Small Craft Advisory 641, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKDT Sat Jul 22 2023

.ANALYSIS AND UPPER LEVELS...

A ridge over western Canada extends into Interior Alaska and then
turns southwest through around Kodiak. A large, nearly stationary
low in the Bering Sea has a front that extends to the southwest
Alaska coast. The interaction of this front and the ridge is
enhancing the winds along this front. In Southcentral Alaska an
upper level low south of the Gulf of Alaska is producing easterly
flow aloft due to the interaction of this low and the interior
ridge. Some weak easterly waves can be seen traversing the Copper
River Basin today. Some large areas of stratus are over the Gulf
of Alaska.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement the next few days.
Agreement is also very good between the models for the convective
parameters over Southcentral Alaska and the eastern part of the
Kuskokwim Valley through Monday.

&&

.AVIATION...

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

&&

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

High pressure continues over Southcentral with a train of easterly
waves rotating around the northern periphery of a Gulf low.
Generally dry conditions are anticipated the next few days. Strong
daytime heating will boost afternoon high temperatures into the
70s for parts of Southcentral with some areas of the Copper River
Basin potentially breaking into the 80s. With the passage of each
easterly wave a threat for thunderstorms is forecast over the
latter half of each day. Initiation should occur over the mountain
slopes initially, with storm and shower activity potentially
moving off the high terrain during the evening hours.

For tonight, fog will redevelop over the Shelikof Strait and much
of Cook Inlet. Up-inlet flow will advect the fog northward,
eventually skirting just west of Anchorage during the morning
hours on Sunday. Locations from Kodiak, to Homer and up to Kenai
will be susceptible to reduced visibilities until daytime heating
eventually works away fog mid to late morning on Sunday.

&&

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

An occluded low pressure system in the central Bering Sea will
remain quasi-stationary as it decays through Monday night. It will
push a front slowly onto the Southwest coast this evening, the
front edge of the rain bands wavering in and out with weak
disturbances tracking along the front. This means the timing of
rainfall, and indeed the overall chance of rainfall, for many
locations is fairly uncertain at this time. Looking at the
broader trends, rain gradually pushes eastward to reach Bethel
tonight and Dillingham by Sunday. The leading edge eventually
reaches about as far as King Salmon on Monday before precipitation
diminishes significantly. Despite some creeping eastward progress,
broadly, the front will be nearly stalled out as it remains
blocked by high pressure extending over Interior Alaska. This will
allow light rainfall to remain in place along the coast, leading
to 0.5 to 0.75 inches of accumulation through Monday, with lower
amounts further inland.

Meanwhile, inland locations can expect warm temperatures under
the influence of the high pressure. The hottest day will be
Sunday, when some areas in the Lower Kuskokwim Valley are expected
to reach into the upper 70s. The warm temperatures will also
contribute to a slight chance for thunderstorms along the northern
section of the Western Alaska Range each afternoon, beginning
Sunday. By Monday afternoon, though the front moving in over
Southwest Alaska falls apart and stops producing rain, the
accompanying shortwave and cooler airmass continues to nudge
slowly eastward, contributing to a gradual cooling trend for
inland areas on Monday and Tuesday.

Over the Bering Sea, fog will be a continual nuisance. The most
favored areas are the Pribilof Islands tonight, as well as the
western Aleutians this evening through Sunday. A new disturbance
enters the region on Sunday and pushes rain along the Aleutian
Chain through Monday. By that point, the disturbance moves into
the western/central Bering and takes the place of the previous
low. The front continues north to the Southwest coast and brings
a brief round of rain Monday night into Tuesday morning, before
weakening.

&&

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

Models and ensemble means are in pretty good agreement for the
long-term. The main feature of interest continues to be a broad
upper-level trough with a low embedded within located in the
Bering. Several upper-level shortwaves rotating around the
aforementioned low will bring unsettled conditions to the
Aleutian Chain and the Southwest coast. Gusty southerly winds
and rain are likely over the eastern Bering Wednesday into the
weekend as a front associated with this low and trough move inland.

Meanwhile farther east, a ridge of high pressure will try to
maintain its hold over the northern Alaska-Yukon border. While
the aforementioned ridge attempts to build southward over
Southcentral on Thursday, models suggest that a few weak easterly
waves will develop and propagate from the Yukon westward into the
Copper River Basin. This introduces the potential for scattered
showers and possibly thunderstorms for the end of next week.
However, the aforementioned trough will gradually win and push
eastward inland towards Southcentral as we head into the weekend.
Overall, expect clouds, rain, and winds to return to much of
southern Alaska in the long-term.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...BL/PP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...NS

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