Gale Warning 510, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 PM AKDT Thu Sep 7 2023

.ANALYSIS AND UPPER LEVELS...
An anomalously deep upper level low sits over the southern half of
the state. A 537 dm low, even for early September is around 150 m
lower than average for the 1991-2020 period. The temperatures
aloft(500mb) are around -30C, quite cold for fall. This cold pool
within the trough is responsible for steep lapse rates throughout
the lower and middle atmosphere. Several smaller scale
disturbances are rotating around the trough. The cold front
associated with the system has dropped well into the northern
Pacific and western Gulf. The colder/drier air has cut the area
off from the deep sub-tropical moisture and atmospheric river
characteristics, with satellite estimates of .4 to .6 inches of
precipitable water which is very near normal for this time of
year.

What does this mean for sensible weather? Overall, cooler,
cloudier and showery. The previously mentioned instability and
cold air aloft encourages showers and thunderstorms, while the
disturbances rotating around the trough help focus them. Wednesday
evening's showers and thunderstorms also brought the snow level
down to about 4500 ft or so. The freezing level on this morning's
sounding was 4800 ft.

&&

.MODEL DISCUSSION...
Guidance is in good agreement about the large scale evolution of
the trough, though with this showery pattern, the devil is in the
details. How the small scale disturbances develop and feed on
instability will determine the placement and strength of shower
and thunderstorm activity. The next synoptic system is fairly well
agreed upon, wrapping into the northern Gulf from the eastern Gulf
on Saturday morning, moving inland over northern Cook Inlet.

&&

.AVIATION...
PANC...Lower ceilings and shower activity will be the main concern
for the terminal. Generally weak wind flow in the low levels and
shower activity keeping moisture at the surface MVFR ceilings will
be in and out of the terminal. It is likely that showers will
move off the mountains and toward the terminal through about 03z.

&&

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

An upper-level low encompasses most of the Alaska Mainland this
afternoon while its center resides near Southcentral. Temperatures
aloft continue to cool as the low and trough slowly nudge farther
south into the Gulf of Alaska. Upper shortwaves embedded within
the flow pinwheel around the low and combine with the increasingly
unstable atmosphere to maintain cooler and showery conditions
across the region. As such, snow levels continue to lower and
light snow accumulation can be expected at elevation. The cool and
showery weather conditions are generally anticipated to continue
through the end of the week as the upper low lingers near
Southcentral tonight and Friday.

The upper trough will continue to drop slowly southward into the
Gulf through Saturday and become more negatively tilted. A surface
low will then merge with another low pressure system that tracks
out of the North Pacific and into the Central Gulf on Saturday.
This newly formed low is advertised to bring a round of steadier
precipitation and stronger winds to the northern and eastern
Southcentral coast areas. Some precipitation is expected to spill
northward across Mainland Southcentral, as well. Moving into
Sunday, the surface low weakens and a stronger system approaches
Southcentral from the west, spreading steady precipitation across
Kodiak Island on Sunday morning and then northeast into the
Susitna Valley and Southcentral coastal areas throughout the rest
of the day. Current thinking is that the western Kenai Peninsula,
Anchorage Bowl, and Matanuska Valley will stay on the drier side
due to downsloping. Models are struggling to resolve just how dry
the Copper River Basin stays as this system tracks to the
northeast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Sunday morning)...

Currently, an amplified yet progressive pattern across Southwest
Alaska and the Bering Sea is observed, leading to conditions
varying from west-east across our region. This afternoon,
northerly winds through Southwest Alaska, largely along the
coastline, along with gusty winds out of the southern gaps and
passes of the Eastern Aleutians and Alaska Peninsula. Winds from
the north are fairly dry, leading to some showers for this area.
Comparatively, in the Western Aleutians, a leading front from a
Kamchatka low is pulling southerly winds up through the Aleutian
Chain into the Western Bering Sea. Moist southerly air drawn up
with this flow is leading to more widespread showers as well as
promoting areas of fog behind the front, currently stretched from
Amchitka to the northwest. Between these two systems, is a weak,
transient ridge that is promoting clear conditions and calm wind
in the Central Bering and Central Aleutians.

For Southwest Alaska, the current pattern is expected to ease,
becoming increasingly calm as influence from the low in the Gulf
of Alaska wanes as that system departs into the Northeastern
Pacific. As that system departs, the weak ridging previously
mentioned will move in closer to Southwest Alaska. This shift will
allow for clearer conditions across Southwest and portions of the
Alaska Peninsula into early Saturday afternoon. During this
period, winds will become lighter, skies clearer, which will allow
for some slightly warmer temperatures Friday afternoon and cooler
overnight temperatures tonight and Friday night.

In the Bering, the front currently in the Western Bering will
continue to march southeastward, bringing southerly winds and rain to
Adak by Friday morning, extending to the Pribilof Islands and
Unalaska by Friday afternoon. The core of strongest winds will be
gale force gusts along the leading frontal boundary, which will
cover the Pribilofs by Friday night. Behind the front, winds will
be initially southwesterly becoming westerly as the front
continues to track eastward. A core of gusty westerly winds may
also develop along the the southern side of the Central Aleutians
by late Friday night into Saturday morning as the parent low
continues to dive southeastward.

By Saturday morning, the elongated low pressure system is expected
to begin its crossing of the Aleutian Chain, over the Eastern
Aleutians and southern Alaska Peninsula. This will result in a
period of moderate to heavy rainfall from Sand Point to Nikolski
from very early Saturday morning until the late morning. By
Saturday afternoon and evening, the low pressure system is
expected to have departed to the Southeast into the Southern Gulf
of Alaska/Northeastern Pacific. Southwest Alaska is currently
expected to dodge the brunt of the rainfall associated with this
system, however by Saturday there will most likely be a return to
cloudier skies for much of the region. The Bering may see a weak
surface feature enter by Sunday morning, however model agreement
on this is low, indicating the potential for a break in activity
the latter half of the weekend.

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

An elongated trough stretches from Siberia into the Gulf of
Alaska through the forecast period. Small variations in the
models, but the overall pattern remains intact. Widespread low
cloudiness, locally heavy rain and gusty winds approaching gale
force sweep across the Southern Bering, Aleutians and Alaska
Peninsula through Thursday.

The existing low in the Gulf of Alaska get reinforcements as
shortwaves exit the Bering into the Western Gulf. Gale force gap
winds start around Kodiak Island early Monday, then as the low
crosses the Gulf, will bring an Easterly barrier jet along the
North Gulf Coast late Monday into Tuesday, subsiding Tuesday.
Frequent heavy rains are expected across the Southcentral coasts,
with lesser amounts inland, diminishing late Tuesday. the low will
weaken as it moves into Prince William Sound through Thursday.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...AF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...BL