Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
210 PM akdt Wednesday Aug 27 2014
Analysis and upper levels...
A very progressive westerly split-flow wave pattern continues
across the Anchorage forecast office area of responsibility this
morning and afternoon. Low pressure system nearing Bristol Bay
early this afternoon with downstream ridging over the eastern Gulf
and along the alcan border puts the western Gulf and most of
southcentral in a diffluent upper level pattern. In addition there
are at least two...if not more...embedded waves moving north and
east across the Cook Inlet region today...and with ample
instability in the lower to middle levels of the atmosphere light to
moderate rain showers continue.
Out west...weak transitory ridging over the south-central Bering
and central Aleutians is quickly giving way to a front associated
with a low about 510 nm west-southwest of Attu...keeping the
southern Bering and most of the Aleutian chain in rain and rain
Models are in good agreement. The latest model trends suggest the
surface low that will race east across the Gulf of Alaska tonight
into early Thursday might be a bit farther north than earlier
predicted. This would result in stronger barrier-jet winds to gale
force especially in the Barren Islands region. Out west...models
are in good agreement in pushing a front well ahead of the next
low that is located well west-southwest of Attu that will
gradually weaken as it moves into the central Bering by Friday
Short term forecast...
Southwest flow aloft down to mountaintop level...with decent upper
level diffluent and antecedent instability have resulted in light
to occasional moderate rain showers over the Cook Inlet and mat-
su valley regions this morning. These conditions will lift north
as more stable maritime air begins to filter into the region from
the south through the day. In addition...surface flow will
gradually shift to offshore as low pressure moves from Bristol Bay
through the Gulf of Alaska...resulting in more of a traditional
stable regime overrunning scenario by tonight and Thursday as the
next northern-stream system approaches from the west for the day
This Thursday system will bring more of a steady rain to Anchorage
for the day on Thursday...with lightest amounts over east
Anchorage where some downslope drying will take place. Conditions
gradually improve Friday as the upper level trough passes through
all but the Copper River basin and west to northwest flow aloft
begins to dry things out...resulting in dry conditions for
Saturday over nearly all of southcentral.
Wet conditions will continue Thursday night most locations...as
the low pressure system over the eastern Bering and Bristol Bay
gives way to the northern stream trough Thursday. Conditions
improve Friday into Saturday as cool northwesterly flow aloft
combines with offshore flow near the surface.
Alternating northern and southern stream systems in fast westerly
flow aloft will keep the Bering Sea and Aleutians unsettled
through Friday...with gradual improvement from west to east
Saturday into Sunday.
Long term forecast...
The dominant feature to begin the extended term will be a large
upper level low anchored over the Chukchi Sea with a broad
longwave trough extending over Mainland Alaska as another in a
series of surface lows moves eastward across the Gulf this
weekend. This will keep a chance of rain in the forecast across
much of the southern Mainland into early next week...particularly
along the northern Gulf Coast. The trough axis will gradually
progress eastward and give way to broad west to northwesterly
upper level flow which will yield generally drier conditions
by the middle of next week.
Marine...Gale Warning 130 131.
Cirrocumulus/cumulonimbus Aug 14