Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 260021
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKDT SUN SEP 25 2016
.ANALYSIS AND UPPER LEVELS...
A short-wave trough is making it`s way across the western Alaska
Range into Southcentral Alaska this morning, with a band of of
showers oriented south to north from Cook Inlet to the Susitna
Valley. The southern end of the short-wave is digging slightly
into the western Gulf leading to a slight negative tilt. Expect
this to lead to a change in orientation of the bands of showers
and lead to greater coverage of showers this afternoon from the
Western Kenai to Anchorage and the Mat Valley. More disorganized
convection continues over the northern Gulf and Prince William
Sound as weak impulses moving through the broader upper trough
in a moderately unstable air mass.
Meanwhile out west, an upper low and amplifying trough with
strengthening jet streak moving through the back side of the
trough, are tracking eastward across the Bering Sea. There is a
reflection of the low at the surface, though it is on the weak
side. A long cold front is sweeping eastward across the western
Bering Sea and Aleutians. This storm system is leading to
strengthening winds and deteriorating conditions across most
of the region.
Lastly, quiet conditions dominate Southwest Alaska as a short-wave
ridge moves overhead. The morning began with isolated areas of
fog, though most of this had eroded by mid-day.
Models remain in excellent agreement with large scale features
over the next couple days and overall forecast confidence is
high. The precip forecast over Southcentral continues to be a
challenge with varying guidance in location of showers as the
upper level short-wave continues eastward through Tonight.
PANC...This is a very challenging precip forecast for this
afternoon through this evening. It appears most of the shower
activity will remain to the south and west right ahead of the
main trough axis, with secondary area of showers along the
mountains. If this works out ceilings will safely stay VFR.
If on the other hand, more showers move over the airport may
see some periods of MVFR ceilings. In any case, confidence
increases overnight that ceilings will gradually lift. There
may be some patchy fog toward morning, but there should be
enough mid to higher clouds remaining to prevent more widespread
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Tonight will be the last gasp of the trough that has been
controlling Southcentral weather for the past week or so. The thin
line of convective showers stretching from the western Susitna
southward through the western Kenai, will continue slowly to the
east while dissipating this evening. More scattered to numerous
showers, some with small hail, will affect the inland Kenai
northward through Anchorage and the Mat-Su. This evening is the
last of the convective showers however as the trough moves into
the gulf and out of our area. A cooler air mass comes in behind
this trough, and could bring some low-lying areas down to the
freezing mark or close to it by Tuesday. A separate trough comes
in from the northwest late Monday but looks pretty dry at this
point. Winds will remain light in all areas and temperatures very
.SHORT TERM FORECAST SOUTHWEST ALASKA...
The low over the northern Bering continues to track to the
southeast as it makes its way towards the eastern Bering and
eventually across the Alaska Peninsula Monday. As it passes it
will bring rain to areas near the coast in southwest Alaska with
the Kuskokwim Delta already seeing some of the rain move in.
Following the low, ridging will move in over the area from the
west and will remain until Wednesday morning when a trough pushes
in over the Kuskokwim Delta. The ridging will help in fog
formation Tuesday night.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The low pushing southeast through the Bering is bringing light
rain and winds to the Pribilof Islands that will continue for much
of the night and spread south over the Eastern Aleutians and
Alaska Peninsula later this evening. To the west ridging is
building in over the western Bering and it will track east and
dominate much of the Bering through the early part of the week. A
system moving into the far western Bering will push its front
through the Western Aleutians Monday night and then over the
central Bering Tuesday evening bringing more light rain to the
.LONG TERM FORECAST (Days 3 through 7)...
The extended term forecast will begin on Tuesday night with
transitory upper level ridging in place over mainland Alaska,
promoting mainly dry conditions across the Southcentral region
beneath clearing skies and offshore flow. A shortwave trough
moving across the northern half of the mainland on Wednesday will
flatten out the ridge and drag a front through the southwest part
of the state, bringing a chance of rain before quickly moving off
to the east. The pattern then becomes much more amplified during
the latter half of the week as an upper level trough deepens over
eastern Russia and a high amplitude ridge builds over the mainland
from the central Pacific. This will support a continuation of the
mostly dry and clear conditions across the southern mainland
through the end of the work week, allowing overnight low
temperatures to dip near or below freezing in many locations as
conditions become optimal for radiational cooling. Meanwhile,
another shortwave rounding the base of the eastern Russia trough
will help to spin up a deep surface low moving into the Bering in
the Thursday/Friday timeframe, bringing rainy and windy conditions
to much of the Bering region. This system also looks to swing a
front over the southwest mainland Friday night and into the Gulf
of Alaska by Saturday, bringing a return of precipitation to the
forecast for Southcentral by next weekend.
SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK