Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 281200
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 AM AKDT Fri Apr 28 2017

.ANALYSIS AND UPPER LEVELS...
Another interesting day played out yesterday weatherwise across
Southcentral. The northward building of strong ridging across the
Gulf yesterday afternoon caused the light southeast or variable
winds earlier in the day to turn southwesterly in the afternoon.
Meanwhile, pressure gradients remained favorable for light
southeasterly flow to continue across the Kenai and Chugach
mountains at the same time. Thus, a convergence zone set up right
along the Kenai mountains by late in the afternoon. Coupled with
orographic uplift, strong shower activity quickly formed in the
afternoon along a line paralleling the Kenai Mountains.
Convergence was weaker initially further north for Anchorage and
the Matanuska Valley, but increased through the late afternoon,
resulting in a nearly stationary line of showers from south of the
Sterling Highway on the Kenai Peninsula all the way through
Palmer. The showers grew upscale with locally enhanced convergence
along the Turnagain Arm towards evening, which led to showers
able to generate impressive downpours for April across Anchorage
through the evening and into the overnight.

Showers and convective development also appeared once again over
Southwest Alaska yesterday, but their coverage was much sparser
than across Southcentral. This was largely due to the atmosphere
being somewhat drier across the area, and with no extra
convergence zones to aid in convective development across the
region. For the eastern Bering, the stacked low continued to
weaken and drift eastward through the day, keeping a similar
weather pattern over the region, though the weaker low was less
able to produce the coverage of shower activity and associated
clouds than in previous days. Over the western Bering, a strong
front continues to rapidly press eastward, leading to short
duration gale force winds and heavy rain as it passed through.

&&

.MODEL DISCUSSION...
The models start out in close agreement regarding the next low
and associated front moving across the Bering, but diverge a bit
Saturday on how quickly the parent low transfers its energy
towards the front and becomes stacked across the central Bering on
Saturday. Since this divergence is somewhat minimal and for most
populated areas does not represent a significant difference in the
overall forecast from model to model, the higher resolution NAM
was the choice for this morning`s forecast. The primary forecast
challenge today remains much the same, determining the coverage
and intensity of shower activity. Other challenges are determining
the impacts from the front moving across the Bering and wind
funneling through the gaps in Southcentral, particularly Turnagain
Arm.

&&

.AVIATION...PANC remains on the edge of VFR condition, but
occasionally lower to MVFR as passing showers move through today.
Look for the winds to remain out of the south, but will increase
by late morning resulting in gusty conditions before weakening by
this evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The cloudy, showery pattern will continue today before drying
slightly over the weekend. Plentiful moisture, cold air aloft,
and seasonable temperatures will continue rain showers through the
day today. The coverage and intensity should be a bit weaker than
the last couple days as forcing is weaker. A weak low pressure
system forming in the Gulf on Saturday will reverse the pressure
gradient, bringing a bit more cross barrier flow to the Cook Inlet
region. Drying should occur from north of Kenai through the
Matanuska Valley with only mid-high level clouds remaining. The
approaching disturbance will affect the Kachemak Bay area late
tonight before the system moves northward and dissipates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Low pressure remains dominant through the weekend bringing
continued chances for rain showers. Weak surface flow across
Southwest becomes southeasterly on Saturday and Sunday as a front
moves into the central Bering. Patchy fog is expected to re-
develop along the Kuskokwim Delta coast and potentially spreading
inland during the early morning hours. Fog conditions will
quickly improve by late morning with diurnal heating.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A Gale force front currently bringing gusty winds and moderate
rain to the Western Aleutians/Bering will move into the Central
Aleutians/Bering by this afternoon. The gusty winds and rain
associated with this system will push eastward into the Eastern
Aleutians/Bering Saturday, as the parent low weakens and moves
over the Western Aleutians. By Sunday, the low pressure system
weakens further with scattered showers lingering across the Bering
and Aleutians as weak short waves propagate around the associated
upper low.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The weekend will be characterized by a low moving through the
southern Bering that will keep cloud cover over much of the
southern mainland and Aleutians with continued shower activity.
As the low tracks across the Aleutians and into the Pacific on
Monday it will push a front into Southcentral Alaska bringing more
steady rain to the north Gulf coast. After this point the models
vary greatly on what to do with the low and another low
approaching the area from the south. Ensemble guidance was used
for mid week which brought a more middle ground and consolidated
low position into the Gulf Wednesday, which is also slightly
weaker than some of the operational models.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING...172 174 177 411.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...DK


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