Flood in Alaska, September 26 to 30, 2020

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
1054 PM AKDT Fri Sep 25 2020

.ANALYSIS AND UPPER LEVELS...
A broad upper trough extends from eastern Russia, over the Bering
Sea, to over the southern Gulf of Alaska. There is also weaker
troughing across the the southern Mainland. A fairly defined
easterly zonal jet extended across the North Pacific south of
50N.

A series of weak lows were evident at the surface as well as over
the Bering Sea. As expected, generally mostly cloudy skies with
areas of showers prevailed across the Mainland and Bering Sea in
conjunction with upper troughing. Some low stratus and fog formed
across Southcentral during the morning hours as breaks in clouds
allowed for spotty cooling and fog formation.

&&

.MODEL DISCUSSION...
Models are in fairly good agreement with the storm/hurricane force
low moving into the Gulf over the weekend so confidence is high
with this system.

&&

.AVIATION...
PANC...Generally VFR conditions with a few passing showers are
expected through today. MVFR ceiling are expected overnight
tonight. Patchy fog may develop during Saturday morning and bring
brief periods of IFR conditions. Light winds should prevail.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Monday)...

A broad upper-level trough keeps much of the Southcentral under
relatively mild conditions through tomorrow. Weak waves embedded
in the upper flow combined with weak instability will continue to
produce largely unorganized areas of showers across the region
tonight, with widespread clouds and patchy fog. As we head into
Saturday, a trough lifting northward across the Gulf will pull the
remnant low over the Gulf westward toward Cook Inlet. This will
provide a more focused area of rain from the north end of Kodiak
Island to the Kenai Peninsula. In addition, expect a brief period
of gusty winds through Passage Canal and Turnagain Arm Saturday
afternoon in the wake of the low passing to the south.

By late Saturday night a strong North Pacific low enters the
Southern Gulf. Widespread gales and storm force winds accompany
the low, with the strongest winds out ahead of the leading front.
Also, expect the formation of a barrier jet, with a brief period
of hurricane force winds along the north Gulf Coast Sunday
morning. Peak strength will be reached as the low enters the
southern Gulf Sunday morning, becoming occluded shortly after and
beginning to weaken as it travels towards the northern Gulf. The
front will rapidly weaken as it reaches the north Gulf coast and
Prince William Sound Sunday afternoon and evening, with nothing to
drive the front onshore.

Based on the low track, the prevailing low level flow over
Southcentral will be northeasterly, which is generally not a
great direction for heavy rain along the coast. This flow will
favor the mountains of western Prince William Sound and the
northeast end of Kodiak Island for heavy rain Saturday night
through Sunday. Rainfall totals will be quite a bit lighter as you
head inland across the Kenai Peninsula as well as for the
Valdez/Cordova areas.

The low will head inland in the vicinity of Cordova Sunday night
and dissipate, with areas of rain spreading into the Copper River
Basin. Cooler, more unstable air moving in behind the low combined
with strong pressure rises will lead to gusty southerly winds
across the Gulf waters. This southerly flow will also keep rain
and or showers going along the Gulf Coast and Prince William
Sound.

&&

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

A trough in the northern Bering is allowing for offshore north-
northeasterly winds to continue through Saturday afternoon. A low
in the Gulf of Alaska on Sunday will then continue this north-
northeasterly offshore flow on Sunday into early Monday. There
will be some precipitation along the Alaska Range Saturday night
into early Sunday. Precipitation will remain as rain near the base
of the mountains and snow for elevations above 4,000ft. The
highest precipitation amounts will be confined to the Alaska Range
as downsloping will cut down on precipitation amounts farther
west. With respect to models, the GFS/NAM blend was favored as
these models showed a high degree of consistency.

&&

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

An unsettled weather pattern will continue over the next 3 days as
waves of upper level energy bring showers to the area. Winds will
remain under gale force with the exception for coastal waters on
the Pacific side of the AKPEN east of 155W Saturday night through
Sunday afternoon.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...

A Gulf low weakens and moves into Prince William Sound then
inland Mon. Its front extends across the Gulf with southwesterly
gales dissipating late Mon. A new North Pacific front pushes
widespread easterly gales across the Gulf from south to north for
Tue and extends around Kodiak Island and the Alaska Peninsula
Coast by Wed.

For the Bering Sea and Aleutians, a polar shortwave drives
southward across the eastern Bering into the North Pacific, with
gusty small craft northerlies dissipates late Mon. High pressure
settles over the Bering through Wed. A strong North Pacific low
and front moves into the western Aleutians and Bering. The low
tracks south of the Aleutians through Wed with widespread east to
southeast gales over the western Aleutians late Mon and Tue, and
dissipating into the North Pacific by Wed.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...
A fairly active and amplified pattern will continue next week. A
short wave rounding the base of the upper trough brings a strong
surface low north toward the Alaska Peninsula Tuesday. Model
spread is fairly broad with this feature with the Canadian NH and
ECMWF the fastest and the GFS the slowest. While confidence is
low on the timing and positioning of this system, all solutions
indicate a large storm system affecting the southern Mainland of
Alaska Tuesday through Wednesday night. Widespread strong winds
and heavy rain are likely. This system weakens Wednesday
night/Thursday morning. Yet another developing low then heads
north toward the Alaska/western Gulf Thursday afternoon and
Friday. Continuing chances of strong winds and widespread rain to
the Alaska Peninsula, Gulf, and southern Mainland.

Out west, the remnants of Tropical Storm Dolphin will transition
to an extratropical low and pass just south of the western
Aleutians Monday through Tuesday. This will bring a quick shot
of rain and wind. Otherwise, with the primary storm track much
farther east, conditions across the western Bering and Aleutian
Chain will remain relatively quiet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 119 131.
Gale Warning: 120 132 136 150.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...KM/SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...MK/BG/SEB