Flood in Alaska, September 21, 2023

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
616 PM AKDT Wed Sep 20 2023

.ANALYSIS AND UPPER LEVELS...
A large upper level low is over mainland Alaska today with a sharp
trough extending southward out to it and over the easternmost part
of the Bristol Bay coast. The Front associated with this low has
pushed east of Prince William Sound, but the moisture and lifting
around the upper level low is keeping precipitation around
Southcentral as well as the Bristol Bay and Alaska Peninsula
regions. A ridge of high pressure is over the Central Bering Sea
west of the upper level low.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement. While that is good, it
also reflects that thy all have shifted the trough location
farther northward tonight into Thursday as it extends over Kodiak
Island and that is a change from yesterday and actually closer to
what was shown 2 days ago.

&&

.AVIATION...
PANC...Rain is expected to persist through the night. MVFR
ceilings are expected into thursday morning with the potential for
occasional IFR ceilings. VFR conditions are expected by Thursday
afternoon and beyond.

&&

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

A low centered over the Alaska Peninsula Wednesday afternoon will
move eastward into the Gulf of Alaska. Widespread showers will
continue through Thursday morning for much of Southcentral.
Precipitation is anticipated to be lighter than this morning's
showers. The low will merge with another low dropping south across
central Alaska overnight into Friday morning.

Models struggle to resolve the low track and the merging systems
beyond Friday. Temperatures will remain seasonal with an
increasing chance for partly sunny to partly cloudy skies Friday
and Saturday. Saturday afternoon could bring the next round of
precipitation as the next low moves north from the Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: This
afternoon through Saturday afternoon)...

Key Messages:
* Rain showers (with snow showers at higher elevations) taper off
  through tomorrow morning across Southwest Alaska.

* Isolated thunderstorms expected in the Western Bering/Aleutians
  through at least tomorrow morning.

* A gale force front (with gusts as high as 40-50 mph and moderate
  to heavy rain) moves across the Western and Central
  Bering/Aleutians through Thursday morning.

A low lingering over the Alaska Peninsula/Bristol Bay continues to
influence weather across the Eastern Aleutians and Southwest
Alaska. With this system weakening and slowly exiting into the
Gulf, expect winds to lighten and rain to taper off through
tomorrow morning. The potential remains for rain/snow mix or snow
along Western Alaska Range (in the Lime Village/Sparrevohn area)
late tonight into early tomorrow morning as lingering showers
occur in tandem with cold air advection behind the low. As was
mentioned in the previous discussion, marginally cold air
temperatures and warmer, early season ground temperatures mean
that little to no snow accumulations are expected for Lower
Kuskokwim Valley.

With the Bristol Bay low exiting the region, the next weather
maker to watch will be a gale force front extending from a
Kamchatka Peninsula low. Things remain largely on track for this
system; the primary changes with this forecast package have been
to slow down the front and to weaken it more quickly as it runs up
against the ridge located over Southwest Alaska.

One other change with this afternoon package has been an update
to the convective forecast. With observations showing lightning
and isolated thunderstorms embedded in the front, have added
isolated thunderstorms to the forecast for the Western Aleutians
and Western Bering Sea. There is some uncertainty regarding where
these thunderstorms will occur and for how long they will persist.
This forecast uses the Total Totals index and 850 mb equivalent
potential temperature as a guide; these parameters align best with
where lightning was observed earlier today, but they don't align
perfectly. As such, there is lower confidence in the
lightning/convective forecast. Although lightning is currently
only in the forecast until late tonight, cold air advection around
the Kamchatka low and the resulting thermal instability could
lead to more marine thunderstorms in the Bering Sea through
Friday.

Aside from marine thunderstorms, the front will bring gusty winds
and steady rain, with the strongest winds and heaviest rain
expected for the Western and Central Bering/Aleutians. By the time
the front approaches Southwest Alaska on Friday, it will have
weakened and slowed significantly. Current thinking is that while
there may be some light rain along the Southwest coastline, the
interior should stay quite dry and potentially sunny.

&&

.LONG TERM FORECAST (Days 4 through 7; Sunday through
Wednesday)...

Long range guidance continues to cluster on a deep longwave trough
extending from near the pole south over Mainland Alaska, the Gulf
of Alaska and into the Northeastern Pacific and a second but
weaker longwave trough over the Western Aleutians, Western Bering
and Eastern Siberia. Model spread on the 20/12z model suite
remains minimal through the weekend and into Monday before
significant spread enters the suite by Tuesday. By mid week next
week the GFS attempts to build a ride over Western Alaska while
both the ECMWF and Canadian prefer a shortwave eroding a broad
upper level ride extending north out of the Northern Pacific into
the Aleutians. The ECMWF model indicates a deep trough developing
in the Western Aleutians by late Wednesday and Thursday of next
week. An ensemble approach with a heavier weighting of ECMWF is
preferred this forecast cycle for the extended forecast over any
of the deterministic models.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...ER
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...CCC