Gale Warning 497, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
620 PM AKDT Mon Sep 18 2023

.ANALYSIS AND UPPER LEVELS...
A progressive pattern remains over southern Alaska with a low in
the Gulf of Alaska moving eastward, a ridge over Southwest Mainland
Alaska, and then another low in the Bering Sea. An upper level
trough connecting the low in the Gulf to the main upper level low
in Northwest Alaska is helping to bring rain to the Matanuska
Valley this afternoon. there is a sharp boundary to this rain with
Palmer being in the rain and Wasilla mostly missing it. The ridge
over Southwest Alaska is moving over Southcentral, and with cold
air aloft over Southwest and Southcentral, temperatures are likely
to be the coldest of the season so far in many areas.

The low in the Bering Sea has cold air aloft associated with it
to the point there have been thunderstorms through the day over
parts of the Southwest Bering Sea and Western Aleutian Islands.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement the next few days. Of
note is that, while all the models elongate the low as it crosses
from Bristol Bay into the Gulf, the GFS digs the low farther
south on Wednesday and Wednesday night than other models. The
greatest impact on the forecast with this solution would be on
Kodiak Island as the low center with the GFS is just south of
Kodiak while other models keep it just north of Kodiak and
Afognak Islands. Model agreement does begin to significantly
deteriorate toward the weekend especially in regards to the low
that will be in the Bering Sea.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

Monday night, ridging will amplify across the area bringing a
brief break from the rain. Seasonably cool air in place with
clear skies overnight are the right conditions for temperatures to
drop near or below freezing by dawn Tuesday across Southcentral.
Confidence is high for clear to mostly sunny skies Tuesday.

The next low advances from the southwest late Tuesday, arriving
overnight into Wednesday morning, bringing widespread
rainfall...and for elevations above 3000ft, snow. Rain could
continue into Thursday, but drier conditions are expected to
return from Thursday afternoon through the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: This afternoon through Thursday afternoon)...

Main weather messages for Southwest Alaska and Bering Sea:
* Tonight: Temperatures will be a bit colder than yesterday for
  interior Southwest Alaska, with some locations dipping towards
  freezing.
* Tonight through Tuesday: Steady rain across Southwest Alaska and
  the Aleutians will gradually transition to rain showers.
* Wednesday through Thursday afternoon: A gale force front will
  bring moderate to heavy rain as it moves eastward across the
  Bering Sea.

A broad low currently over the Central Bering Sea will be the
primary weather maker through Tuesday. Cold air wrapping around
the low and moving over warmer ocean waters is producing steep
lapse rates; this thermal instability is allowing for the
development of isolated marine thunderstorms and sporadic
lightning strikes across the Southern Bering Sea. Isolated marine
thunderstorms remain possible through at least tomorrow morning.

Most areas, however, won't see much lightning. Instead, the main
story with this low will be the rain that it brings in,
especially in combination with a weak surface low that skirts
along the Alaska Peninsula on Tuesday. There remains some
uncertainty regarding rain amounts from these two systems. Rain
totals will ultimately depend on how far inland the weak low's
front will make it before the low itself exits into Southcentral,
as well as the duration of showers from the Bering Sea low.
Overall, with rain intensity looking to remain on the lighter end,
rain totals should remain within the norm for this time of year.

Speaking of time of year, let's talk a bit about the dreaded 's'
word: snow. With temperatures dipping towards freezing tonight,
there is some question of whether the lower elevations will see
snow as the Bering Sea low pushes its front into Southwest Alaska.
Right now, it's looking unlikely as the front arrives later
tomorrow morning, giving temperatures a chance to rise. Warm air
will also move in with the front, further raising snow levels. As
such, it's likely that mountain tops and areas at higher elevation
will get a dusting of snow, but unlikely that anything but rain
will be seen at lower elevations.

Looking ahead, a strong low extending from Kamchatka Peninsula
will push its front across the Bering Sea. Confidence is high that
southerly gales (winds 35+ kt) will accompany this front, as well
as a hefty plume of moisture.

&&
.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Long term guidance continues to show a strong low pressure system
dropping south from eastern Siberia towards the Bering Sea Friday
night into Saturday morning. There was quite a bit of model
disagreement yesterday, with the EC drifting south towards the
North Pacific and the GFS stalling south of Saint Lawrence Island.
Most recent model runs show somewhat better agreement. The EC now
takes a slower path towards the North Pacific and the GFS is
trending towards a similar, but faster moving, course. The
associated front is represented relatively well across all models,
showing a widespread band of precipitation tracking eastward
across the Aleutian Chain toward Southwest Alaska. The front will
fizzle out quickly as it moves onshore and rain, if any, further
inland will be light. Additionally, there remains the hint of a
triple point low developing near the Alaska Peninsula on Friday
that will merge with a developing North Pacific low.

Two additional systems are expected to impact the marine areas by
early next week. One low develops west of the Western Aleutians
and will move into the Bering Sea by Monday. The second moves from
the North Pacific into the southern Gulf of Alaska Monday night
into Tuesday morning. There is quite a bit of uncertainty about
how far north the low tracks and whether or not impacts will
affect Southcentral or stay strictly marine.

&&

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