Gale Warning 449, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
539 AM AKDT Wed Aug 30 2023

.ANALYSIS AND UPPER LEVELS...

An upper low over the eastern Bering Sea is now drifting near the
Kuskokwim Delta region this morning as multiple shortwave troughs
continue to rotate around it to the south and east. One of these
shortwaves is now moving up along the Alaska Peninsula into
Bristol Bay as areas of showers follow along the remnants of an
occluded front closer to the surface between Kodiak Island and
Dillingham. A second negatively tilted shortwave is lifting from
the Kenai Peninsula and Mat-Su Valleys into the Copper Basin this
morning, with areas of steady rain now shifting into the eastern
Prince William Sound and parts of the Copper Basin.

Looking back to the west, the well-advertised extratropical
remnants of Damrey are beginning to show signs of rapid
organization into a potent low, with the center now moving a ways
to the south of the Aleutian Chain. The vortex structure leftover
from the former typhoon is phasing/merging with a deep upper
trough moving near and south of the western Aleutians. The surface
low is expected to deepen to Hurricane force as it matures and
reaches peak intensity just off to the south of the
Bering/Aleutians outlook area later this afternoon.

&&

.MODEL DISCUSSION...

Models continue to show improvement for the progression of the low
associated with Damrey's remnants as it tracks into the Aleutians
later today, then into Bristol Bay by Thursday evening. There are
sill some differences in terms of how quickly the low moves inland
over Southwest, with the NAM and Canadian models bringing the low
north towards the Interior faster than the GFS and ECMWF by Friday
morning. This has relatively minor implications from a forecast
standpoint, with the more significant impacts from this system in
the near term becoming much better dialed in.

&&

.AVIATION...
PANC...Some MVFR ceiling may move in and out through the morning
hours, but should improve to solid VFR as southeasterly winds
increase by noon.

&&

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

The front that has brought a good amount of rain to the Prince
William Sound area is lifting northward this morning and that
should bring in some temporary relief for the rain around that
area. However, the next system that is approaching the Alaska
Peninsula form the southwest, will track farther eastward than was
expected the past few days and this will have a significant impact
of the forecast for the region especially for Thursday and Friday.
The main issue is that this will bring in more rainfall to
Southcentral for Thursday and Friday. While total amounts should
be less than the past few days, eastern Prince William Sound may
see another 2 to 3 inches of rain before the weekend.

Part of the reason this next low is tracking farther east is due
to the upper level low which is also farther east Thursday night
into Friday. This will bring showers to the Susitna Valley as the
cool upper level trough moves through. This is worth mentioning
since there may be some localized areas of heavy rainfall Thursday
night into Friday with this set-up. Seeing that there remains a
flood warning out until this evening for the Susitna, Yentna, and
Skwentna rivers, this is a situation worth paying attention to.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 2: This morning through Friday morning)...

A deepening North Pacific low will cross between the Central and
Eastern Aleutians later this evening at around 965 mb. Strong
winds in the form of gales and storm force winds are expected to
follow the Aleutian Chain, beginning this morning and eventually
reaching the Alaska Peninsula by Thursday morning. Precluding the
issuance of a high wind warning, in particular for locations such
as Unalaska within the Eastern Aleutians, is the fact that the low
is forecast to occlude before crossing into the southern Bering,
thereby weakening the thermal gradient within the low. In
response, the low is forecast to weaken into the mid to upper 970
mb range by late Thursday morning. Peak forecast winds for the
duration of the low's passage should remain around 40 to 50 knots
with peak gusts of 65 knots possible, but likely remaining to the
south of the Aleutians on the Pacific side. Otherwise, heavy rains
will also be possible with the Eastern Aleutians expected to see
the greatest rainfall totals ranging from 1.00 to 1.50 inches
through Thursday.

Southwest Alaska will also join in the fun as the low and its
front finally begin to move onshore of the coast Thursday morning.
35 to 45 mph south to southeasterly winds will span from Bristol
Bay briefly into the Kuskokwim Bay. A steady flow of 35 to 45 mph
southeasterly winds are also anticipated out of the Kamishak Gap
into southwest portions of the Lower Kuskokwim Valley. Late
Thursday into Friday the flow begins to change as the low tracks
into interior Alaska. West to southwest winds will remain elevated
across Bristol Bay and on the Pacific side of the Alaska
Peninsula through Friday morning. Rainfall amounts for Southwest
Alaska won't be quite as high as parts of the Aleutian Islands,
but a broad region of 0.50 to 1.00 inch rainfall amounts are
forecast across all of Southwest Alaska through Friday morning.

&&

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

The progressive and active weather pattern looks to continue into
the long-term for both Southcentral, Southwest, and the Aleutians.
Zonal west-to-east flow over the Gulf of Alaska and Southcentral
Alaska on Saturday will give way to more of a southwesterly flow
regime for Sunday and Monday as a trough looks to dig southward
through the area. Thus, expect quick hitting systems with rain for
Sunday and Monday as multiple triple-point lows associated with
strong Bering Sea front look to move into the Gulf. There is a
little more disagreement for Tuesday with regards to the strength
and positioning of a potential Gulf low, even in the ensemble
data. Therefore, confidence is quite low regarding the
precipitation forecast for Tuesday.

Heading to Southwest Alaska and the Aleutians, confidence remains
relatively high in a number of low pressure systems tracking
eastward from Kamchatka, through the Bering Sea, and into western
Alaska. Saturday and Sunday are of note as the first of these
tracks through the northern Bering while the gale-force front
looks to sweep the Aleutian Chain and eventually making its way to
Southwest Alaska for Sunday. In addition to gusty winds, moderate rain
will also accompany this front. Pockets of heavier rain are not
out of the question, especially for the Aleutian Chain and perhaps
the Pribilof Islands for Saturday. The wet day for Southwest looks
to be Sunday with this system. Confidence decreases somewhat for
Monday into Tuesday. There is loose agreement that another
Kamchatka low looks to push another front eastward across the
Aleutian Chain Monday into Tuesday; however, there are differing
solutions amongst guidance in the southern extent of the front. As
for Southwest, Monday and Tuesday look somewhat quieter with upper
ridging trying to build back in from the Bering.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
LONG TERM...DAN