Small Craft Advisory 751, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKDT Sat Sep 24 2022

.ANALYSIS AND UPPER LEVELS...

A large, stacked low remains embedded within the broader envelope
of a longwave trough stretching across the outlook area, with the
center now drifting just west of Bristol Bay. A plethora of
smaller embedded waves continue to rotate around the primary low,
and these smaller swirls of energy are helping to kick off
periodic showers at times for just about everywhere near and east
of the Bering this morning. Cool air pooling across warm sea
surface temperatures (nearing the climatological maximum for the
year) around the Bering low continues to support an unstable low
level atmospheric profile, especially where the coolest
temperatures are located in the mid to upper levels around the
low center. The convective shower activity this instability is
facilitating has been deeply developed at times to produce
lightning overnight, namely over Bristol Bay and just south of the
Alaska Peninsula (AKPen). Farther east across Southcentral,
showers continue to push north with more southerly and onshore
flow and embedded areas of shortwave energy. Across the higher
elevations of the various mountain ranges, some of this
precipitation continues to fall in the form of snow as snow levels
become seasonably lower in altitude, including up around Hatcher
Pass, Paxson and Eureka. Winds across the entire outlook area are
relatively light at this time, with even west to northwest winds
across the Bering and along the Aleutian Chain dropping below gale
force.

&&

.MODEL DISCUSSION...

Model agreement remains good at the large scale over the next
several days, but, as is often the case, the smaller scale details
are a little less clear. In the near term for today and tonight,
this is largely limited to difference for the timing of
precipitation associated with the numerous small waves pivoting
across both Southcentral and Southwest around the upper level low.
Some bigger discrepancies appear with a new low expected to trek
swiftly north into the eastern Gulf by Sunday. The latest few GFS
runs are a clear outlier for this system, moving the center up
towards Cordova by early Monday morning. The remainder of current
model solutions show a track farther east and outside of the
outlook area into Yakutat. This track discrepancy has significant
implications for the Southcentral forecast, with much wetter,
windier (and perhaps snowier) conditions possible between Sunday
and Monday across eastern parts of Southcentral if the more east
GFS scenario comes to fruition.

&&

.AVIATION...

PANC....Light winds out of the southeast may still gust up to
around 20 kts for a few hours later this morning, but will become
calmer again for most of the afternoon. Another batch of rain
showers is expected to move in tonight, but it does not appear
likely for the onset of shower activity to drop ceilings
significantly at this time. Gusts again to around 20 kts out of
the Turnagain Arm should resume at the terminal once more by
early Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Sat
through Mon)...
It will be a mixed bag of clouds, rain, snow, and some potential
sun breaks for the weekend. A broad trough remains parked along
the west coast of AK. A broad upper level low remains with the
base of the trough. The low continues to shuttle several
shortwaves from south to north from the Gulf over the coast. One
wave moved through the Copper River Basin overnight, leading to
some light snow for the Paxson area with temperatures holding
right around the freezing mark. In the wake of this wave, some
breezy southeast winds will develop through the typical gaps (Knik
Arm, Copper River Basin, etc).

Onshore flow will keep precipitation more steady along the Gulf
Coast, especially from Prince William Sound over to the southern
Kenai. For inland areas though, the atmosphere will trend slightly
drier and more unstable. This should yield more showery
precipitation, but also the potential for some sunny breaks in the
clouds. Over the Gulf, there will be enough instability over the
warmer waters that we have introduced isolated thunderstorms
through at least this evening. The next wave will lift north
across Cook Inlet tonight into early Sun morning bringing the next
best chance of organized showers. By Sun afternoon, generally
drier (relative) conditions will develop across most of the area.
However, given the unstable nature of the atmosphere, any sun
breaks could be enough to trigger shower development in the
afternoon, especially near higher terrain.

By Monday, models continue to struggle mightily with the placement
of a surface low near Yakutat. So, PoPs and winds have been broad
brushed with low confidence in the forecast for that time period.

&&

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

A longwave trough is centered offshore of Cape Newenham this
morning with areas of cloudiness and isolated light showers
spreading into Southwest downstream of the trough's axis. This
trough continues to promote largely benign conditions across
Southwest with light and variable winds and sparse showers. The
trough's slow eastward progress will see conditions change little
through the weekend. The most notable feature is a smaller
shortwave trough currently pushing across the Alaska Peninsula
near Nelson Lagoon. While no major impacts are anticipated, an
uptick in shower activity can be expected Sunday into Monday as
the wave is lifted northward across Southwest within downstream
southerly flow aloft. Otherwise, daily high temperatures across
Southwest should remain in the mid to upper 40's while lows will
continue in the 30's.

&&

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

The axis of a longwave trough is positioned over the eastern
Bering Sea this morning. West and upstream of the trough, cold air
advection and northerly flow permeates the western and central
Bering with scattered convection and low stratus. Northerly small
craft winds extend from Saint Matthew Island down into the Central
Aleutians while calmer conditions are noted east of the trough
axis from Nunivak Island into Bristol Bay. Winds will remain
elevated over the rest of the Bering Sea into Sunday. By late
Sunday, however, a Kamchatka low is forecast to dig into the
Western Aleutians with the return of gales to the forecast. The
winds will accompany an occluded front while the low itself tracks
eastward through the Aleutian Chain into Monday. Seas over the
western Bering Sea could approach 20 feet in some areas.

&&

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

Gulf of Alaska: A well developed low and front moves over the
Eastern Gulf by Tuesday. High end gales spread across the Central
Gulf through Tuesday, diminishing. Seas to 18 feet, subsiding. A
second front moves into the Central Gulf by Wednesday. Widespread
gales with the front. Seas to to 18 feet Wednesday.

Bering Sea/Aleutians: Models in good agreement. A well developed
low and front moves along the Aleutians into the Alaska Peninsula
through Wednesday. Widespread gales wraps the low and front
through Wednesday. Seas to 22 feet through Tuesday, subsiding.
A new low and front enters the Western Aleutians and Bering
Wednesday. Winds increase to gales, with seas 10 feet Wednesday.

&&

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

Moderate model agreement through this timeframe in the forecast.
A spatially large upper trough sets up midweek in the eastern
Bering Sea and the Alaska Peninsula with a weak ridge over the
western Aleutians. Southwesterly flow becomes southerly in the
Gulf and Southcentral Alaska as the eastern Bering low moves into
the Gulf by Wednesday afternoon. This will keep the persistent
wet and unsettled pattern across the Gulf of Alaska where it is
looking to remain through Friday. Another low is expected to
develop to the western Aleutians through Thursday, pushing the
ridge east. Model agreement begins to break down a little bit from
Thursday into Friday, primarily with the timing, and therefore
location, of the low pressure centers and the amplitude of the
pattern. Generally though, there is still reasonably good
agreement in the overall features. Going into Thursday night, we
are monitoring the potential for a storm system to move out of
the Pacific into the Bering Sea, with relatively higher winds,
precipitation, and seas possible. It is way to early to even
speculate on potential weather impacts. Stay tuned as we'll
provide updates as we get closer in time.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 178,411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
MARINE/LONG TERM...KZ/VR