Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
341 PM akdt Tuesday Jul 7 2015
Analysis and upper levels...
broad cyclonic flow in the upper levels associated with a low
anchored over the Bering Sea continues across the forecast area.
Upper waves and associated surface features...embedded in the
flow propagate west to east from the Bering Sea to the
Mainland/Gulf. Water vapor imagery shows the an upper trough
associated with the frontal system which moved through SW Alaska
yesterday and southcentral overnight to be clearing the Copper
River basin this afternoon. This front brought areas of wetting
rains to SW Alaska Monday and over southcentral overnight/this
morning...except for the Copper River basin. Conditions on the
backside of the front are more showery and generally not very well
organized. Gusty gap flows across Turnagain Arm...Portage
Valley...and the Knik River Valley have diminished from overnight
highs. The Copper River basin continues to see gusty south winds
which will likely hold into the evening hours tonight. Areas of
gusty south to southwest winds associated with weak troughs are
evident across the Bering Sea/Aleutians and coastal SW Alaska
Model discussion (day 1 and 2)
models are in fairly good agreement over the next couple of days.
They continue to hold the main upper center over the Bering Sea
and persist with broad cyclonic flow aloft and shower activity
across the majority of the forecast area.
The main upper low is anchored over the western Bering Sea. This
is resulting in south southwest flow over the Mainland. An upper
trough is rotating across the southern Mainland now. A second
upper trough is digging across the eastern Aleutians and will
rotate northeast across Kodiak Island to the north Gulf Coast
Wednesday night into Thursday morning. The net result of all this
is a continuation of the cool moist stable pattern over the
southern Mainland and southwest Alaska through Thursday. A weak
upper level ridge will build into the Copper River basin Thursday
night and Friday. This will usher in slightly warmer and drier
conditions to the Copper River basin for this coming weekend.
Short term forecast southcentral Alaska (days 1 and 2)...
with the front over the Copper River basin continuing its
northward progression into the interior...more unstable and
showery conditions are expected over much of southcentral
through Thursday. Diminishing high pressure along with onshore
flow over the area...will usher in cooler temperatures and
resupply moisture over southcentral through Thursday.
Additionally with this pattern change and no major pressure
centers near southcentral...winds will begin to diminish and
remain light through Thursday. However...models show a weak
gradient redeveloping each afternoon between Prince William
Sound and the Copper River basin...keeping some gusty winds
along the Copper River and northeast Prince William Sound
each afternoon/evening through Thursday.
Short term forecast southwest Alaska (days 1 and 2)...
the Mainland is getting into a stagnant pattern aloft with a large
stacked low pressure system controlling the pattern in the Bering.
Cloudy conditions will prevail for the next couple days with
periodic breaks of sun as well as rain showers. The rain showers
will be driven by weak disturbances propagating through the flow
around the upper circulation...but will be enhanced by diurnal
heating and local terrain features. Temperatures will hover around
normal with generally light southerly winds for the next couple
days. Expect the first disturbance to move ashore in Bristol Bay
early Wednesday morning and enhance shower activity along higher
terrain through the afternoon and evening hours. Another stronger
disturbance follows the first Wednesday night into Thursday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
cyclonic flow encompasses much of the Bering Sea with rain showers
but a couple of areas of steadier rain will be moving across the
central Bering Sea associated with some upper level disturbances.
Persistent southeasterly flow over the eastern Bering Sea will
bring some higher dew point air over cooler waters of the
northeastern Bering Sea. The combination of these conditions
should bring a widespread area of dense fog from Saint Matthew
Island through Nunivak Island extending southward to just north of
the Pribilof Islands.
Long term forecast (days 3 through 7)...
A persistent large area of low pressure will remain centered over
the Bering through at least the weekend...with an attendant weak
upper level low at times extending into the Gulf especially late
this week into the first part of the weekend. As long as the
attendant weak upper level low persists in the Gulf...strong
onshore flow will be significantly...but not
totally...reduced...with easterly waves and some diurnal heating
largely dictating where this shower activity will be favored each
day. The weak upper level low over the Gulf also allows for
ridging toward the interior to be a bit stronger (though weak) as
A weak front may move north and east toward the southern
Mainland toward Sunday and Monday of next week and bring a more
sustained period of rain especially along the coast...but
confidence in this particular feature is low right now.
As far as impacts...the cloudy and cooler...and showery conditions
mean fire weather conditions will continue to be more favorable
for wildfire suppression efforts over southwest and southcentral
Synopsis and model discussion...British Columbia
southwest Alaska/Bering Sea/Aleutians...ml