Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 281220
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 AM AKDT Tue Mar 28 2017
.ANALYSIS AND UPPER LEVELS...
Upper level ridging persists over the Bering with a cold upper
level trough running north to south across mainland Alaska. A
surface low to the southwest of Shemya is extending a weak frontal
system across the western Aleutians. The surface low south of the
eastern Aleutians has dropped far enough south that its
associated surface front remains south of the Alaska Peninsula. A
broad surface trough containing several small low pressure
circulations extend from the southwestern Gulf north to the Kenai
Peninsula. Another trough/front stretches from Prince William
Sound along the northern Gulf Coast.
Models continue to be challenged in the short term by the array
of various weak troughs and small lows rippling through the Gulf.
The EC appears to have the best initialization of the low east of
Kodiak based on scatterometer winds, while the 06Z NAM looks
closest with the low currently in Prince William Sound. Based on
the weakness of the forcing and continued diversity of model
solutions, confidence continues to remain low on the on the
forecast details through around Wednesday. As a deep north
Pacific low and strong frontal system approach Wednesday night,
models come into significantly better agreement with increased
forecast confidence from Thursday through the end of the week.
PANC...With some light precipitation overrunning the Chugach
mountains this morning, periods of light snow are possible at
PANC, however with a dry air mass in place, accumulations should
be minimal with VFR conditions prevailing. A better chance for
accumulating snow will start after midnight tonight and continue
through through Wednesday morning...however confidence remains low
on both the timing of onset of overnight/Wednesday morning snow
and where exactly the snow band will set up.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Numerous weak features and subtle differences in model solutions
make for a challenging snow forecast for Southcentral. A weak low
over Western Prince William Sound weakens through this morning.
Snow with this feature is expected to be concentrated over the
north Gulf (mixed or rain in Cordova), eastern Kenai. Light snow
however is expected across the Anchorage Bowl and Matanuska Valley
through today. Yet another low heads north tonight and ends up
near the Southern Coast of the Kenai Peninsula and eventually into
Western Prince William Sound or there abouts during the early
morning hours Wednesday. Overall, tonight through Wednesday
afternoon appears to be the best chance for moderate accumulating
snow across the Anchorage Bowl and Matanuska Valley. Wednesday
night, the flow aloft takes a more southwesterly shift and should
push the heavier snow potential north of the Anchorage Bowl.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The majority of the Southwest Alaska region remains with offshore
flow resulting in drier conditions. The Gulf of Alaska has a weak
low tracking into the northern Gulf before moving inland near
Prince William Sound region by Wednesday morning. Therefore, look
for wrap-around moisture from this system to spread over into the
Lower Kuskokwim Valley southward into the Bristol Bay interior.
The majority of the aforementioned moisture will remain along the
Alaska/Aleutian Range through the midweek as the weak low exits
north into the central Alaska interior.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The blocking high pressure over the Bering Sea continues to keep
low pressure systems on the periphery south of the Aleutian Chain.
By Wednesday, the jet stream transitions from zonal to meridional
southwesterly flow into the Gulf. While upstream a 170 knot
northwesterly jet streak digs into the base of the longwave
trough over the North Pacific ocean. This results in a storm force
low kicking out from the base of the longwave trough northward
towards the Central Aleutians. By Wednesday evening a 967 mb low
tracks near 47N 170W with an associated weather front stretching
through the Central/Eastern Aleutians. This brings gale force
northeasterly winds along the aforementioned region with
widespread moisture accompanied by warmer temperatures. As this
system intensifies, the pressure gradient will strengthen
resulting in storm force northeasterly winds on the Pacific side
from Dutch Harbor eastward towards the Shumagin Islands by
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast looks to be quite an active one for much of
Alaska. Starting late Wednesday into Thursday, an upper level
trough extending over the Bering Sea and northern Pacific Ocean
will continue to amplify and elongate as an influx of energy from
the coast of Russia moves into the Pacific Ocean. As the trough
amplifies, a warmer and more moist air mass originating in the
subtropics will begin to be advected north toward the gulf coast
of Alaska. While models have struggled recently with the pattern
change, models are in near lock-step through Friday before some
discrepancies begin to develop.
The big story on Thursday and Friday will be a powerful low
pressure system moving northward toward the Alaska Peninsula which
will likely slam the gulf coast with a mixture of rain and snow.
Model guidance shows sfc to 500 mb thicknesses increasing and 850
mb temperatures approaching 0 degrees C which indicates a change
to rainfall along the coast. However, interior locations that see
precipitation will likely remain snow. The low track will be
important as this far west track will likely keep Anchorage and
normal downsloped locations on the dry side. Heading into the
weekend, models begin to struggle with the orientation of the
upper level trough as the ECMWF tries to flatten the trough out
which brings all the energy toward Juneau while the GFS keeps the
energy in the Gulf of Alaska. The take-away story here is that
this active pattern looks to be here to stay for the next 7 to 10
days but it`s difficult to see as of now which areas may see the
most rainfall/snowfall. Stay tuned.
MARINE...Gales 172 174 176.
Heavy Freezing Spray Warning 179 185 412 414.
SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD