Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
438 PM akdt Tuesday Jun 30 2015
Analysis and upper levels...
the upper level low centered southwest of Kodiak Island continues
to slowly weaken as it drifts to the east northeast. This low is
spinning easterly waves across the southern Mainland spreading
clouds and showery conditions across the Copper River basin and
Susitna Valley while keeping Prince William Sound cool and rainy.
Surface ridging building in along the north Gulf Coast is
producing the typical gusty channeled wind along Turnagain Arm.
Over the eastern Bering Sea the ridging continues to hold low
clouds and areas of fog along the north side of the Alaska
Peninsula the eastern Aleutians and Pribilof Islands. A low is
moving south of the western Aleutians resulting in a tighter
pressure gradient along the Aleutians that is expected to push the
winds up to minimum gales.
in the short term the models remain in very good agreement at the
synoptic level through middle week with the expected dissipation of
the southwestern Gulf low and are in excellent agreement with the
track of the next low into the eastern Gulf on Wednesday. The
models are also in good agreement with the amplification of the
ridge over southwest Alaska Thursday. By Thursday the models are
in good agreement with moving the surface low in the Gulf inland
with a trough trailing to the southwest into the northern Gulf of
Alaska. This coupled with the ridging over western Alaska will
result in offshore flow along the Gulf Coast Wednesday and Thursday.
for the southcentral...relative humidity and wind values will
remain above critical thresholds through the later part of this
week...as the Gulf low and upcoming easterly wave usher in more
stratiform cloud cover and rain over the area starting this
evening. Gusty gap flow along the Gulf Coast...Turnagain Arm...and
the Copper River will persist through this evening before
diminishing as the coastal ridge and thermal trough over the
interior continue to weaken.
For southwest Alaska...while the Gulf of Alaska system is moving
east...another upper level trough will keep
cooler...cloudier...moist conditions over a good portion of the
Mainland. High pressure from the Bering Sea will build at the
surface keeping winds generally light through Thursday. Isolated
wet thunderstorms remain possible...mainly over the Alaska Range
this evening and tomorrow afternoon/evening but most of the
precipitation will be showery.
Short term forecast southcentral Alaska (days 1 and 2)...
a stacked surface low south of Kodiak Island will continue to
drift southeast under the upper center and weaken...along with the
associated front currently lingering over the northern Gulf Coast.
Rain is expected across much of the Gulf Coast with this system
with showers inland. This low will eventually phase with another
system quickly tracking through the North Pacific from the south
by Wednesday morning. The precipitation will become more
stratiform from the western Copper River basin westward as an
easterly wave moves in from the northeast...bringing some wetting
rains to the area Wednesday. Areas west of the Copper River basin
will see drier conditions spreading in from the west with a
building ridge late Wednesday night. The thermal trough will continue to
weaken each day...keeping abundant cloud cover over much of south
central through Thursday. The main area for isolated wet
thunderstorms will be along the mountains(ak range/talkeetna) in
the Copper River/eastern Susitna Valley for this evening. The
increased cloud cover along with decreasing instability over the
area will limit any thunderstorm development through Thursday.
Brisk gap winds are expected to persist over the favored
locations(turnagain arm/Portage Valley/Knik Arm/copper river) this
evening as the coastal ridge remains in place. The ridge begins to
weaken through Wednesday afternoon...progressively weakening The
Gap flow over the aforementioned areas. However...increasing
northwesterly gap flows are expected across the eastern Kenai
Peninsula and eastern Aleutian Range...as the ridge builds over
the southwest coast and the low over the Gulf tracks further
eastward into the Panhandle.
Short term forecast southwest Alaska (days 1 and 2)...
the eastern portion of the domain...along the Alaska/Aleutians
ranges are still under the influence of the departing upper level
low with scattered showers. The lingering instability will also
make isolated thunder a possibility along the Alaska Range this
evening. Showers will continue tomorrow afternoon for the
Kuskokwim valley and interior Bristol Bay zones before drying out
on Thursday. The Kuskokwim Delta will remain dry the next couple
days and the marine layer will pulse in from the Bering Sea on a
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
an upper disturbance and front is affecting the western Aleutians
bringing widespread rain to the area. The front will slowly move
eastward over the central Aleutians tonight and eastern Aleutians
Long term forecast (days 3 through 7)...
Thursday night into Friday a longwave trough digs from the
northwest coast. This will keep light rain chances across the
southern Mainland for the end of the week. Most of the
precipitation will be light with higher amounts along mountain
ranges. To the west...a low pressure system across the central
Aleutians moves a front into the southern Bering Thursday night
which extends toward the Alaska Peninsula. This front moves into
the eastern and northern Bering by Friday night.
Long term forecast for Saturday into the beginning of next week
remains highly uncertain as models continue to have a large spread
in solutions. The model choice for the extended range was the
European model (ecmwf) with only minor changes made. The general pattern into the
next week keeps diurnal showers along the mountains and generally
weak surface flow. The main challenge for the upcoming weekend
will be a front that moves into the southern Gulf on Saturday. The
European model (ecmwf) pushes this front into the coast Saturday night and the GFS
diminishes it...while the Gem trended with a Middle Range
solution. The GFS continues to present the warmest solution for
the beginning of next week...but will continue to be conservative
with forecast influences as the other operational models remain
Synopsis and model discussion...surface observation
southwest Alaska/Bering Sea/Aleutians...mtl