Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 290500 AAA
Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
900 PM AKDT Fri Apr 28 2017
UPDATED LONG TERM FORECAST
ANALYSIS AND UPPER LEVELS... A northward and slow-moving shortwave
over Southcentral Alaska continued to bring rain through the
morning over Anchorage and the MatSu valleys. Rainfall ranged from
one-tenth to about one- half inch near the mountains. Light
southeasterly flow continues over the region with mid-level
moisture decreasing in the wake of the shortwave. This is allowing
sun to break through and rebuild a healthy field of convective
clouds and showers.
The upper-level flow consists of a low and shortwave stretching
from the Northern Bering to the Alaska Peninsula and into the
North Pacific. The main upper-level low remains closer to the
coast of the Kamchatka Peninsula.
At the surface, an occluded front is crossing the Aleutians from
West to East. The front is drawing in a warmer and drier air mass
ahead of the occlusion, with improved conditions in the
Models initialized well and were in general agreement through the
weekend on the placement of the Bering Sea and Gulf of Alaska
lows. The timing of the front and troughs were in general
agreement as well, though the GFS was more consistent in timing
the front and placement of multiple low centers in the Bering.
The Nam Nest was used for timing and strength of the wind details
Forecast challenges in Southcentral will continue to be timing and
extent of convective showers and strength of the Turnagain Arm
wind. For Saturday, the juxtaposition of off-mountain winds
versus precipitation will be challenging as well.
Forecast challenges in Southwest and the Bering Sea region will be
timing of the front and the predicting the placement of multiple
low centers through the progression.
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Another evening of diurnal rain showers will occur over much of
Southcentral as cold air aloft associated with an upper level low
approaches from the western Gulf. This should however be the last
evening of showers for Anchorage and the Matanuska Valley as a
weak low moving in the western Gulf during the afternoon should
keep any rainfall confined to the Gulf Coast, with downsloping on
the lee of the Chugach Mountains. Beginning Sunday morning,
confidence very good that widespread rains will move into Kodiak
and spread northward to the Gulf Coast on Sunday evening. It still
appears however that downsloping from this system will keep most
of the Anchorage Bowl and Matanuska Valley dry through the
SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The showery regime will continue at least through Sat
afternoon/evening. The area will remain under the influence of
moist, warm, south to southeasterly flow as a front approaches
from the Bering. Shower activity will peak during the afternoon
and evenings, especially over terrain, before waning during the
overnight hours. Then by late Sat into early Sun, the Bering front
will working towards Bristol Bay. As it does so, southeast winds
will increase, especially through southeast to northwest gaps in
the Aleutian Range. More steady rain will move into the Bristol
Bay coast Sun morning while some showers will make it further
inland. Locations on the northwest side of the mountains will stay
a bit drier as downsloping conditions develop.
SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
The warm occluded front will continue its track through the
Aleutians as a broad low sets up across the Bering Sea. The
occlusion that is passing through Atka and Adak now will make its
way to Dutch by late tonight and then into the Alaska Peninsula by
Sat afternoon. As it shifts east, it will continue to produce
small pockets of gale-force winds and plenty of rain, especially
on the Pacific-facing islands. The current low pressure center
west of Shemya will yield to a developing triple-point low near
the Central Aleutians. This developing low will then become the
driving-force for the weather into the weekend. It will
effectively park itself near the Pribilof Islands and produce
broad cyclonic flow across the region as it weakens in place.
LONG TERM FORECAST (Days 3 through 7)...
The trough centered over the Alaska Peninsula at the beginning of
next week will slowly migrate eastward towards the gulf by the
middle of the week, with a new surface low developing over the
North Pacific and pushing into the central gulf Tuesday night into
Wednesday. In terms of sensible weather impacts, this will
largely mean more of the same for the southern mainland with
steadier rain focused along the gulf coast and more sporadic
showery conditions inland as temperatures continue to run near or
slightly above seasonal normal values. Weak high pressure will
build in across the Bering Sea in the wake of the trough, yielding
fairly benign conditions. Another low looks to approach the
Western Aleutians late next week and bring a better chance of
precipitation to the chain, however confidence in this scenario
remains on the low end as model spread is still fairly large.
MARINE...Gale warning 155 170 172 174
SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO