Area Forecast Discussions

000
FXAK68 PAFC 271331
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
531 AM AKDT Mon Jun 27 2022

.ANALYSIS AND UPPER LEVELS...

A large area of high pressure persists across Southcentral and
the Gulf of Alaska as an upper-level ridge extends from the region
east across the Eastern Mainland. The interior areas of
Southcentral are seeing generally clear skies this morning which
will persist through the day as a result of the ridge. Widespread
marine stratus continues to cover most of the Gulf but is less
extensive across Cook Inlet and Prince William sound compared to
the past few mornings. During the overnight hours, low clouds
developed over the western Kenai Peninsula again and then extended
north into the Anchorage and Birchwood areas. The stratus is
expected to break up by early afternoon today.

Upstream of the ridge, an upper trough is exiting to the
northeast over southwest Alaska as an associated surface front
pushes into the ridge and decays over the region. This is
maintaining cloud cover for Southwest as well as some much needed
rainfall across the Lower Kuskokwim Valley, interior Bristol Bay
locations, and near the Kuskokwim Mountains. Farther west, a large
upper-level low is located over the western Bering while a
surface front pushes across the Central Aleutians. Thus, overcast
skies, areas of fog, and rain with the front are dominating the
far western domain this morning.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement for the next few days, so
forecast confidence is high. All models depict a low spinning over
the western Bering while its front progresses east across the
Central and Eastern Aleutians through Wednesday morning. However,
there are some differences regarding an upper-level disturbance
that develops over southwest Alaska on Tuesday afternoon.
Depending on how this disturbance evolves could have an impact on
thunderstorm potential and coverage along the Alaska Range that
day.

&&

.AVIATION...
PANC...VFR conditions and light westerly winds will persist
through the TAF period. Areas of marine stratus may drop ceilings
to MVFR or IFR at times, but any lingering stratus is expected to
break up by early afternoon today.

&&

.FIRE WEATHER...

Southcentral: Fire weather conditions will deteriorate as we head
through the week, with a return to widespread hot and dry
conditions. Also, generally expect an increase in thunderstorm
activity as we head through the week. The key forecast problem is
the areal extent of thunderstorms and resultant lighting activity
level (LAL), which will help determine whether Red Flag Warnings
should be issued. At this point in time, the forecast is calling
for isolated to widely scattered thunderstorms with LALs no higher
than 3. This is based on increasing airmass instability, focused
along the thermal trough. At this point in time, there does not
seem to be any other forcing mechanisms (such as a short-wave
trough). While that is not absolutely necessary to reach an LAL of
4 or higher, it means you need greater instability to produce
more thunderstorms and lightning. Model guidance does vary in some
key stability parameters, but has generally been trending toward
more unstable conditions. The area of most concern for a lot of
lightning is the northeastern Copper River Basin (Tok Cutoff
corridor) on Tuesday, then nearly all of the Copper River Basin on
Wednesday.

Southwest: Rainfall becomes more showery and will continue to
diminish throughout the day today. Residual moisture will linger
where upslope terrain enhancement can occur. Temperatures will
trend a little warmer for Southwest as some of the skies begin to
clear. High resolution models hint at slight convective potential
this afternoon. Any daytime surface heating and residual upper
level forcing would allow for pop up showers to potentially
produce a few lightning strikes near Lime Village east towards the
Western Alaska Range. Wednesday is another potential day for
isolated thunderstorms, but the convective forecast continues to
be tricky with regards to the strength of instability and
available forcing for convective initiation, especially for
Tuesday afternoon near the Western Alaska Range.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

The marine layer stratus deck is much less extensive in Cook
Inlet and Prince William Sound then it was over the weekend. As a
result, expect the areas of low clouds that do exist to clear out
more quickly today, leading to marginally warmer temperatures
along and near the coast. Hot and dry conditions will persist
inland, especially across the Copper River Basin. The thermal
trough, which has been centered over the Yukon, will shift
westward toward the eastern Copper River Basin today, with an
uptick in diurnal thunderstorms expected from the north side of
the Wrangell Mountains up to the Alaska Range.

The big story as we head through Tuesday and Wednesday will be a
return to hot and dry conditions across Southcentral along with
increasing thunderstorm activity each day. The thermal trough will
shift westward across the Copper River Basin to the Talkeetna
Mountains Tuesday, then encompass nearly all of Southcentral by
Wednesday. Weak offshore flow will develop tonight through Tuesday
in response to a building surface ridge from Southwest Alaska
into the Interior. This will contribute to warming temperatures
and further destabilization of the atmosphere along with locally
gusty winds along the coast (namely Seward and Whittier). The best
chance of thunderstorms will remain over interior Southcentral
Tuesday, but it looks good for isolated thunderstorms farther
south on Wednesday. However, based on the flow (which will be
weak), any storms which do form will likely be focused along the
Kenai and Chugach Mountains. This mean little or no rainfall for
the very dry areas of the western Kenai Peninsula, Anchorage, and
southern Mat-Su.

&&

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

Cloudy skies are expected today with rain showers gradually
decreasing throughout the day and become more isolated tonight and
Tuesday as the upper trough lifts north. Enough daytime surface
heating along with residual upper level forcing lingering near the
Western Alaska Range this afternoon and evening may be enough for
isolated thunderstorms to form from around Lime Village eastward.

A shortwave ridge builds over Southwest Tuesday afternoon to help
clear some of the clouds inland and warm temperatures slightly.
There is uncertainty in the stability and forcing for the
potential for thunderstorms. A drier and warmer weather trend will
continue Wednesday. The convective potential for thunderstorms
increases on Wednesday and with skies being a little more clear,
the surface will warm more efficiently and aid with instability.
For Thursday, southerly flow will return to the coast and
temperatures will begin to cool again as a front approaches
Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A low over the western Bering Sea remains stalled over the area
for the next few days. Its front stretches west of the Pribilofs
to near Adak and into the North Pacific. The orientation of this
front will allow rainfall and southerly winds approaching gale
force to sweep across the Aleutians today through Tuesday. The
front is not expected to cross the Pribilofs until late Tuesday
and at which point starts to weaken as rain shower potential and
decreasing winds continues eastward into the eastern Bering.
Wednesday afternoon, another broad swath of rain and wind lifts
northward near Atka and reaches the Pribilofs after midnight
Wednesday into Thursday before progressing eastward Thursday
night. Temperatures stay moderated in the 40s and 50s.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Bering Sea/Aleutians: Wednesday night, a weak triple point low
develops over the central Aleutians and tracks into the Bering
Sea, pushing a front from the central Aleutians to the Alaska
Peninsula through Friday. Confidence remains moderate that gale
force winds will develop and persist along this front. Localized
gale force gap winds, however, will be likely from Atka eastward
into Bristol Bay Wednesday through Friday. Seas are not expected
to exceed 20 feet.

Gulf of Alaska: With high pressure remaining in place over the
Gulf, winds less than gale force and seas less than 20 feet are
expected for the days 3 through 5 period.

&&

.Long Term Discussion (Days 4 through 7: Thursday through Sunday)...

Looking ahead into the extended forecast, the latest operational
model runs are in good agreement with the evolution and
progression of the synoptic scale features, but diverge in the
specific details towards the end of the period. A strong ridge of
high pressure seems to be a key dominant player influencing
Mainland Alaska with temperatures expected to be above normal and
precipitation below normal across Southcentral. With the exception
of the higher terrains and favored upslope areas, the potential
for any diurnally driven convection associated with weak
shortwaves seems to be low given the subsidence aloft and weak
instability. By midweek, the ridge axis slowly shifts east and
stalls in the eastern mainland and parts of the Panhandle in
response to a deepening trough and embedded upper low in the
Bering Sea. Most of the ensemble runs are taking the Bering upper
low a more northerly track rather than northeasterly between
Thursday to Friday. Another upper low forms at the base of the
trough and moves from the North Pacific into the Gulf Alaska late
next week. Meanwhile, the approaching trough and a surface low
will lead to increasing moisture and rainfall chances first for
the Aleutian Islands and then spreading into Southwest Alaska
along and ahead of the attendant front. The aforementioned ridge
may begin to break down across much of the mainland over the
weekend but confidence is low if any precipitation makes its way
over the region.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AF
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...VR
MARINE/LONG TERM...MF