Flooding in Alaska, May 1 to 10, 2020

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
356 PM AKDT Thu Apr 30 2020

.ANALYSIS AND UPPER LEVELS...
The upper trough which dropped into Southcentral Alaska on
Wednesday has now split into two pieces, with a closed low along
the ALCAN border lifting northward while a new closed low tracks
westward across Southwest Alaska. The remaining trough axis over
Southcentral is now lifting back northward. Meanwhile, the low
that had been sitting over the eastern Gulf on Wednesday has now
largely dissipated along the northeast Gulf coast. Some of the
moisture with this is being pulled northward into Southcentral
behind the retreating upper trough. Radar imagery shows steady
rain across eastern Prince William Sound (including Cordova and
Valdez) up into the very southern portion of the Copper River
Basin (in some locations as snow). There are also some clouds and
isolated to scattered showers associated with the retreating
trough, from the Kenai Peninsula up to the Mat-Su. Satellite and
radar imagery show many of these showers forming on the area
mountains, then tracking off the mountains in a general
northwesterly direction.

Clear skies dominate most of Southwest Alaska despite the upper
low passing overhead. Onshore flow across Bristol Bay has advected
low clouds and fog inland, which is now clearing to the coast.
Out west, a large nearly stationary low continues to spin north of
the western Aleutian islands. There are still some small craft
winds and rain showers along the central to western Aleutians, but
the occluded front that wrapped around the Bering Sea has
completely dissipated. There is a layer of stratus sitting over
most of the northern and eastern bering due to the persistent
ridge that has been sitting overhead for days.

&&

.MODEL DISCUSSION...
Numerical guidance is in good agreement with the overall evolution
of the pattern through the weekend and forecast confidence is
generally high. Do expect models will continue to struggle a bit
with handling of weak short-waves aloft with very little steering
flow. Still, in this pattern don't expect any big "surprises" that
would change the forecast very much.

&&

.AVIATION...
PANC...There will be a few showers in the vicinity associated with
the upper trough lifting northward, but conditions will remain
solidly in the VFR category. The main impact for the terminal
could be a period of southeasterly Turnagain Arm winds, though
confidence is pretty low as to exactly when they might arrive and
how strong they will be. The winds seem most likely during the
evening hours as high pressure moves in behind the trough. This
will also coincide with peak heating over the Mat-Su, which is
what causes the wind exiting Turnagain Arm to bend into Anchorage.
Nonetheless, this looks like a weak and short-lived event.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3...Tonight through Sun)...
A front along the Northeast Gulf will bring rain to the coastal
locations through Friday before diminishing early Saturday. A few
showers will develop inland as upper level southwesterly flow
advects moisture inland and weak disturbances move overhead. South
to easterly flow prevails through Friday with a few gusts tonight
along Turnagain Arm. On Saturday, offshore flow returns with a
weather pattern shift. The upper level ridge centered along the
west coast builds across the western interior while a low persists
over the Alcan border. Gusty flow is expected along typical
coastal gaps as energy slides toward the Gulf between these two
synoptic features.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The forecast for the next few days in Southwest Alaska will
remain mostly benign with lows around freezing and highs around
50. A lack of active weather can be attributed to the presence of
a broad area of high pressure across the northern half of the
state and the main jetstream positioned across 40N. These two
factors are yielding a fairly ambiguous wind pattern, little
moisture to work with, and a series of minor disturbances that we
might squeeze a drop or two of rain out of. The biggest weather
story today for Southwest Alaska is an area of stratus and fog in
the Bristol Bay area, which will likely be a presence the next
couple of days with light winds and ample moisture from melting
snow. The challenge here will be whether the sun can burn off the
clouds. The Kuskokwim Delta and lower Kusko Valley should escape
the fog with stronger winds and a dryer atmosphere closer to the
high pressure. As the high pressure retreats north, a few easterly
shortwave disturbances will travel through the area. The first
such wave will be Saturday morning with another Sunday morning.
These waves have little moisture to work with and will likely show
up with clouds and scant precipitation from a few isolated
showers that might get some enhancement from elevated terrain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A stacked 994 mb low in the western Bering with a smaller closed
low rotating around its eastern periphery over the eastern
Aleutians and western AKPEN are the primary drivers for weather
this afternoon in and around the Bering/Aleutians. As mentioned
previously, the jetstream is too far south to provide meaningful
support and these two features are drifting somewhat listlessly
eastward. The main low in the west will absorb the smaller feature
by Saturday morning and push south of the eastern Aleutians by
Saturday afternoon. The path of this low is somewhat contingent on
the high situated over northern Alaska which tries to expand
southward into the northern Bering by Sunday, which will act to
nudge the Aleutian low southward. Shower activity on the north
side of the low will likely be the main event Friday and Saturday
across much of the Aleutian chain and AKPEN in the form of mostly
rain with temperatures near 40, while a few snow showers are
possible in the overnight hours with low temperatures around
freezing. Things dry out some by Sunday which will be a mostly
quiet day across the Bering/Aleutians. A frontal boundary enters
the western Aleutians on Sunday and will bring the next
opportunity for measurable precipitation.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...
Expect a fast moving low to track along the Aleutians Sunday
through Monday morning before exiting quickly southward into the
Pacific. Looks like mostly small craft advisory winds, but there
is potential for gale force winds, especially when the low first
reaches the western Aleutians on Sunday.

Meanwhile, forecast remains on track for a low to track westward
across the Gulf of Alaska Sunday night through Monday. However,
there is still a lot of uncertainty in both the track and strength
of this system. Will continue to advertise small craft advisory
winds or weaker. Weather over the northeast Pacific looks like it
will be active, so there is also potential for another storm to
move northward and affect the Gulf waters Tuesday and beyond.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...
To start out the new work week, expect a low tracking westward
across the Gulf of Alaska. While there is some uncertainty in the
details of this storm, based on the general track expect most of
the winds and precipitation with this system will be along the
Gulf coast and Kodiak Island. Clouds and a few showers may quickly
skirt inland areas of Southcentral Monday and Southwest Alaska
Monday night through Tuesday.

In the wake of this first storm system, model solutions rapidly
diverge and depict radically different solutions as we head
through the week. Thus, forecast confidence is low and model
ensembles will be heavily relied on. There is general agreement
that the upper level flow over the North Pacific will become more
amplified, with potential for one or more large and deep storm
systems to affect the region. Of course, the use of ensembles will
largely wash out any indication of a strong storm. Will just need
to monitor this over the next couple days to see if the signal in
the models becomes any clearer.

The other general area of agreement in the models is that the
favored areas for storm development and track will be over the
North Pacific (and perhaps the western Bering Sea) and Mainland
Alaska will likely stay on the periphery of any storms that do
develop. In fact, generally expect some sort of ridging to
dominate mainland Alaska, with occasional upper waves tracking
around the ridge. Based on this, would expect a mix of sun and
clouds, mostly dry conditions (typical for this time of year),
and warming temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 152 155.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

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