Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 241242
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKDT TUE MAY 24 2016
.ANALYSIS AND UPPER LEVELS...
The trough that moved a front into the southwest mainland on
Monday is currently across southcentral early this morning. The
front is rapidly dissipating as it moves further from the parent
low which is stationary around the Chukchi Sea. Across the
southwest mainland, onshore flow is relatively light as a surface
ridge slowly builds in from the Alaska Peninsula. A stable air
mass across the eastern Bering and Peninsula is keeping low
stratus and fog over the area. The next front to impact the
Bering is approaching the western Aleutians this morning. This
system is organizing as a lobe of low pressure closes off from the
parent low to the north.
Models continue in good agreement with the overall synoptic
pattern through Thursday night. The NAM and GFS were used to
update the morning package with only minor changes needed.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)..
A shortwave trough will move into Southcentral today, spreading
increasing chances of rain/showers from west to east. The track
of this system will keep the best chances for precipitation from the
Matanuska Valley northward(mainly upslope along the talkeetna
mountains), though some showers are possible along the Chugach
Mountains east of Anchorage and the Kenai Mountains. Rain and
showers will continue across the Copper Basin today with clearing
and only isolated showers westward. The flow aloft becomes more
westerly Wednesday so expect mainly clouds with relatively low
probabilities of showers. The low over the North Gulf strengthens
today and will tighten the offshore pressure gradient and give
rise to north to west gap flows across the Eastern Kenai/Western
Prince William Sound. Outflow winds will keep afternoon sea
breezes at bay in these areas and allow for some locally toasty
temperatures this afternoon (Seward/Whittier). Gap flows will
diminish overnight tonight over the Eastern Kenai/Western Prince
William Sound. Northerly gap winds however will increase briefly
along the North Gulf Coast late tonight/Wednesday morning before
returning to sea breezes Wednesday afternoon.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
High pressure has begun to build across the tip of the Alaska
Peninsula this morning and will continue to across Southwest
Alaska through the forecast period. The upper level ridge lags
slightly behind the surface ridge which will allow a few showers
to traverse the zonal upper level flow and impact parts of the
Kuskokwim Delta and Lower Kuskokwim Valley today and Wednesday.
Temperatures will finally begin to rebound as high pressure builds
across the region and a combination of warming 850 mb temperatures
and offshore flow developing. In lieu of recent rainfall, it
would not be surprising to see fog across the southwest mainland
during the overnight hours tonight and again on Wednesday before
offshore flow begins to dry out the surface layer.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An area of low pressure located near Shemya will push a front
across the western Bering Sea through Thursday before it begins to
collides with ridging over the eastern half of the Bering Sea. By
this point the front will have occluded and be weakening as it
pushes eastward. The parent low will remain in the vicinity of
Shemya through Thursday while gradually weakening. The low and
front will bring gale force winds to the Western and Central
Aleutians over the next 24 to 48 hours before the front weakens
and is no longer able to support gale force winds. A very moist
air mass will be advected across the Bering Sea and model PWAT
values show well over twice the average values for this time of
year. This should bring some moderate to heavy rainfall to the
Western and Central Aleutians through Thursday as well. Fog will
become an issue over the eastern half of the Bering Sea and the
Eastern Aleutians with building high pressure. The fog will likely
mix out during the afternoon hours but should quickly re-develop
following the loss of diurnal heating.
.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...Gales 175 176 177 178.
SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC