Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 250030
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKDT FRI JUN 24 2016
.ANALYSIS AND UPPER LEVELS...
There is an upper level low over southwest Alaska and Kodiak
Island that is moving southeast into the Gulf. There are several
vort-maxes associated with this low. On the western side of the
low, one of these vort-maxes is moving into the Alaska Peninsula,
and one is over Kuskokwim Bay. Over the Gulf, there is one to the
southeast of the Kenai Peninsula moving slowly north, and one
south of Kodiak moving east. High pressure is over central
Alaska, just nudging into the northern Copper River basin. A
large upper level ridge of high pressure is building over the
Western Bering Sea.
The numerical models are in good agreement for the short term
portion of the forecast (through Sunday afternoon). There are of
course some differences in the exact location of vort-maxes and
how they handle the convection and moisture, but these differences
are not large. Therefore the forecast confidence is above normal.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The showery weather regime continues through the weekend as an
upper level low remains in the Gulf of Alaska. Several shortwaves
will spin-around this low resulting in the majority of the
precipitation along the northern coastal communities through the
weekend. Look for gusty gap winds through Turnagain Arm region
through the weekend before the pressure gradient weakens. The
models currently indicate that instability will be greater and
support more vigorous convection on Saturday, with a few more
lighting strikes possible along the Alaska Range and the Talkeetna
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The pattern will get a bit more interesting over the weekend. The
upper level low is currently spinning through the area. It has
brought widespread cloud cover and light rain to several locations
over the past 24 hours. In fact, the cloud cover is so widespread
and thick that we have pulled back the thunderstorm coverage for
this evening. Right now, it looks like the only place that might
see some activity will be right along the Western Alaska Range
in the evening.
But that all changes tomorrow. The afore mentioned upper level low
pulls away to the south and east over early Sat. This opens much
of the mainland to long duration overland flow. This will bring
drier air, clearer skies, and markedly warmer temperatures. The
thermal trough should not have much trouble establishing itself
Sat afternoon and evening. The interesting aspect of this is that
there will be a pocket of cold air near the surface as some marine
air remains entrenched over the Yukon Kuskokwim Delta. This cold
air could serve as a forcing mechanism for the thunderstorms that
do fire. So expect scattered thunderstorm coverage from the Alaska
Range to the Kilbuck Mountains. Further south over Bristol Bay,
some breaks in the clouds early on Sat will give way to afternoon
and evening showers as cyclonic flow wraps around the low.
Sun will be much of the same. The best chance for thunderstorms
will be slightly further east and south stretching from the
middle Kuskokwim Valley over to the Alaska Range and south toward
Dillingham. While instability associated with the thermal trough
does not look quite as impressive as Sat, the potential for any
thunderstorms does spread out to a broader area. The YK Delta
should see enough stability move in from the ridge over the Bering
and the marine air to keep things dry out there.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
High pressure will slowly drift east from the Western Bering to
the Central Bering by late Sun. This will make for light winds
across much of the area, but also allow fog and stratus to set up
easily in many places. Some of this fog could be dense at times.
Over the Eastern Aleutians and Alaska Peninsula, northwest flow
will continue to allow pulses of energy to crash into the Bering
side of the chain. This will keep rain showers going and make for
occasional gusty winds.
By Sat afternoon, a new warm occlusion will approach the Western
Aleutians. This will cause the winds to shift to southeasterly and
should be yet another source of fog for the chain. This system
will only bring light rain as it falls apart while drifting into
the Western Bering on Sun. Another slightly stronger system will
be on its heels Sun night.
The thermal trough will establish itself over the weekend. This
will bring the potential for scattered thunderstorms both Sat and
Sun. The best potential on Sat will start over the Middle
Kuskokwim Valley. Storm motion will be from northeast to
southeast, so expect storms to track along the terrain toward the
Ahklun Mountains. Sun shows slightly more stability at this time
and focuses the potential further to the east. There is potential
for the indices to increase over the next couple days, so we will
be watching them closely. The over-riding factor in all of this
is that with the recent cool, wet pattern, conditions are not
overly favorable for starts.
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast will be characterized by mostly cloudy
conditions with afternoon showers along higher terrain, however
some breaks of sunshine will be possible. Sunday afternoon starts
the forecast with the mainland in between a couple of systems.
These conditions will be conducive to mountains showers that move
off the terrain in the evening hours as well as along the north
gulf coast. Monday afternoon holds more of the same with a little
more activity in the northern Gulf. Daily gap winds through
Turnagain Arm, the Knik River Valley, and Copper River Valley, to
name a few will pulse in the afternoon/evening while subsiding
overnight. Mid to late next week, cloud cover and rain chances
increase as the gulf system pushes ashore. Temperatures should be
right around average for this time of year with generally light
Technical details...The upcoming week will be a battle between two
systems, a closed low spinning in the gulf, and a building ridge
over the Bering Sea that will be moving onshore. The gulf low will
be continually reinforced by disturbances within the jet to the
south while the ridge will try and build over the southwest
mainland. The trend in the models has been to move the ridge
farther and farther north as weak trough digs and shears out ahead
of it. That will allow the gulf low to impose it`s cyclonic
circulation over the southern mainland. The pattern will feature a
weak baroclinic zone aloft underneath the easterly flow that will
promote amplifying shortwaves rotating around the trough. Late in
the week the low will move ashore completely, dropping heights and
some steadier rain.
SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO