southwest Alaska.
A few below zero readings are being reported in Kalskag and Aniak.
Single digits are the norm all the way to the coast of the
Kuskokwim Delta, Lower Kuskokwim Valley, and portions of interior
Bristol Bay. Teens are common over coastal Bristol Bay, with 20s
down along the northern Alaska Peninsula.

Across the Bering, a col persists over the northern Bering, with
a cutoff upper level low and trough over the western Bering. A
strong southwest-to-northeast oriented jet streak is approaching
the central Aleutians, and will be the building mechanism for a
strong ridge over Southcentral in a couple days.

&&

.MODEL DISCUSSION...

The model agreement over the next couple days has significantly
deteriorated compared with the forecasts from previous days. This
is largely to do with the uncertainty with a low that hasn't even
formed yet, which will move into Bristol Bay and will impact much
of Southern Alaska with precipitation of all different varieties
from tonight through Tuesday. The NAM is a big outlier with this
model cycle, keeping the low further east, moving the center
onshore into Southwest Alaska late Tuesday night, far earlier than
the other models which do the same Wednesday afternoon. This
difference would have tremendous impact on the forecast, not only
in Southwest Alaska, but regarding the moisture feed from the Gulf
into Southcentral as well. While the other models agree with each
other better, there remain strength and timing differences even
among them. However, more confidence exists now regarding the
precipitation type, timing, and amounts across Southwest Alaska.
Forecast confidence remains low, but with this next large storm
system due in late tonight through Tuesday, enough confidence was
there to update and issue a new watch, upgrade to a warning, and
issue advisories as well. More on those below.

&&

.AVIATION...
PANC...VFR conditions will persist as winds diminish out of the
south this morning and flip out of the north this afternoon. On
Tuesday morning, the leading edge of the next round of
precipitation may begin during the TAF period. It should begin as
snow, but transition over to rain through the day, which will
cause lowered ceilings and visibilities.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Tuesday night)...

An upper level short-wave trough will slowly exit across the
Copper River Basin today, with showers lingering along and near
the mountains. With an arctic air mass now over Southcentral,
any remaining precipitation will be all snow except right along
the Gulf coast (Cordova area). Meanwhile, a dissipating front will
approach Kodiak Island, bringing some light rain primarily to the
southern half of the island.

Attention will then shift to an amplifying trough over the north
Pacific. A strong southerly jet will develop today and drive a
frontal system northward across the southern mainland tonight
through Tuesday. A surface low will also develop and deepen as
it tracks into the eastern Bering Sea on Tuesday. There is
uncertainty in the track and strength of this low, which
introduces uncertainty into the wind and temperature forecasts
over Southcentral. The further north and east tracks would result
in southeast winds spreading inland sometime Tuesday afternoon
(especially for Anchorage, the Matanuska Valley and the southern
Susitna Valley. There would be a jump in temperatures as high as
the lower to mid 40s and precipitation would fall as rain. On the
other hand, a farther south and west track would keep the bulk of
these winds confined to the Gulf coast as well as Turnagain Arm.
Of course, warming temperatures aloft could then lead to freezing
rain. For now, the forecast represents a middle ground, with some
wind and a changeover to plain rain for the aforementioned areas
as temperatures climb above freezing Tuesday afternoon.

What can be said with higher confidence is that this will be a
fast moving system with heavy precipitation. With strong south to
southeast low level flow, precipitation will be enhanced across
Prince William Sound as well as the Susitna Valley. Though the
southern Susitna Valley may warm Tuesday afternoon, expect the
northern to western valley to hold on to remain cold enough for
all snow. Thus, have issued a Winter Storm Watch for heavy snow.
The limiting factor will be the speed of the front, though
whatever snow does fall will be heavy and will cause impacts
along the Parks Highway from about Talkeetna northward. Strong
low level southeast flow will limit, but not prevent precipitation
for the western Kenai, Anchorage, and Matanuska Valley on
Tuesday. Expect periods of light precipitation as early as Monday
night and continuing through early to mid afternoon Tuesday. The
Copper River Basin will likely see some downsloping, though some
precipitation may make it over the Chugach Mountains into the
southeastern portion of the basin. There is good potential for
southeast winds to develop along the Copper River, in which case
temperatures will likely jump above freezing and result in rain.
This will continue to be evaluated this as we get closer in time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Monday
through Wednesday)...

Weather will start out quiet today for southwest Alaska. This will
quickly change tonight into tomorrow when a low pressure system
rapidly forms over the eastern Aleutians and moves quickly north
into Bristol Bay. This system will be smaller in size, but a
potent one. The system will move quickly north, sending warm air
rapidly into southwest Alaska. With cold air in place over the
region now it will create a myriad of winter weather for
southwest AK.

The Y-K Delta looks to get the worst of the weather with this
system. The low moving north will induce strong, cold
northeasterly winds with gusts up to 60 mph. At the same time, a
large push of moisture will arrive over the area creating heavy
snow. This will make for blizzard conditions tomorrow morning due
to the heavy snow and high winds causing blowing snow. A Blizzard
Warning has been issued for the Kuskokwim Delta zone as a result.

For Dillingham and the northern Bristol Bay area up the Nushagak
River Valley, expect snow starting later tonight. However, the
strong warm air push with this system will change the snow to
freezing rain for a few hours in the early morning, before it
switches to entirely rain. A winter weather advisory was issued
for the Bristol Bay zone to reflect this.

Lastly, interior areas of southwest AK will see the arrival of
snow as well tomorrow. They will stay colder longer with the warm
air taking a little while to reach them. Therefore, it will stay
snow all day for the interior. A switch to freezing precipitation
will occur in the evening hours when the warm air arrives. A
winter weather advisory for snow has been issued for the Lower
Kuskokwim Valley.

Please see the weather hazard statements for further details on
the above. Stay tuned for updates over the next couple of days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Monday through Wednesday)...

A very elongated frontal system is stretched out from NW to SE
across the Bering Sea. Behind it, a mature low pressure is moving
into the western Aleutians and is well defined on satellite. It
will be this system that sets up the broad area of low pressure
that will bring storm and gale force winds to much of the Bering
and Aleutians for the next couple of days. Low level cloud cover,
rain and patchy fog can also be expected over the next couple of
days.

Early tomorrow morning, a small but powerful low will quickly form
within this broad low over the eastern Aleutians. Models are
consistent in this bringing storm force winds into Bristol Bay,
as well as the southwest of the Alaska Peninsula. So storms have
been added into the forecast tonight for Bristol Bay and with an
expanded area of storms on the southern side of eastern
Aleutians. This low will be a fast mover and rotate quickly north
along the Bering Sea coast. It will also lose strength quickly,
with winds dropping to small craft advisory level by Wednesday.

&&

.MARINE (Days 3 through 5)...

Beginning the extended period marine forecast on Wednesday, there
will be a weakening gale force low moving inland across Southwest
Alaska as it exits the Bering Sea, with the associated near gale
force front moving across the Gulf of Alaska. Attention turns to a
second low which will move into the Aleutians Wednesday night and
Thursday. There still is lower confidence with the strength and
track of this low, but the likelihood is increasing that this low
will reach at least gale force, and possibly low end storm force,
as it rapidly tracks east into the eastern Bering Sea, and then
the coast of Southwest Alaska by Thursday. Expect the highest
winds and seas across Bristol Bay and along/south of the eastern
Aleutians and the Alaska Peninsula. The associated gale force
front will move into the Gulf of Alaska Thursday with southwest
flow lasting over the Gulf through Friday. Expect a period of
quieter weather heading into the weekend as the storm track
briefly subsides over the Bering Sea.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended forecast period Wednesday evening, The
North Pacific storm track will be unleashing its fury across
Southern Alaska as another strong low and warm front will be
plowing through the Southern Mainland. Expect warmer than average
temps to be in place along with inland rain/snow and coastal rain,
although the heaviest rain will be shifting east into the Yukon.
Attention very quickly turns to the very next storm which will
quickly amplify along the NPAC jet stream and move across the
Aleutian Chain into Southern Alaska. While uncertainty still
exists, this low will likely bottom out sub-960 hpa, and will make
landfall across Southwest Alaska Thursday morning, then quickly
move into Southcentral later in the afternoon. This low looks
rather warm, but given the challenges of local microclimates,
expect another rainbow (myriad) of precipitation types ranging
from rain, to rain/snow mix, to possible freezing precipitation
for inland areas. However, most inland locales near the coast will
be rather warm, so most areas will likely just be rain during the
onset.

The upper level trough gradually moves east Friday and Saturday,
so temperatures will gradually cool aloft again. Showers and
clouds will still be in place, so a gradual changeover back to
snow or rain/snow showers will occur. Thereafter, a lot of
uncertainty develops, but the latest ECMWF/GFS are depicting the
chance for subtle basal shortwave troughs to undercut the larger
trough Sat/Sun, which would be a favorable pattern for a return to
accumulating snow threats across parts of the Southern Mainland,
but at this juncture, that is effectively fairy-tale land in the
world of atmospheric modeling.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
Winter Storm Watch: 145.
Winter Weather Advisory: 152, 161.

MARINE...Storm Warning: 150-165, 412.
Gale Warning: 119, 120, 125, 130-136, 138-140, 172, 176-185, 351,
			  352, 411, 413, 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...JA