NWS Anchorage, AK wrote its last area forecast discussion before the first event at Sep 23, 3:44 AM AKST, 8 hours 15 minutes earlier.
Area forecast discussion, Sep 23, 3:44 AM AKST, 8 hours 15 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Thu Sep 23 2021
.ANALYSIS AND UPPER LEVELS...
The main weather driver and system of interest continues to be an
elongated low south of the Aleutians and the newly developing
system at the base of its trough. The new low develops just south
of Kodiak Island and makes its way north towards Prince William
Sound, reaching the Sound by Thursday night. There are still
quite a few inconsistencies across the models with this low in
terms of speed and center location, but there seems to be good
agreement again by early Friday morning. This system deepens as it
pushes towards the North Gulf Coast and then stalls over PWS as it
begins weakening. An upper level arctic trough moves across
Southern Alaska, transporting cold northerly air across the
region, and the upper level low sets up over the Anchorage/Mat-
Valley area. As colder air gets wrapped around the low, the chance
for snow increases for the overnight hours. Ultimately, a large
part of determining the precipitation type for this type of setup
during this time of year will be the intensity of the precip.
Further west, ridging sets up over the Bering just ahead of a low
that will end up traveling south of the Aleutians. It is unlikely
that this low will have much of an impact aside from pulling
pieces of energy southwards across the Bering. Thus, creating
strong, northerly flow that will become increasingly gusty
through the typical gaps and passes of the Eastern Aleutians and
Southern AKPen.
&&
.MODEL DISCUSSION...
Models remain in good overall synoptic agreement through the short
term. There is still quite a bit of struggle with the upcoming
Gulf low. Track and strength of the system continue to be a non-
issue amongst solutions, while center location and speed remain
problematic. The NAM initially aligns well with the Canadian/EC
solutions, and stays in agreement until later Thursday afternoon,
with the GFS further south and west than the other three. By
Thursday evening, the NAM begins to slow down, matching better
with the GFS, while the Canadian/EC continue to fast track across
Prince William Sound. Model agreement improves again by Friday
morning, with all solutions showing the low stalling out over
Prince William Sound.
&&
.AVIATION...
PANC...VFR conditions and light winds will persist through late
afternoon. Rain is expected by late this afternoon, with cloud
ceilings dropping below 5000 ft and will likely last through the
evening hours. Late tonight, cloud ceilings drop to MVFR and rain
may transition to a wintery rain/snow mix.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...
Another significant fall storm is in store for Southcentral Alaska
over the next 24 hours, bringing strong winds and high seas to
Gulf coastal marine areas and another round of wintery precipitation
to portions of Southcentral. A strong upper level jet streak is
driving multiple surface lows northeastward into the Gulf. A
short-wave trough over Bristol Bay will dig southeastward across
the Alaska Peninsula and into the Gulf, leading to intensification
of the low through this evening. While model guidance has
struggled with determining which of the current lows will deepen,
the impacts will be similar no matter which one becomes the
primary low. The amplifying flow will pull the low westward toward
the northwest Gulf today, then somewhere in Prince William Sound
tonight as the low becomes vertically stacked.
The big change to the forecast was to go with a deeper low and
farther west storm track. As a result, have strengthened winds
across the Gulf waters, especially across the Kodiak Island
waters northward into Cook Inlet. As arctic air advects across the
Alaska Peninsula and into the Gulf tonight very strong gap winds
will develop across the western Gulf. As for precipitation, the
more westward low track will bring heavier precipitation to the
western Kenai Peninsula and Anchorage. Short-waves tracking
northeastward this morning will rotate northwestward across
Southcentral this afternoon and evening as the Bristol Bay
short-wave digs underneath the surface low. In the low levels,
the occluded front out ahead of the deepening low will similarly
rotate northwestward across the northern Gulf and Prince William
Sound. While the front won't survive the journey intact as it
heads inland, a surface trough will rotate across the Kenai
Peninsula and Anchorage, with strong low level convergence along
this trough. Because of the uncertainty in the low track as it
nears Prince William Sound, there is also some question about the
exact placement of the surface trough and band of heavier
precipitation. However, unless the track were to somehow shift
eastward, which doesn't appear likely, Anchorage should see a
period of moderate to heavy precipitation this evening and into
the overnight hours, especially along the mountains.
This brings us to the most challenging part of the forecast, which
is precipitation type. The air mass in place out ahead of this
storm is cold enough to support primarily snow inland. However,
warmer air east of the low track, will warp around the north side
of the low center, helping to moderate the air mass across most
of the region today. The warmest air will advect across Prince
William Sound and into the southeastern Copper River Basin,
meaning this will be primarily a rain event for these areas. It
gets a little trickier as you head north and west from Prince
William Sound because the airmass will undergo less moderation.
Still early in the season, solar radiation is still strong enough
to produce heating near sea level, which will make it hard to
produce snow during the afternoon hours. However, as heavier
precipitation moves in tonight, the atmosphere will cool as the
low levels first become saturated (evaporative cooling), then by
the process of melting (melting snow aloft cools the atmosphere
and gradually allows snow to fall lower and lower in the
atmosphere). Colder air will also start to filter in from the
west overnight through Friday morning. As a result, would expect
a transition from rain to snow tonight through Friday morning for
Anchorage and the western Kenai Peninsula, beginning along the
hillside this evening and dropping to sea level by Friday morning.
Snow accumulation is most likely along the Anchorage hillside and
other areas above sea level. In fact, if the heavier
precipitation does settle on the hillside, this could be another
early season significant accumulation (of 6 inches or more). The
Mat-Su looks like it will be positioned north and west of the best
forcing, so even if snow does fall it shouldn't be much.
The surface low and upper trough looks to linger in the region
through Friday night or Saturday, so additional light precipitation
is likely. It looks like things will finally dry out by Saturday
night as the trough exits into the Gulf. With a cold air mass in
place, it could be another chilly night Saturday night.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level low over Southwest continues to move eastward as an
upper level ridge over the Bering advances inland, leading to a
clearing trend for Southwest over the next day. Residual showers
will continue this morning across the Bristol Bay region before
dissipating by the afternoon. Colder northerly flow will continue
through the overnight hours tonight and skies will gradually
clear by early Friday. A few snow showers may still linger along
the foothills of the Alaska Range overnight tonight before
tapering off early tomorrow. Fair conditions can be expected
across Southwest into Saturday as a ridge builds over the region.
An weak upper level shortwave arrives over the Kuskokwim Delta
Coast Saturday evening, introducing the chance for scattered
showers. Given the colder temperatures in place, there is a chance
for a rain/snow mix, though little to no accumulation is expected
at this time.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The combination of a building ridge over the Bering and a low in
the Gulf will keep gusty northwesterly winds in place across the
central and eastern Aleutians this morning. As the Gulf low moves
eastward, winds will eventually weaken and diminish overnight into
early Friday. Meanwhile, an upper level wave over the western
Aleutians is quickly ushering a surface low across the North
Pacific, just south of the Chain. The western Aleutians can expect
to see showers developing today with precipitation filling in
across the Chain from west to east. By Friday morning, increasing
chances for precipitation are expected across the central Bering
and Pribilofs as a stronger upper level trough over Siberia drops
southward. This feature will also introduce colder northerly flow
to the western Bering Saturday morning and allow for a return of
gusty northerly winds across the central and eastern Aleutians
Saturday afternoon. The next low arrives late Saturday as a
leading frontal boundary approaches the western Aleutians.
&&
.MARINE (Days 3 through 5: Saturday through Monday)...
Gulf of Alaska.... As a low over the Gulf begins to diminish,
east winds will decrease with waves subsiding through Saturday.
Northerly winds along the Southcentral coast will increase on
Sunday and continue through Monday with the potential of gale
force gusts as a low tracks east in the southern Gulf. Waves from
the Barren Islands into the central Gulf will build in response.
Bering Sea.... A low tracks east to the western Aleutians Sunday
and will move to the southern Bering Monday. South gale force
winds ahead of the low and north gale force winds behind the low
will result in elevated seas measuring up to 13 ft in the southern
Bering late Saturday into Sunday.
&&
.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...
Models are in generally good agreement through Monday with model
spread increasing by Tuesday. A continuation of the current
amplified pattern is anticipated to prevail with a broad longwave
trough centered over much of the mainland with multiple embedded
shortwaves and a building ridge in the Bering. Colder
temperatures will persist across the Southern Mainland as arctic
air is drawn down from the interior due to a northerly flow aloft.
A shortwave trough and surface low are expected to drop south
over the eastern Bering and Aleutians from Saturday to Sunday.
This has the potential of bringing small craft to locally gale
force winds and rain for a brief period as it dives south.
Confidence begins to diminish as models struggle to position the
axis of the ridge and trough. Even with the details for individual
synoptic systems less clear, there is a fairly good signal that
the Pacific jet and prevailing storm track will stay relatively
far south from this weekend through early next week. Global models
and their respective ensemble suites have the North Pacific low
moving along the Aleutian Chain Sunday and Monday and entering the
southern Gulf Tuesday as it tracks around the base of the
longwave trough axis. This should keep conditions much less
active over the southern mainland, albeit still chilly by late
September standards, with abnormally cool low level temperatures
likely to remain over much of the state through nearly the entire
extended period. Another low could track across parts of the
Bering between Tuesday and Wednesday, although any details by this
point are far from being resolved.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 119 120 130 131 132 351
Gale 125 136 137 138 139 140 150
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...TM/NS