Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 260039
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT THU AUG 25 2016
.ANALYSIS AND UPPER LEVELS...
The steady moisture stream aloft is still affecting Southcentral
Alaska while the remnants of the elongated upper level low
pressure are spinning over the southwest Mainland, beginning to
move over the Alaska Range. The stout northeast Pacific ridge
remains in place, and is a big reason for the moisture stream
pointed right at us. Another upper level low spins over the
western Bering Sea dropping southward. Dry air aloft is beginning
to intrude the Southwest as the trough moves to the east.
The main forcing of the upper level trough appears to be only
affecting the north Gulf Coast region as the trough continues
east. The plentiful low level moisture nearly everywhere has aided
in development in low stratus/drizzle and visibility issues as
the mid-level moisture has vacated.
The major differences among models have to do with a couple of
upper level features. The first is an elongated vorticity maximum
left over by the decaying trough. The 00z/12z ECMWF and now the
18z NAM develop this trough into a consolidated upper low and move
it along a low-level baroclinic zone toward Kodiak Island. The
forecast will trend toward this solution. The second is the system
currently over the Bering Sea. Most guidance wants to sweep it
eastward into the cyclonic flow over the mainland while the ECMWF
keeps weakening it in place.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A transition to a more showery regime is underway this afternoon
and will continue through the evening hours as the heavier
stratiform rain pushes off to the east and the upper level trough
approaches from the west. An intrusion of dry air in the mid
levels ahead of the trough has yielded partial clearing over the
Susitna Valley, where isolated thunderstorms will be possible
through this evening as the breaks in the cloud cover aid in
destabilization of the atmosphere. Thunderstorms are not expected
farther south towards the Anchorage bowl as lingering low level
cloudiness through much of the day has led to more stable
conditions than previous thought, as noted on the afternoon
The passage of the trough tonight will usher in a drier offshore
flow pattern, bringing a period of more pleasant conditions
characterized by clearing skies and warmer temperatures from
Friday into the weekend. This will also yield gusty outflow winds
through many of the favored areas along the north Gulf Coast. The
exception to the drying trend will be Kodiak, where a compact
disturbance moving through the gulf will bring a chance of rain
Friday into Friday night.
.SHORT TERM FORECAST SOUTHWEST ALASKA...
An upper level shortwave pushes through the Lower Kuskokwim Valley
resulting in a slight chance for isolated thunderstorms during the
early evening hours tonight. Meanwhile, a weak ridge tracks over
the Southwest Alaska region combined with southwesterly onshore
flow will aide in developing dense fog by the early Friday morning
time-frame. Look for the dense fog to cover portions of the
Kuskokwim Delta and the Bristol Bay region before dissipating by
mid-day. Look for marine layer stratus to cover the region before
offshore flow develops and begins to clear out the marine layer
stratus as we head into the weekend. The temperatures will
continue to remain in the upper 60s except for the lower 70s in
the Bristol Bay region through Friday. Look for cooler temperatures
to develop as the offshore flow develops accompanied by colder air
filtering into the region from the north.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
There is a weak 1007 mb low pressure system tracking eastward over
the western Bering before entering the central Bering by late
tonight. The associated weather front is approaching the Pribilof
Island before stalling and dissipating by Saturday. On the
backside of this system an upper level ridge builds into the
Western Aleutians. This will bring marine layer stratus/fog to the
aforementioned region heading into the weekend.
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast is quite interesting today with lots of
discrepancies in the mid to upper levels with little variance in
sensible weather. This means that although models are struggling
resolving several mid to upper level features (700 to 500 mb) the
impact on the surface weather appears quite marginal. Based on
current model guidance there is high confidence that the offshore
wind event expected to start on Friday will continue through at
least Wednesday of next week. This will keep the majority of the
mainland on the dry side but there will be some gusty winds
through channeled terrain as a decent pressure gradient sets up
across the state. The pesky upper level low over the Alaska/Canada
border will continue to make the forecast difficult but a few
rounds of rain (especially on Sunday) will be possible over the
eastern half of the Copper River Basin through the middle of next
The Bering Sea is a much more challenging forecast as models
struggle with a cut off low over the northern Bering Sea and how
that energy is either absorbed or displaced. This has repercussions
in how the upper level trough expands over the western Bering Sea
and drives a front through the region. While timing and strength
of this front is severely in question, the one fact that the
models agree on is that it is 24 hours slower than initially
thought yesterday. High pressure over the eastern Bering will
likely continue to create fog and low stratus issues through much
of next week.
PUBLIC...Flood Advisory 145.
SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD