Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 181323
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
523 AM AKDT Mon Sep 18 2017

.ANALYSIS AND UPPER LEVELS...
Water Vapor satellite imagery shows a longwave trough over the
west coast and the eastern Bering with the parent low north of
Wrangle Island. Another low is spinning in place south of the
central Aleutians as a ridge centered over the Central Pacific
blocks its progression. A second ridge axis is along the western
Bering as a northern jet stream nudges in from Kamchatka. At the
surface a cold front is moving in from the northern Bering into
the southwest mainland. Rain is developing along the frontal
boundary which pushed through Bethel to Togiak early this morning.
A few showers are also developing along a line from the Anchorage
bowl to the Susitna Valley as an easterly upper level wave
propagates away from a low in the eastern Gulf of Alaska.

&&

.MODEL DISCUSSION...
Models continue in good agreement with the general synoptic
pattern through Tuesday night. There is still some uncertainty
between models as they handle two upper level lows sending fronts
toward the southern mainland heading into mid week.

&&

.AVIATION...
PANC...VFR conditions and light winds will be the prevailing
conditions through the next TAF package. Ceilings may low to MVFR
this morning as an easterly disturbance moves across Anchorage.
Light rain is expected through this evening then tapers off
tonight as a front pushes off to the east.

&&

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

An upper level low near the southwest mainland will drift into
the north Gulf of Alaska coast today. While the system itself is
only moderately strong, the associated surface low will move
across the north Gulf Coast today and create only very weak
pressure gradients. As a result, there will be little to no
downsloping, and the typically downsloped areas of the Anchorage
Bowl, western Kenai Peninsula and Matanuska Valley will see
periods of rain starting by noon. Conditions will finally clear by
Tuesday as the low continues to drift to the southeast away from
the Southcentral mainland. However, with a progressive pattern in
place, a new low pressure system will nudge into Southcentral by
Thursday, bringing widespread rain in once more.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest Alaska region remains under the influence of a
midlevel low. With a cold pool of air aloft expect showers
intermittently today. This low will rapidly trek southeastward
and weak ridging will follow in its wake. This setup will be
conducive for fog formation not only along the coastline but also
for interior locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
At the onset of the forecast period there is an area of zonal flow
over the Central Bering with a vortex to the north and a closed
low south of the Western Aleutians. High pressure will continue to
influence the steering flow and the low south of the archipelago
will follow the path of least resistance. The dome of high
pressure will keep any North Pacific storms from entering the
Bering going into the midweek time-frame. All the while, the
northern Bering has a low dropping off the Kamchatka Peninsula by
Monday as it track eastward into the Kuskokwim Delta.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Tuesday night, there will
be a shortwave trough traversing the northern two-thirds of the
state with a stout Northeast Pacific subtropical ridge axis south
of Kodiak Island. At the west end of that ridge will be a North
Pacific closed low and associated subtropical warm front
stretching across the Eastern and Central Aleutians. With the
exception of the Alaska Range and Northern Susitna Valley/Copper
Basin, Wednesday will be mostly dry across Southern Alaska.
However, this brief reprieve and quiet weather pattern will
quickly shift to a broad southwest flow regime as the North
Pacific front merges with a trough over the state of Alaska. There
is now rather high confidence in this pattern as the ECMWF, GFS
and ensemble systems have shifted toward a more amplified pattern
and increased ridging over the NE Pacific. What this means is
increased chances for widespread rain beginning Thursday and
likely continuing into the weekend. By the weekend, uncertainty
grows with the placement and strength of the arctic trough, but
eventually cooler air will filter in as the warm front shifts
east, so temps will begin to feel rather cool along with higher
elevation snow.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 130.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...JA


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