Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 240047
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 PM AKDT THU JUN 23 2016
.ANALYSIS AND UPPER LEVELS...
There is a negatively tilted upper level low over the eastern
Bering Sea and Southwest Alaska, with its axis running from Saint
Lawrence Island through Bristol Bay to just southwest of Kodiak
Island. There are no well organized fronts with this low, but a
few upper level waves (vorticity maxima) surround it. The
strongest of these is just moving into the northwest Bering Sea
near 180 degrees longitude. Moisture and showers are increasing
across southern Alaska as this low moves east. There is a
flattening and "dirty" ridge over south central Alaska with the
atmosphere destabilizing as it weakens. A large upper level ridge
is building into the western Bering Sea.
The numerical models are in good agreement for the short term
portion of the forecast (through Saturday afternoon). The
exception is the Canadian Gem, which handles the low moving into
the Gulf on Saturday a little differently. The other models are
tightly clustered with this feature. There are also naturally some
differences in stability indexes and precipitation forecasts, but
these are minor. Due to the good model agreement, the forecast
confidence is above normal today - except for near normal for
Marginal Red Flag conditions are expected in the Copper River
Basin this afternoon into early evening. All three parameters -
winds, temperatures, and humidities - will be just at critical
levels. On Saturday, winds will be stronger, but temperatures
will be lower and humidities higher.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southcentral Alaska will transition to a slightly more active
pattern over the next several days as the ridge shifts to the east
and an upper low drops into the southern Gulf from Y-K Delta
region. This will support intermittent rain along the gulf coast
through Saturday, with generally cloudy and showery conditions
farther inland. Several weak waves embedded in easterly flow over
the mainland coupled with modest instability along the Alaska
Range will promote the possibility for a few thunderstorms for the
next several afternoons over mainly northern portions of the
Susitna Valley and Copper River Basin. The primary limiting factor
for thunderstorm potential will be persistent cloud cover
resulting from broad cyclonic flow and a relatively moist airmass
in place over the region.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Fri and Sat)...
Periods of light rain and some embedded showers are pushing inland
this evening. These are courtesy of the upper level low centered
near Nunivak Island. This low will pivot there as some Arctic
energy drives down its western flank. Then the entire trough will
tilt to a more northwest to southeast orientation. This will
spill some more cold air aloft into far inland portions of the
Kuskokwim Valley and ultimately increase lapse rates. All that to
say there is potential for isolated thunderstorms along the Alaska
Range this evening.
The low dives quickly to the southeast on Fri and should allow
for a decent fetch of overland flow for inland areas. With drier
air moving through expect some more breaks in the clouds and
better instability on Fri. This should allow thunderstorms to
spread out to the Kilbuck Mountains and further south on the
Alaska Range. By Sat, the thermal trough will be firmly in place.
This should bring the best thunderstorm potential yet as we head
into peak heating Sat afternoon and evening.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...fri and
High pressure will remain in control of the Western Bering while
waves of energy from the Arctic continue to move through the
Eastern Bering. Out west, small craft winds will become light and
variable on Fri as the high sets up. Some westerly flow is helping
to break up some of the fog and stratus this evening, but with
warming 850 mb temperatures it could easily make a rapid return.
By early Sat an extremely weak occluded front limps its way toward
the Western Aleutians. This should bring another chance for some
light rain and switch the winds to easterly.
Over the Eastern Bering a few pieces of robust energy drop in
from the Arctic. These should help the modest westerly winds to
become slightly more gusty from the northwest. They will also
reinforce rain on the Bering side of the Alaska Peninsula. The
first impulse comes in before daybreak on Fri with the next one
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast begins on Saturday afternoon with a
shower/thunderstorm regime in place across the mainland. Expect
any thunderstorms to mainly be over the Talkeetna Mountains and
Kuskokwim Mountains/Valley, even into Greater Bristol Bay.
Elsewhere precipitation will be showery. Sunday brings the nearly
the same forecast with more shower activity in the northern Gulf
of Alaska extending up into the Copper River Basin. Into next
week, the same pattern holds pretty firm; showers (with a chance
for thunderstorms) will form on the mountains in the
afternoon/evening hours, then move off to the west. Cloud cover
will be highly variable, pulsing with activity each day,
flattening out over night. The pattern should provide opportunity
for periods of sun. Temperatures will be near to slightly above
normal, with generally light winds. Daily enhanced winds through
favored gaps will persist.
Into the details...a stacked, closed off low will be settling into
the Gulf for most of the extended forecast. Weak ridging over the
mainland, combined with moisture and easterly disturbances from
the low pressure will provide the necessary ingredients for
diurnal mountain based convection. The southwestern mainland will
have the thermal trough extending through the Kuskokwim Valley to
provide low level support in the absence of the upper low. This
general pattern will persist through next week until a stronger
upper ridge tries to build over the mainland on Wednesday, but the
active weather will be between the ridge over the interior and the
low in the gulf, leaving the mainland in an easterly wave pattern.
FIRE WEATHER...Red Flag Warning 141.
SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO