Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 230009
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
409 PM AKDT FRI JUL 22 2016
.ANALYSIS AND UPPER LEVELS...
IR and WV satellite show a mid level low spinning over the
Kuskokwim Delta this afternoon with a front stretching eastward to
the Copper River Valley. A ridge upstream from this feature has an
axis along Kamchatka with zonal flow across the Bering. At the
surface, a frontal boundary pushed from the southwest mainland
across the Kenai into the southern mainland by the late morning
hours. Rainfall amounts averaged up to one tenth of an inch with
the frontal passage. A surface low associated with the upper level
system, is centered near Bethel with rain slowly spreading south
and east early this afternoon.
To the west, a surface ridge is keeping low clouds an patchy fog
across the eastern Bering. Westerly flow is the dominant wind
direction with gusty winds along the Aleutian Chain as the gradient
is squeezed between high pressure to the north and low pressure
along the North pacific.
Models are generally in good agreement with the synoptic pattern
bringing an active weather regime through the upcoming weekend.
There are differences between the surface pressure centers on the
00Z and 12Z models runs which leads to some uncertainty with the
highest rainfall accumulations tonight into Saturday as the low
center moves from the Kuskokwim Delta to the Gulf. The main
forecast adjustments were made to reflect higher accumulations
tonight into Saturday across Southcentral and the Gulf using the
Canadian Regional guidance. The Southwest mainland and Bering only
needed minor changes and were updated with the NAM.
A wetter pattern remains on tap into the weekend across the
southern mainland. The upper low across the southwest mainland
slowly shifts to the gulf on Saturday with a broad surface low
developing across Southcentral tonight. This low becomes better
organized on Saturday with a center focusing along the Northeast
Gulf. Once the low moves into the Gulf, Southcentral areas will
see breaks in the clouds with occasional showers developing in the
wrap around flow. A second upper level system drives a front into
southwest Alaska early Saturday morning. This system will stall as
it re-organizes into Sunday. Expect directional wind changes
through the weekend with gusts along frontal passages. Also,
stronger winds increase on Saturday across the Southwest and
Kodiak Island as low pressure sets up in the gulf.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
As the upper level low slides into the Gulf tonight, a plume of
moisture will take aim at Southcentral Ak. Rain...heavy at times
overnight...will fall over most of the region before tapering off
to a few showers Sat morning. A few embedded thunderstorms are
also possible after midnight tonight over the marine waters south
of Prince William Sound.
Some ridging will occur during the day on Sat and this will
reduce precipitation to shower activity. The next upper wave then
begins to swing through the region, with more significant rainfall
expected to begin Sat evening and persist through Sun.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The upper level trough will continue to push southeastward through
the Southwest mainland this evening, and into the western gulf by
early Saturday morning. Widespread showery conditions will briefly
lighten up over much of the area overnight tonight, with
northwesterly flow increasing. However, we will continue to see
upsloping along the mountains in this northwesterly flow, which
will keep areas of steady rainfall along the mountains.
A shortwave trough traveling through this northwesterly flow will
reach the Kuskokwim Delta Saturday morning, then rotate across
the remainder of Southwest Mainland through Saturday night,
bringing another round of steady rainfall to the area. The
associated upper level low will settle south to become centered
over Southwest Alaska Sunday and Monday.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Flat upper level ridging will build over the Bering tonight as
the upper low to the east exits into the Western Gulf. Persistent
Northwesterly flow aloft will bring a couple of troughs southward
along the Southwest Coast and Eastern Bering, while generally west
to east running frontal boundaries linger south of the Aleutians.
The system in particular diving down from the northwest on
Saturday will bring Wet and windy conditions to the Eastern Bering
and Aleutians through Sunday night. Whereas further west, a
frontal system on the northern periphery of the high over the
North Pacific will bring rain to the Western and Central Aleutians
Saturday night into Sunday morning.
.LONG TERM FORECAST (Days 3 through 7)...
Late Sunday there will be a northerly 80 knot jet streak over the
Bering Strait and northerly 90 knot jet stream over the Kamchatka
Peninsula that will transition to a westerly flow over the
Western ALeutians by Sunday night. These synoptic features merge
into one jet streak Monday as they dig into the base of the
longwave trough located south of the Central Aleutians. By Tuesday
evening through Wednesday this jet streak transitions to a 110
knot southwesterly jet streak south of the Eastern ALeutians
extending into the Gulf of Alaska. Meanwhile, an upper level
closed low over the Lower Kuskokwim Valley becomes elongated as it
tracks south of Eastern Aleutians by Tuesday afternoon. This will
result in a deep southwesterly flow in the North Pacific ocean as
the jet stream moves into the Gulf, and extends into the Southeast
Panhandle by Thursday. Therefore, look for a wetter pattern as
shortwaves track through the Southwest Alaska region, and swing
around the base of the longwave trough before moving through the
Southcentral region. This synoptic scenario will bring abundant
moisture, and precipitation to the Southwest/Southcentral regions
MARINE...Gale Warning 130 131 132.
SYNOPSIS AND MODEL DISCUSSION...kh
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...pepe