NWS Anchorage, AK wrote its last area forecast discussion before the first event at Jul 22, 4:35 AM AKST, 6 hours 49 minutes earlier.
Area forecast discussion, Jul 22, 4:35 AM AKST, 6 hours 49 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKDT Sat Jul 22 2023
.ANALYSIS AND UPPER LEVELS...
Most of the southern Mainland is experiencing some of the driest
and calmest conditions we've seen so far this summer as a strong
ridge of high pressure sitting over Northwest Canada continues to
expand west into Southcentral and portions of Southwest. Areas of
mid-level cloud cover near a weak shortwave trough moving east
near the Kenai Peninsula have persisted overnight, but most of the
Copper Basin, Lower Kuskokwim Valley and Bristol Bay have remained
fairly clear with calm winds. Out in the west, a stacked low
continues to spin over the western Bering Sea, with a leading
front stretching out north of the Pribilof Islands and out over
the AKPen, where bands of light to moderate rain are progressing
slowly eastward. Aside from these slightly inclement conditions
out west, a shift to a more warm and dry summer pattern is really
beginning to take shape across the outlook area.
&&
.MODEL DISCUSSION...
Models are in very good agreement at the larger scale through the
start of next week, but a few forecast challenges remain. Models
are coming into better alignment with the degree of destabilization
over the Copper River Basin. The GFS and NAM are now both showing
very high surface based CAPE values by Alaska standards
developing on Sunday and Monday, or above 1000 J/Kg near the peak
of daytime heating across namely the northern Copper Basin on
Monday. However, there remain two aspects that will affect
thunderstorm coverage and intensity that models are struggling
with a bit - the degree of capping under the southern edge of the
ridging aloft, and the track and timing of very weak easterly
waves rotating into Southcentral. Any stronger shortwave moving
past and enhancing lift and upper level flow could greatly
increase the chance for more numerous and stronger thunderstorm
coverage across interior parts of Southcentral on Sunday or
Monday.
&&
.AVIATION...
PANC...VFR conditions and light winds will persist.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Monday)...
A ridge of high pressure will continue to build into the interior
and across the ALCAN, spreading south and west as it brings warm
temperatures and generally dry conditions to Southcentral. To the
south, over the Gulf, a low continues to slowly churn, with an
easterly flow regime trapped in between these two features. It is
this boundary that is bringing fog and low stratus to coastal
locations such as Kodiak and Cordova this morning, though some
afternoon clearing is in store.
The main story is the aforementioned easterly flow regime, as
easterly waves propagate through each day, bringing a chance for
scattered showers and wet thunderstorms across the mountains each
afternoon and evening. Each day's thunderstorm threat will
increase slightly in coverage and severity as CAPE values increase
to several hundred J/kg, leading towards the possibility for
frequent lightning, heavy rain, and gusty winds by Monday across
the Wrangell and Talkeetna Mountains, where afternoon temperatures
could reach into the upper 70s and low 80s, especially along
south-facing slopes. Otherwise, calm conditions are in store, for
some quieter summer weather these next few days.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA AND BERING SEA/ALEUTIANS
(Days 1 through 3: Today through Tuesday morning)...
A deepening low pressure system in the western Bering is, and will
serve as, the driving force for the upcoming persistent active
pattern for the Bering and Aleutians. The other major factor in
consideration is the quickly building ridge from Interior into
eastern Southwest Alaska. Currently, the frontal system associated
with the Bering low has mostly cleared the Pribilof Islands and
is approaching the coast of Southwest on its eastward journey.
This front is expected to be stalled just off the coast by the
shielding ridge that has built into eastern Southwest and most of
interior Alaska. As such, a corridor of moderate rainfall will
stretch from the Kuskokwim Delta coast to the Southern AKPen, with
the chance for periods of heavy rainfall between False Pass and
Nelson Lagoon. This stalled out front is expected to continue to
produce moderate rainfall through Sunday afternoon, with a total
of 1 to 1.5 inches through the rainfall period in the heavier
locations of the AKPen. As this is happening, the Central and
Western Aleutians will likely see little precipitation, however
cloudy skies will be prevalent and cooler temperatures will be
widespread.
By Sunday afternoon, yet another disturbance is aiming
to enter the western Bering Sea, reinforcing the trough in the
area, and associated light rainfall. This feature looks to sweep
the Chain, reaching Adak by Monday morning. Meanwhile, coastal
Southwest and and the southern AKPen look to remain showery Sunday
and most of Monday as moisture gets re-entrained from the North
Pacific and lifted northward by the upper low in the central
Bering. There may be a brief dry period for the coast Monday
afternoon and overnight before the next Bering sea front arrives
for Tuesday, bringing more clouds and showers back into the
forecast.
In contrast to the unsettled conditions of the coast, AKPen,
Bering Sea, and Aleutian Chain, inland areas of Southwest Alaska
will likely enjoy some relatively clearer conditions this weekend
and the start of the next work week with drier air and steady
southerly flow. Given the clearer skies, there is a chance for
some convective showers to pop up during peak heating hours each
late afternoon, especially Sunday and Monday. However confidence
is lower regarding thunderstorm development currently.
&&.LONG TERM FORECAST (Days 4 though 7: Tuesday through
Friday)...
The key feature in the long term is a broad upper level low in
place over the western Bering. Several upper level shortwaves
rotating around this low will bring unsettled conditions to the
Aleutian Chain and the Southwest coast. Gusty southerly winds and
rain are likely over the eastern Bering Wednesday into Thursday as
a front associated with this low moves inland. At this time, the
system is looking quite progressive and fast moving, so while
ample rainfall is expected, it should be quick to move through. By
next weekend, a second round of precipitation is possible for the
Aleutians and Alaska Peninsula, however models are struggling a
bit with whether this tracks inland or tracks into the Gulf of
Alaska.
Meanwhile farther east, a ridge of high pressure looks to
maintain its hold over the northern Alaska- Yukon border. While
this ridge attempts to build southward over Southcentral on
Thursday, models suggest that a few weak easterly waves will
develop and propagate from the Yukon westward into the Copper
River Basin. This introduces the potential for scattered showers
and possibly thunderstorms for the end of next week. By the
weekend, the remnants of the frontal boundary arriving over
Southwest will reach Southcentral, bringing increasing chances for
rain.
&&
SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...AB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DAN
LONG TERM...MB