Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 210103
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKDT Sun Aug 20 2017

.ANALYSIS AND UPPER LEVELS...
Another cold upper level trough that originated over the Arctic is
currently located over the Interior and digging toward the Southwest
mainland of the state. A fairly strong ridge over the central
Bering Sea is pushing into the trough causing gusty north to
northwest winds over the eastern Bering Sea and Southwest Alaska.
The surface low is in the Gulf of Alaska and is strong enough to
bring northerly winds to much of Southcentral Alaska as well.
One interesting note with the pattern over Southcentral is that,
if it was wintertime, it is a pattern that would bring in a lot
of snow to the main population centers of Southcentral Alaska. All
the precipitation will be rain with this system, but the overall
pattern would be a snowy one if it was winter.

&&

.MODEL DISCUSSION...
Models remain in good agreement both in regards to one-another
and with run-to-run consistency. All show this latest Arctic
trough dropping out of the interior of the state, through the
Bristol Bay region overnight and into the Gulf of Alaska for
Monday and Tuesday. By Wednesday models are also all indicating
moist southwest flow returning to Southwest and Southcentral
Alaska and therefore more rain. Confidence is increasing for this
mid-week pattern to develop.

&&

.AVIATION...
PANC...Rain will persist but the low level flow is from the north
instead of the southwest with this system which is different from
most of the recent rainfalls. What is pertinent about this is
ceiling heights generally are higher with northerly flow than
southwest flow.

&&

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

Widespread steady rain will persist across most of Southcentral
Alaska tonight as a surface low tracks northward across the Gulf
out ahead of a strong upper level trough/low dropping south across
Southwest Alaska and into the southwest Gulf. The upper level low
will track south of Kodiak Island late tonight then continue
east-southeast across the southern Gulf on Monday. The surface low
will be pulled southward toward the upper low leading to a
gradual tapering off of rain over Southcentral on Monday,
lingering longest along the Gulf coast. The northern half of the
Copper River Basin, from Glennallen northward, will remain on the
northern periphery of the rain shield, so expect little or no rain
from this system.

Aside from the rain, also expect robust winds with this storm
system. Expect widespread Small Craft to Gale Force winds across
the Gulf coastal waters tonight through Monday. The strongest
winds will be along the south side of the Alaska Peninsula,
including Shelikof Strait. The passage of the upper low and
accompanying jet streak will combine with tightening low level
pressure gradients to really accelerate winds through bays and
passes.

Look for a warmer, drier day across Southcentral on Tuesday as the
Gulf storm exits and weak ridging slides in from the west. Expect
increasing sunshine from the western Gulf to Cook Inlet (including
Anchorage and the Matanuska Valley) with favorable north to west
flow. It looks like clouds may hold in farther north and east on
the periphery of the exiting low and a trough crossing interior
Alaska.

There is high confidence in another widespread rain event beginning
Wednesday afternoon as a broad trough moves in from the Bering
Sea, setting up deep southwest flow. The rain will likely linger
through late in the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A highly amplified pattern is on tap with an upper low over
southwest AK and the ridge axis to the west. The models continue
to drop this low south-southeastward. In the wake of the low
passage, high pressure will build in and expect a tight pressure
gradient from Bristol Bay to the AK Pen where gusty winds are
anticipated. Additionally, as the ridge moves inland expect the
skies to become partly cloudy and clear in several locations on
Monday. Tuesday will be mild and pleasant.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure will continue to build over the Bering Sea,
resulting in widespread stable conditions over much of the area.
The caveat being the eastern Bering/Aleutians and Alaska
Peninsula...where a tight pressure gradient will set up.
Anticipating gap winds south of the Alaska Peninsula to ramp up
tonight and reaching Gale force by tomorrow morning. As time
elapses a new low will approach the archipelago from the
southwest bringing rain to the western Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Wednesday night, there will
be a deepening upper level trough over the state of Alaska with a
subtropical warm front at the base of this trough over the North
Pacific. Some of this moisture will be moving north toward the
Gulf, so this system will have some enhanced moisture aiding it.
Rather widespread rain in southwesterly flow aloft will be
impacting most of Southern Alaska through Thursday night. However,
by Friday, the main upper low and associated trough position will
dig into the Gulf of Alaska, so the steadiest rainfall will begin
to shift towards the coastal areas and Bristol Bay while more
showery conditions (instead of steady rain) develop over interior
locations. All the global numerical models depict a similar fate
to the upper trough as it weakens in place over the North Gulf
Coast through the weekend, likely keeping some clouds and shower
threats through the weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...119 120 130 131 139 150.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...JA


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