Flood in Alaska, April 19 to 24, 2021

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Mon Apr 19 2021

.ANALYSIS AND UPPER LEVELS...

The blocked pattern featuring a prolonged period of warm and dry
conditions across much of the mainland continues today. A strong
omega-shaped ridge remains anchored in place from the Canadian
Rockies to the eastern interior. A weakness in the ridge that
allowed upper clouds and moisture to infiltrate parts of
Southcentral on Sunday has since filled in, with predominantly
clear skies now returning to most of the area. Areas that
remained free of clouds farther east yesterday have continued to
observe soaring temperatures following peak daytime heating. This
included a high of 71 F at the Cordova airport on Saturday, where
weak northeasterly winds and subsidence likely aided temperatures
locally with downslope warming.

To the south of the ridge, a large, stacked low continues
to swirl south of the Gulf as it retrogrades northwest towards
the southern AKPen. The pressure gradient between the low and the
ridging farther north is helping to drive northeasterly small
craft to low gale force winds over the western Gulf and across
the southern Bering. The low's occluded front is also beginning
to bring rain showers into the southern AKPen and Eastern
Aleutians. Conditions continue to be about as low-impact and calm
as one could expect elsewhere for this point in the year.

&&

.MODEL DISCUSSION...

Models are still in very good agreement at the synoptic level, and
overall forecast confidence is high. However, the fate of an
Arctic trough dropping into the eastern mainland near the AlCan
border on Tuesday and Wednesday remains somewhat contested. The
ECMWF/Canadian continue to show a more progressive and north
track for this feature, keeping weak ridging intact over
Southcentral through midweek. The GFS/NAM show the wave axis drop
farther south, ultimately becoming entangled with the easterly
wave moving across the AKPen on Tuesday. The trough then pushes
more into the Susitna Valley through Tuesday night with this
scenario, resulting in some precipitation potential from the
Susitna Valley to the Copper River Basin. This could involve some
mixing issues, especially for the Copper River Basin where the
coldest air would spread into from Tuesday to early Wednesday.

&&

.AVIATION...

PANC...VFR conditions will persist. Southeast winds out of the
Turnagain Arm of around 10 to 15 kts may impact the terminal at
times from this afternoon through early Tuesday.

&&

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

A vertically stacked low situated well south of Kodiak Island this
morning will lift a front north into the western Gulf today and
eventually into the northern Gulf by this evening. This system
will bring rain and higher elevation rain/snow first to Kodiak
Island then to coastal Southcentral. For interior locations,
another mainly clear and dry day is on tap as the weather over a
majority of the state is dominated by an expansive area of high
pressure. This ridge, however, is moving east, and that will allow
a southerly flow to develop across Southcentral by the afternoon
which will allow for some higher clouds to advance across the
interior.

As the frontal system approaches, its associated upper-level
shortwave will also move inland. This feature is expected to help
the increasing southeasterly gap winds ahead of the surface front
mix down and allow for a southerly wind to develop for places like
Anchorage and Palmer this afternoon. The front is expected to
stall out and weaken along the coast, keeping a lingering threat
for coastal rain showers and higher elevation rain/snow showers in
the forecast through late Tuesday.

Also on Tuesday, a shortwave and associated cold front will drop
from northwest Alaska to the Alaska Range. There are still
uncertainties as to timing of this feature and just how far south
it digs before exiting to the east. Right now, it looks like an
increased chance for mixed rain/snow showers across the northern
half of the Susitna Valley, Talkeetna Mountains, and Copper River
Basin Tuesday afternoon through early Wednesday. Temperatures
Tuesday will be cooler and more seasonal for this time of year as
the front ushers in cooler air and increased cloud cover. However,
the Rex Block pattern looks to re-establish by Wednesday,
allowing for a return to clear and dry conditions along with above
normal temperatures for the second half of the week.

&&

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

The upper level high pressure ridge gets shoved off to the east
late this afternoon by a longwave trough extending off a polar
low. This allows a wave of energy along a trough on the backside
of the North Pacific low to move across Southwest Alaska. The main
impacts will be increased winds across Bristol Bay, especially
through the gaps in the Alaska Range and across the Kuskokwim
Delta. A majority of the precip with this system will get caught
up along the Aleutian Range. Most of Southwest Alaska is expected
to be dry, with relatively clear skies throughout the day Monday
and clouds rolling in late Monday night.

&&

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

The front extending off the North Pacific low moves across the
Southern AKPen and into the Bering this afternoon, producing rain
showers across the Eastern and Central Aleutians. The backside of
the low consists of gusty northerly winds across much of the
Bering, with areas of gales in the northern Bering and south of
the AKPen from Monday night into Tuesday morning. This transition
to colder northerly flow allows for some heavy freezing spray
concerns along the ice edge in the northern Bering late Monday
night and Tuesday morning. As winds begin to diminish Tuesday
afternoon, the threat of heavy freezing spray will subside.

A secondary North Pacific low develops Tuesday night and rotates
around our existing low before merging and moving south of the
AKPen. As the two systems become one, the new low strengthens and
brings high end gales south of the AKPen and Aleutian Chain.
There is the possibility for a period of storm force winds on
Wednesday as well, however, there is still some model
inconsistency with the higher end winds. Additionally, expect
another round of showers to spread across the southern AKPen to
the Central Aleutians.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

Shelikof Strait and the Gulf of Alaska southeast Kodiak looks
active late Tuesday and Wednesday. Expect gusty and enhanced seas
for this vicinity. With a tight pressure gradient in place, there
is potential for gales to develop south of the Alaska Peninsula,
and also the Eastern Aleutians on both the Pacific and Bering
side. Tue through Thur...the seas over the Bering should stay
below 11 feet. However, seas south of the Eastern and Central
Aleutian could be upwards of 17 ft.

&&

.Long Term Forecast (Days 4 through 7: Thursday through
Sunday)...

This forecast package was composed using a blend of models. There
are a few items that jump out. Temperatures in Southcentral and SW
AK will be spring like and numerous locations will hit or exceed
the 50 F benchmark. There are a couple of systems in the North
Pacific that the models don't have a precise track locked on yet.
Widespread subsidence from an upper level high will be the cause
of the the warm and dry trend over Southcentral and SW AK.
Overall, it will be a mild forecast period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 138,150,155,181,185.
		 Heavy Freezing Spray: 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...PJS