Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 232341
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
341 PM AKDT MON MAY 23 2016
.ANALYSIS AND UPPER LEVELS...
An active pattern remains in place over southern Alaska as short
waves continue to cross the area. The shortwave that caused
showers overnight across the Kenai Peninsula, Matanuska Valley,
Susitna Valley and Anchorage Bowl is now bringing clouds and
widespread showers across the Copper River Basin this afternoon.
Another upper level shortwave is following quickly on its heels
and bringing steady rain to the Kuskokwim Delta and Kuskokwim
Valley today. Meanwhile, high pressure dominates much of the
central and western Bering which is supporting widespread stratus.
Models are in great agreement again today with several prominent
features. The upper level wave currently over the Southwest
Mainland is expected to continue to travel eastward, and bring
showers to south central Alaska overnight tonight. While there
are slight differences in the timing and location of showers,
rainfall is expected to be light. Due to good consistency and
agreement, a blend of models was chosen.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)..
A shortwave trough will move into Southcentral tonight, spreading
increasing rain chances from west to east. The track of this
system will keep the best chances for rain from the Matanuska
Valley northward with mostly just clouds farther south. Rain and
showers will continue across the Copper Basin Tuesday with
clearing and only isolated showers westward. This low will also
tighten the offshore pressure gradient tonight, with north to west
gap winds along the coast increasing and becoming rather strong
for this time of year. The shortwave trough will move out Tuesday
night with dry northwest flow aloft keeping gap winds along the
coast through Wednesday. High clouds will increase on Wednesday as
a Bering low spreads high level moisture eastward.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showers over the Southwest Mainland have started to taper off
this evening as the weak upper level trough track further
Eastward, with some lingering showers along the mountains lasting
through early tomorrow morning. High pressure from the North
Pacific will build across much of the Southwest Mainland Tuesday
morning, and continue to build across the Kuskokwim Delta and
Kuskokwim Valley by Tuesday afternoon. This high pressure will
help keep the weather on the dry side through Wednesday for much
of the area. The high pressure overall is rather unimpressive
which will allow some rainfall associated with weak shortwaves
traversing the zonal flow over the Bering Sea to return to the
Kuskokwim Delta and Kuskokwim Valley Tuesday night into Wednesday
Temperatures have been slower to warm up than expected because a
tap into the arctic air mass over the Beaufort and Chukchi Seas
continues. This feed of cooler air should get cut off Tuesday
afternoon which will allow temperatures to warm back to near
normal temperatures by Wednesday.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Zonal flow will continue across the Bering Sea through Tuesday
afternoon before a deepening upper level low moves to near
Shemya. Rain chances through Tuesday morning will be largely
suppressed by weak high pressure building across the region from
the North Pacific. This high pressure will act as a double-edged
sword as it should allow fog to develop with subsidence, keeping
moisture trapped at the lowest levels of the atmosphere. A front
associated with a low approaching the Western Aleutians tonight
will begin to bring gusty winds and rain back to the Western and
Central Aleutians through Wednesday. this should act to mix out
any foggy conditions over the area, as it pushes the more stable
High pressure eastward over the Eastern Aleutians.
.LONG TERM FORECAST (Days 3 through 7)...
The ridge will continue to build over southwest Alaska and the
eastern Bering Sea into a closed high pressure aloft at 500 mb by
early Friday morning. This high pressure will become amplified and
extend into the Western Siberian area as we head into the weekend.
The ECMWF and the Canadian-NH models also build the ridge into
the south central Alaska, versus the GFS which is keeping the
ridge only in the southwest. Even with these model differences
the temperatures will be warming into the lower to mid 60s heading
into Memorial weekend for both the Southcentral/Southwest Alaska
regions. The other concern will be the increased marine layer
stratus/fog developing under the inversion as the high pressure
builds through the week. Look for patchy-areas of fog developing
in the northern-eastern Bering sea by the midweek time-frame
through the weekend. The ridge will move inland along the
Kuskokwim Delta, and the Bristol Bay regions with continued
onshore flow through the weekend. There will be minor shortwaves
tracking from the north and will mainly affect the Alaska Range
Thursday with showers through the weekend. The Western Aleutians
will have a low pressure system track just southwest of Shemya by
Wednesday and remain somewhat stationary as it runs into the
blocking high pressure. This will bring a showery regime to the
Western Bering/Aleutians through the Friday before filling and
dissipating by Saturday. Then another low pressure system will
track off the Kamchatka Peninsula, and approach the Western
Aleutians by Sunday. The forecast confidence remains near average
as we head into the weekend.
MARINE...Gale Warning 177 178.
SYNOPSIS AND MODEL DISCUSSION...en
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...pepe