Area Forecast Discussion
Issued by NWS Anchorage, AK

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587
FXAK68 PAFC 241247
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKDT Wed May 24 2017

.ANALYSIS AND UPPER LEVELS...
Looking at the latest IR image on GOES-15...the cloud shield from
this mature low is already as far inland as Denali National Park
with a cold front bisecting the AK Peninsula and a low anchoring
the occlusion near St Lawrence Island. The Bering Sea is flush
with cold air stratocumulus. The most recent ASCAT data really
captures the changes in the wind field for a portion of the cold
front south of the Aleutians. This system is tracking eastward as
advertised.


&&

.MODEL DISCUSSION...
The models are in decent agreement with the major synoptic features
at 500 mb through Saturday. The highlights include broad troughing
over the Bering and into the Aleutians...a ridge axis into SE AK
and widespread southwest flow into southcentral Alaska. Several
shortwaves on the backside of the trough enter the domain but the
timing and the amplitude varies with these smaller features. Still
waiting for the details to flesh out for this weekend but
generally expecting cool and wet pattern.

&&

.AVIATION...

PANC...The latest image from the Kenai radar clearly depicts
several bands of precipitation and the latest metar at PANC has
-RA in the observation. Expect reduced cigs and precipitation for
 the ensuing 36 hr period as the frontal boundary passes through
 the region. Additional impacts from this storm include gusty
 winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Rain will overspread most of Southcentral this morning as a high
amplitude upper level trough and surface cold front approaches
from the west. Strong and moist southerly flow will lead to
heavier precipitation along the south sides of mountain ranges as
well as across the Susitna Valley. A shallow layer of southeasterly
flow along the Kenai and western Chugach Mountains will keep
rainfall a little lighter near the mountains, with marginally
higher rainfall amounts expected near the Inlet. The Copper River
Basin and northern half of Kodiak Island will remain notable drier
with this system due to downslope flow.

A short-wave moving through the larger trough will quickly track
northward into Interior Alaska tonight, helping to drive the
trough and surface front eastward. This will bring a quick end to
rain tonight for all but the upslope areas along the Alaska Range
and eastern Prince William Sound. While wetting rains are still
expected, rainfall totals will not be as high as first thought a
few days ago when it looked like a slower progression of the
upper levels.

Another potent upper level low and short-wave moving through the
long-wave trough will then move to Southcentral on Thursday. With
500mb temperatures dropping to -35 degrees Celsius or colder mid
level lapse rates will rapidly steepen. When combined with
impressive dynamical lift and strong south to southwest winds,
conditions look favorable for heavy showers, particularly along
the Kenai, Chugach, and Talkeetna Mountains. Have introduced a
slight chance of thunderstorms for these areas and others nearby.
Meanwhile, low level cold advection will cause snow levels to drop
to 2000 feet or lower by Thursday night. Showers will be tapering
off overnight, but there is potential for snow accumulation on
area mountains before showers end.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The frontal system stretched across Southwest Alaska and the
Alaska Peninsula this morning will swing east through the day,
shifting the areas of heavier rain to the west slopes of the
Alaska and Aleutian Ranges. An unseasonably cold upper level
trough behind the frontal system will swing onshore this afternoon
and evening, bringing the potential for isolated thunderstorms and
then changing the precipitation type over to snow showers
overnight. A deeper secondary trough will rotate onshore over the
Kuskokwim Delta early Thursday morning and continue across
southwest Alaska through the day Thursday. This trough will both
increase shower coverage again and bring a good potential for
thunderstorms to the Kuskokwim Valley Thursday afternoon and
evening. Showers will taper off to a more scattered coverage
Thursday night through Friday as the parent upper low weakens
somewhat and drifts north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The large vertically stacked low centered over the northern Bering
will open to a trough which will reach the Kuskokwim Delta Coast
tonight. The main upper low center will shift north to become
vertically stacked with a surface low just north of the Bering
Strait Thursday morning. The entire low complex will then
gradually drift further north through Friday. To the south, a
surface low will approach the western Aleutians this evening will
continue east along the chain to reach the eastern Aleutian
Thursday night and the Alaska Peninsula Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The very cold upper level low over the state will continue to be
the major story into the weekend. Snow levels will gradually rise
but the cold temperatures aloft will keep conditions conducive for
periods of showers over the majority of the region. A cutoff low
will develop from the main upper level low on Sunday and remain
over Kodiak Island as the main low finally lifts back into the
Arctic.

After this the uncertainty really increases as model spread
dramatically increases for the operational runs. The model
ensembles are indicating the next surface low will scoot just a
little south of the Aleutians but that solution is far from
certain.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...EZ



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