Area Forecast Discussion
Issued by NWS Anchorage, AK

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556
FXAK68 PAFC 301326
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
526 AM AKDT Sun Apr 30 2017

.ANALYSIS AND UPPER LEVELS...
The Central Aleutians has an upper level closed low just east
Atka, and is vertically stacked through the lower levels. This
feature is reflected at the surface with a 988 mb low located over
the central Bering with an associated front extending eastward
before crossing through the Eastern Aleutians this morning. This
synoptic feature is bringing gusty small-craft southeasterly winds
accompanied by stratus/rain through the aforementioned region.
The winds on the backside of this low pressure system are
predominantly all westerly, and reflected with the surface
observations along the Chain this morning.

Meanwhile, the northern Gulf has a shortwave pushing onshore, and
the Middleton radar imagery is showing a large swath of rain
spreading into the northern coastal communities. The Anchorage
Bowl will remain downsloped drier conditions through the rest of
the weekend. The extensive cloud cover from the shortwave entering
the northern Gulf is moderating the temperatures along the
Eastern Kenai Peninsula northward through Prince William Sound,
keeping them in the lower to mid 40`s. While farther inland areas
are receiving mid-high clouds streaming from the Gulf with high
temperatures will remain in the 50`s.

&&

.MODEL DISCUSSION...
The models continue in good agreement with large synoptic features
through the weekend. The Southwest Alaska region has a frontal
boundary sweeping through this afternoon before falling apart as
it moves inland. The broad low pressure over the eastern Bering
and Aleutians remains almost stationary before phasing in with a
North Pacific low by Tuesday morning. While the northern Gulf has
a ridge building in briefly with an upstream front moving into the
Gulf by Sunday evening. The next major North Pacific low pressure
system tracks to the lower Gulf by Tuesday afternoon. The GFS was
the preferred model of choice up to this point, but the models
are struggling with the timing/strength of this next system. The
NAM seems to be the outlier with a faster trend, and the GFS,
ECMWF, and Canadian solutions are in better agreement. Therefore,
the forecast confidence starts out above average, but diminishes
with this next system entering the Gulf by Tuesday afternoon time-
frame.

&&

.AVIATION...
PANC...Looks to remain under the influence of VFR conditions. The
only issue will be with the increase of the southerly Turnagain
Arm winds streaming into the airport this afternoon through the
early evening hours. Look for the winds to weaken by late Sunday
evening time-frame.

&&

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

The sounding this morning indicates steep lapse rates and dry air all
the way to 700 mb. As a result, while radar indicates decent
echoes and mid-level moisture, no precipitation is currently
reaching the ground. Convective resolving models such as the HRRR
indicate the possibility of another round of convective showers
developing across the Susitna Valley this afternoon and evening.
It is likely that more moisture will be available in the Susitna
Valley where downsloping and lower level drying will be more
limited.

A front in the southwest corner of the Gulf of Alaska will move
northeastward quickly today towards the northern Gulf Coast. This
front will support gusty east winds first across Kodiak Island
and then through local gaps as it approaches the mainland tonight.
While some gusts are initially anticipated in Anchorage this
evening, Turnagain Arm gap winds are expected to eventually bend
towards the south tonight due to lower pressure in the south.
Perpendicular flow should limit precipitation across the west
Kenai Peninsula and Anchorage Bowl. However, if lapse rates remain
as steep and saturated as models indicate above the mountain
tops, some spillover showers are possible in the Anchorage Bowl
tonight into tomorrow morning. &&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The trough rotating from the Alaska Peninsula, across Bristol Bay
into Southwest Alaska will increase showers across the area this
afternoon and evening. Additionally, as the trough rotates
through, easterly winds will increase with the strongest winds
through gaps and passes in the Aleutian Range. These easterly
winds will lead to downsloping in the lee of the mountains and the
development of a small rain shadow where showers are sparser.
Although showers will become less organized tonight once the
trough is past, they will pick up again Monday afternoon and
evening as the presence of the cool upper low along the Alaska
Peninsula keeps the atmosphere unstable. Diurnal shower activity
will diminish somewhat Tuesday as the upper low swings south in
response to a very strong low tracking into the Gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The broad low complex centered along and north of the central and
eastern Aleutians will gradually fill as it drifts east to be
centered over the Alaska Peninsula tonight. Ridging will build
over the western Aleutians Monday then shift east to the central
Aleutians and central Bering Tuesday and Tuesday night as a low
pressure system approaches and tracks into the western Aleutians.
The upper low of the Alaska Peninsula will drop weaken and south
Tuesday as a very strong low tracks into the Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is fair agreement amongst the models of a deep low moving
into the Gulf around Tuesday evening. However, they are struggling
with the intensity of the low which could be the difference
between a gale force low and a storm force low. For now there is
higher confidence in the low remaining gale force. Regardless,
this system looks to bring rain to areas around Southcentral
Alaska into Thursday, with the heaviest for the north Gulf coast.
Meanwhile ridging will have moved in over the Bering with another
system trying to approach the Aleutians from the west, but being
largely held at bay by the ridge.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 119 125 130 131.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DK



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