Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 231244
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
444 AM AKDT Sat Sep 23 2017

.ANALYSIS AND UPPER LEVELS...

The upper low that has been centered near Norton Sound the past
two days is finally beginning to make its much anticipated move
southward towards Bristol Bay. As with many upper lows the
unsettled weather continues east of the low were warm, moist air
is riding up and over the cooler air mass associated with the
upper level low. As a result, all of the radars from King Salmon
eastward are showing widespread light rain falling over much of
the area. As the upper low shifts southward, a surface low that
has been gradually shifting eastward along the Aleutians will
pivot northward into the western Gulf. The associated
precipitation is reorienting from a southwest to northeast
orientation like it is now towards more of an expected southeast
to northwest one. This spells upcoming changes to the
precipitation and wind patterns across all of southern mainland
Alaska as the favorable mountain-parallel southwest flow becomes
mountain-perpendicular southeast flow. More on that in the
forecast discussions below.

With surface high pressure and weak upper level ridging in place over
the western Bering, much of the rest of the Bering is experiencing
a cold north to northwest wind, leading to widespread cellular
stratus with occasional showers, particularly on the windward
sides of the eastern Aleutians and southern Alaska Peninsula.

&&

.MODEL DISCUSSION...

The models start out the forecast period in decent agreement, but
significant differences arise fairly quickly. This is not
surprising given the complex pattern and the low south of the
Aleutians interacting with the terrain of southern Alaska. The GFS
was the most notable outlier, especially by Sunday night,
maintaining a closed low over the northern Gulf while all the
other models preferred a weaker open trough. It also is the
furthest west with the low that develops over southwest Alaska on
the back side of the system by Monday morning.

Temporal continuity is also very poor with the latest model runs
this morning. The consensus of all the models has shifted eastward
and stronger with the low over the western Gulf Sunday. As a
result, there have been substantial changes with regards to the
wind pattern made with the morning package. Due to the back-and-
forth nature of the model consensus, forecast confidence is low.
Thus, potential future changes in the models may result in further
large changes to the forecast.

&&

.AVIATION...

PANC...The main weather problem has shifted from rain to wind
shear this morning. There is a rapid change of wind direction
with height from calm or light northerly near the surface to 25-30
kt southeasterly winds just above the surface layer. Otherwise VFR
ceilings and visibilities will persist through the day into
tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A high amplitude trough digging south of the Alaska Peninsula
this morning will become negatively tilted as it moves into the
Gulf later today, with the remnants of an upper low lifting north
out ahead of it. This will lead to strengthening of a surface
frontal system lifting northward across the Gulf as well as
development of heavy rain. Strong southeast low level flow out
ahead of the front will bring the heaviest rain to western Prince
William Sound as well as the west side of Cook Inlet. On the other
hand, this flow will produce a downslope hole in the lee of the
Kenai and Chugach Mountains, helping to dry out portions of the
western Kenai on up to Anchorage and the Mat-Su.

Low level flow will rapidly weaken as the front moves onshore
along the north Gulf coast this evening. This may allow some areas
of rain to make it inland. There is come uncertainty in the track
of key features. The latest trend in model guidance is to develop
a closed low in the upper levels and drive it inland across Prince
William Sound and into the Copper River Basin, pulling a trailing
surface low near Kodiak Island the same direction. Thus, it is
looking like the Cook Inlet to Susitna Valley corridor may just
see a few showers or brief periods of light rain tonight through
Sunday night, with the bulk of the rain remaining in Prince William
Sound.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Low pressure over the Southwest keeps an ongoing wet pattern
across the region into the beginning of next week. A surface low
just south of Kodiak Island lifts north today spreading rain to
the Kuskokwim Valley. Western areas of the Alaska Peninsula will
see less rain today as the low lifts north, however a developing
low over the Kuskokwim Delta gains southward momentum spreading
rain back across the Peninsula tonight. Northerly flow is expected
to be gusty along the Peninsula as the pressure gradient tightens
between the developing surface low and a ridge holding place over
the western Bering. Patchy fog along and just west of the Aleutian
Range continues to reduce visibilities this morning, then should
improve this afternoon as the surface low lifts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The eastern Bering is expected to continue under a wet pattern as
low pressure develops along the Southwest coast today and shifts
slowly south through the weekend. Gusty northerly flow keeps
small craft winds across much of the eastern waters through
Monday. The western Bering stays under the influence of a ridge
which shifts south of Shemya on Monday with light winds and
patchy fog for prevailing conditions.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Sunday evening, with an
upper level low tracking into the eastern Bering and Bristol Bay
area, then crossing the Alaska Peninsula and heading back into the
north Pacific Tuesday. Model guidance struggles with this feature
as it progresses into the later part of next week. Some solutions
bring the upper low into the Panhandle by the end of next week,
where others curve it into the western Gulf as early as Thursday
afternoon. As can be expected, the surface features are also
showing a decent spread in their solutions, keeping lower
confidence in the forecast for this time frame. What can be said
with some certainty is that there could be a window of slightly
drier conditions during the early part of next week. Rain will
then put an end to these briefly drier conditions by mid-week, as
the north Pacific system pushes a front into Southcentral and
parts of the southwest Mainland through the end of the week.

The potential for gusty winds developing along the Gulf coast and
Cook Inlet region will also largely depend on which model solution
pans out. Depending on the track of the surface low that tracks
from the north Pacific and into the Gulf mid-week, there is some
potential that we can see an increase of gusty northeasterly
winds over the Cook Inlet and Shelikof strait areas, and/or gusty
easterly winds along the northern Gulf coast.

Further west, high pressure will try to build into the Western
Bering/Aleutians through the end of next week, when it starts
getting shoved eastward by a trough tracking in from the west.
models are loosely showing the track of this next front pushing
into the western Aleutians by the end of next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning: 121 125
MARINE...Gales: 119 120

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP



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