Small Craft Advisory 757, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Sun Oct 4 2020

.ANALYSIS AND UPPER LEVELS...
The pattern change is beginning with the upper level trough axis
shifting eastward over Southwest Alaska. This is in conjunction
with the 250 mb jet stream moving eastward as well. The surface
low is perfectly positioned in the upper left exit region of the
jet streak putting it near Middleton Island this afternoon.
Moisture form this low is edging back into the Cook Inlet region
after a period of mostly clear skies that brought numerous below-
freezing temperatures to Southcentral Alaska this morning.

Upper level ridging over the western Bering Sea, combined with the
trough is bringing northerly flow over the Bering. A couple of weak
surface low can also be seen on GOES-17 satellite imagery south
of the Aleutians, and another one near Unimak Pass.

&&

.MODEL DISCUSSION...

Models are in good agreement through the middle of the week. The
next system of note will be moving through the Bering from
northwest to southeast and models are in good agreement with the
movement of both the low and front associated with this low.

Models begin to struggle a bit with the potential development of
a triple point low in the Gulf of Alaska toward next weekend. The
trough axis on the GFS and EC are at least similar so that does
give us something reasonable to work with in spite of other
differences.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A 980 mb low over the northern Gulf continues to lift northward,
bringing rain and gusty easterly winds to the northern Gulf coast.
As this low lifts inland, rainfall will taper through Monday
morning. While there is not abundant cold air aloft, some of the
higher peaks in the Copper River Basin may see a light dusting of
snowfall overnight, especially along the southern corridor of the
Richardson Highway. By Monday morning, this low will be exiting
the area to the east and centered over the Alaska-Yukon border.
Residual showers are likely over the Prince William Sound area,
however much of Southcentral should see improving conditions and
a break from the rain on Monday, as an upper level ridge builds
in from the west. By Tuesday, weak troughing develops over the
northern Gulf, keeping scattered showers over Prince William Sound
and nearby coastal areas. Winds over the water will be minimal,
with flow expected to be below small craft levels with the
exception of winds through Kamishak Bay and across the Barren
Islands Tuesday night into Wednesday, where small craft levels are
expected.

&&

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

Scattered showers will continue near the Kuskokwim Delta coast
through Monday in onshore flow. Otherwise quiet conditions with
partly to mostly cloudy skies are expected across the rest of the
region, although showers or flurries remain possible near higher
elevations under weak upper shortwaves. Any lingering
precipitation will end by Tuesday under a ridge. Clouds will
increase Tuesday evening ahead of a front with moderate beginning
early Wednesday morning along the coast. Brief light snow is
possible farther inland, but will quickly change to rain in a
strong southerly flow.

&&

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

Benign conditions are expected through Monday evening over the
western Aleutians north of high pressure. Scattered showers will
persist through Monday over the central/eastern Aleutians in a
northerly flow, likewise leading to gusty winds out of bays and
passes on the Pacific side. These will gradually taper off from
west to east by Tuesday near the aforementioned high pressure. A
strong front will track over the western Bering Monday evening,
bringing gusty southerly winds (possibly storm force) and moderate
to locally heavy rain into Tuesday. It will slightly weaken as it
moves eastward into mid-week as a result of its parent low
occluding, however similar conditions are to be expected for the
central/eastern Aleutians.

&&

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

Bering/Aleutians

Early Wednesday a low will move into the northwestern Bering,
with its front stretched across the central Bering/Aleutians. Gale
force southerlies are expected ahead of the front across the
central/eastern Bering and Aleutians. Late Wednesday, this front
rapidly heads east of the Bering with winds turning west
southwesterly and decreasing to small craft advisory levels across
the region. For Thursday and Friday the low slowly weakens near
St. Matthew island as winds across the Bering/Aleutians continue
westerly at small craft levels.

Gulf of Alaska

Light westerly flow is expected Wednesday. On Thursday winds will
flip to southerly, with small craft level winds expected across
the western Gulf. On Friday winds turn northerly and fall below
small craft levels.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

A broad upper level trough is expected to extend from the central
Bering through the Gulf of Alaska and Southcentral Thursday
through Sunday. Despite the low 500mb heights, temperatures are
not expected to be much colder as there is minimal cold air
advection expected over Southcentral Alaska due to the center of
the upper low remaining over the Bering Sea. This will result in
prevailing south-southeasterly flow across Southcentral which will
help provide moisture to produce showers through the area. The
strongest cold air advection will occur west of the upper low over
the western Bering/Aleutians. This strong cold air advection is
likely to result in Ocean-Effect showers throughout the western
Bering/Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood advisory zone 121.
MARINE...Storm 178. Gales 119 120 131 177 351 352 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MK/ED