Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 241243
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Mon Jul 24 2017

.ANALYSIS AND UPPER LEVELS...
Precipitation has returned to Southwest Alaska this morning as a
front trailing from a low over the North Slope tracks from the
Kuskokwim Delta into the Bristol Bay region. Marine stratus and
fog maintain a heavy influence ahead of the front as onshore flow
remains focused over Bristol Bay. The front also extends into the
central Bering, bringing rain to the Pribilof Islands and
portions of the Central Aleutians. Despite the advance of the
front, extensive low stratus and fog continue to dominate much of
the Bering beneath the flattening ridge which now roughly
parallels the Aleutian chain as it sinks towards the south.
Southcentral AK is seeing an increase in mid level cloud cover
with the ridge axis shifting southward into the Gulf as an upper
level trough embedded within a quasi-zonal flow pattern moves
across the mainland. The marine layer has begun to push ashore
along the North Gulf Coast and also advance up Cook Inlet aided by
onshore flow, bringing localized fog or lower ceilings to some
locations.

&&

.MODEL DISCUSSION...
Models continue to show good agreement on the large scale features
through the middle of the week. The zonal flow pattern looks to
become much more amplified by Wednesday as an upper low drops out
of the Russian Arctic and intensifies over the west coast of
Alaska, keeping a more active pattern going into the middle of the
week. The primary short term forecast challenges continue to
revolve around the influence of the marine layer in the coastal
regions and precipitation chances as the front progresses
eastward. Continued to primarily utilize Hi-Res NAM guidance for
the morning forecast package.

&&

.AVIATION...
PANC...Gusty southeast winds will continue to come in off Turnagain
Arm through about mid-day Monday at which point gradients will weaken
and flow will shift around to up Inlet. Stratus will make its way
up Cook Inlet early this morning, but the Turnagain winds should
keep it all south and west of Fire Island. Flow will become more
favorable Monday night for a possible push of stratus (low MVFR or
IFR ceilings) into the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A well developed upper level trough is approaching the
Southcentral Region this morning, and supporting showers along the
Alaska Range. However, radar indicates precipitation echoes
weakening fairly quickly as they drift off the mountains. Models
are indicating that this trend will continue through much of the
day. With minimal forcing at the surface, rainfall should be
confined mainly to the Alaska Range and Talkeetna mountains today,
although a shower or two could drift into lower elevations as the
trough pushes east.

The most uncertainty in the forecast is for Tuesday night into
Wednesday morning. While some models indicate precipitation only
at higher elevations, some models indicate widespread
precipitation in both higher elevations and at lower elevations
such as the west Kenai Peninsula and Anchorage Bowl. These are
also the same models that indicate a healthy fetch of
southwesterly flow all along the Inlet, which would provide
favorable low level upslope flow. As this low level flow varies
widely by models, there is uncertainty in rainfall in more
populated areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
A frontal boundary continues to moves from west to east across
the Southwest Alaska region. This synoptic feature enters the
Bristol Bay area by this afternoon bringing stratus accompanied by
rain. Look for heavier precipitation in upslope regions aided by
orographic lift. Otherwise, the rain will inhibit more dense fog
from developing this morning, but the challenge occurs after the
front exits the region. This sets up for onshore flow along the
Southwest Alaska coastline with a weak surface low developing
along the Western Capes by Tuesday morning. Therefore, look for
the marine layer stratus to advect into the AOR, and areas of
dense fog formation.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The western Bering and the Aleutian Chain remain under the
influence of a high pressure ridge, which is accompanied by
marine layer stratus/fog. A weather front is tracking across the
eastern Bering this morning before crossing over the Alaska
Peninsula by Tuesday morning. The biggest challenge remains with
how widespread the marine layer stratus/fog will advect into the
AOR. The Western Aleutians will likely have dense fog issues in
the morning time-frame as the high pressure builds into the Bering
by Tuesday morning. The wind direction remains southwesterly, and
supports the stratus/fog to advect into the central Bering
region.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The forecast period commences with a potent low over the AKPEN
and a ridge axis bisecting the Bering Sea. Generally speaking, the
synoptic pattern is amplified...very dynamic and progressive. At
the onset..sensible weather-wise, anticipate the possibility of
precipitation for the western and south central portions of the
state Wednesday night through Saturday. All the while, the western
and Central Aleutians will be under the influence of the ridge
keeping much of the region dry, however low stratus clouds and fog
will persist under the stable air mass. Late in the period, the
ECMWF and GFS models are in relatively good agreement with the
concept of another cyclone getting spun up over Russia and
traversing the Bering once again bringing a fetch of maritime
moisture into Western Alaska.



&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...SEB
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...PJS



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