Gale Warning 549, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
408 PM AKDT Fri Sep 29 2023

.ANALYSIS AND UPPER LEVELS...

A deep vertically stacked low is centered around 330 nm SSE of
Atka this morning. The low bottomed out last night at hurricane
force (as per satellite-based ASCAT winds), with a central
pressure around 965 mb. The low is tracking slowly off to the
E-NE, remaining south of the Aleutians. The leading frontal system
is lifting northward across the central to eastern Aleutians and
southern Alaska Peninsula, bringing widespread rain and strong
gusty winds. Out ahead of this system, a weak neutrally tilted
trough is crossing southern Alaska, with light rain falling across
much of the Copper River Basin. Snow levels are roughly 3000 to
3500 feet, bringing a fresh coating of snow to surrounding
mountains. The rest of southern Alaska is dry with a mix of sun
and clouds. Benign weather also prevails across the western
Aleutians under the influence of high pressure.

&&

.MODEL DISCUSSION...

While there is consistency with the overall pattern, there
continues to be minor shifts in timing/placement of key features
this weekend into early next week. All solutions show a trend
toward more spacing between the deep low crossing the Gulf Sunday
and the trailing Arctic short-wave. With less interaction of these
two features, the overall QPF forecast for Sunday-Sunday night
looks less impressive. However, there is good agreement on the
progression of the Arctic trough, reinvigoration of precipitation,
and arrival of Arctic air across Southcentral Monday-Monday night.
Minor differences in progression of this trough are key to
figuring out where the heaviest precipitation will fall. At this
point, favored areas include eastern Prince William Sound and the
Copper River Basin.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. A front
lifting northward across the Gulf on Saturday will spread rain
onshore into Southcentral. However, strong easterly flow across
the Kenai and Chugach Mountains should keep things dry at the
terminal. Even if there are some sprinkles or light rain,
ceilings/vis will remain safely in the VFR category.

&&

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

A neutrally tilted, upper level trough tracking across southern
Alaska is promoting light rain showers across much of the Copper
River Basin this afternoon. Lowering snow levels are bringing
light snow to the surrounding mountains. Isolated shower activity
is expected to linger across the higher terrain surrounding the
Copper River Basin into Saturday morning.

A strong, vertically stacked low currently near the Aleutians
will push its surface front from the western Gulf to the northern
Gulf through the day Saturday. Strong winds, including widespread
gales across the open waters, and rain will first affect Kodiak
Island Saturday morning. The front will continue lifting north
toward the Southcentral coast, bringing rain high end gales right
up along the coast late Saturday. Northeasterly winds will develop
across Cook Inlet, the Anchorage Bowl, and the Matanuska Valley
late Saturday into early Sunday as the front continues north and
the pressure gradient between weak ridging over the Yukon and the
low over the AKPen tightens. Strong easterly-southeasterly winds
will also increase for Prince William Sound on Saturday night
with the heaviest rain falling along the immediate coast. Inland
areas, especially those in the lee of the Chugach and Kenai
Mountains, will likely see occasional sprinkles or very light rain
for Saturday due to a strong cross-barrier flow.

On Sunday, the low weakens as it moves east across the Gulf, and
winds and precipitation both subsequently decrease. By Monday,
another upper level trough moves over the region while an
associated weak surface low develops over the northern Gulf/Prince
William Sound area. Thus, expect rain and high elevation snow
across much of Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Tonight through Monday afternoon)...

The forecast remains on track in the short-term for the Bering Sea
region and Southwest Alaska. Currently, a North Pacific low, just
south of the Central Aleutians, is expected to continue on its
east-northeast track to just south of the southern Alaska
Peninsula (AKPEN) by Saturday morning. Gale-force winds and light
to moderate rain will accompany this system, especially along the
occluded front in the south-central to southeastern Bering.
Easterly Kamishak gap winds also pickup tonight into Saturday
morning for interior Bristol Bay before diminishing Saturday night
as the front pushes further into the Gulf of Alaska. Cooler air
will begin to filter in on the backside of the system for Sunday.
This cold air will be re-enforced Sunday night and Monday as an
upper-level shortwave trough digs southward from the Arctic to
over the Southwestern Mainland. The main impact will be increased
gap winds through the favored terrain of the AKPEN. Temperatures
Sunday night/Monday morning look to drop to the lower to middle
30s for the Kuskokwim Delta and Bristol Day, while middle to upper
20s are likely for the Lower Kuskokwim Valley with the coldest
temperatures around the western Alaska Range. Meanwhile, the
weather across the western Bering Sea and western Aleutian Chain
remains relatively benign as high pressure persists into Monday.

&&

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

The start of the long term period next Tuesday begins with poor
deterministic model agreement. Thus, more weight has been given to
the ensemble means. Ensembles depict a large upper low anchored
over the Beaufort Sea on Tuesday. A second upper low is forecast
over the North Pacific well to the south of the Eastern Aleutians
by roughly 600 nm. Through the end of the week a new developing
low pressure center over Kamchatka will overtake the North Pacific
low, with a longwave trough becoming established from the
southern Bering into the North Pacific by next Friday. As the
Kamchatka low moves into the Bering Tuesday and Wednesday, a
large front will spread across the Western and Central Aleutians.
This should bring widespread showers and potentially elevated
winds into the Eastern Aleutians and then northeastward into the
Kuskokwim Delta as the front weakens and begins to dissipate. Much
of Southwest Alaska will be the beneficiary of high pressure,
likely experiencing generally dry conditions through the long term
period. Interior Southcentral may also see initial shower
activity on Tuesday and Wednesday dry out for Thursday and Friday.
Along the coast, however, a wet pattern may be ongoing as
southwesterly flow sees multiple shortwave disturbances digging
into coast through the end of the long term period. It should be
noted that the deterministic GFS has much drier conditions in
place while across the Gulf coast and the Gulf itself, as high
pressure builds back into the region. On the other hand, the EC is
more in line with ensembles. Which bring potential heavy rain
over Prince William Sound and along the coast, with lighter rain
moving further inland into Southcentral. All that being said,
forecast confidence in the long term is low.

&&

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