Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
435 am akdt Tuesday Jul 7 2015
Analysis and upper levels...
A broad upper level low remains centered over the Bering with an
associated frontal system curving from the Seward Peninsula
southeast though south central Alaska and into the central Gulf. A
secondary circulation associated with a shortwave trough is
rotating up from Bristol Bay into southwest Alaska. The frontal
system lifting through south central Alaska is pushing the
previously established high amplitude upper level ridge off to
the east and into Canada where it will flatten. The net result
will be the start of a rather persistent pattern shift ushering in
a cooler...moister and more showery air mass.
Good model agreement continues with the synoptic scale features
and all models supporting the idea of the pattern shift now
underway. While some timing and other fine scale differences do
exist in the individual waves rotating around the large Bering
low forecast confidence is high in the trend towards a cooler and
cloudier weather pattern...with each shortwave trough rotating
through reinforcing low level moisture and bringing chances for
Rain continues across southwest Alaska with the current trough
rotating through bringing another round of widespread wetting
rain. While precipitation amounts with subsequent troughs do look
to be a bit less their frequent intrusions will keep skies
cloudy...temperatures cool and relative humidities high. Further
east in south central Alaska...fire weather conditions will be
moderating as well with the current frontal system pushing the
upper level ridge clear of the area and sweeping in a cooler and
moister air mass. Onshore flow from a surface ridge building into
the Gulf of Alaska will continue to bring breezy conditions to
favored terrain gaps however cooler temperatures and increased low
level moisture will keep relative humidity values well above
Short term forecast southcentral Alaska (days 1 and 2)...
The front moving through the southcentral stretches southeast
from the Susitna Valley into Prince William Sound. It is bringing
rain and gusty winds as it passes and will make its way into the
Copper River basin by this afternoon then head further off to the
northeast. Behind the front southwest flow and unstable conditions
will lead to shower activity through Wednesday. This change in
pattern from the high pressure that has been sitting over the area
will also allow temperatures to fall back down to cooler values.
Additionally...with no major pressure centers close to the
southcentral winds will stay relatively light through Wednesday.
Short term forecast southwest Alaska (days 1 and 2)...
The upper level vertically stacked closed low remains positioned
over the western Bering Sea. This synoptic feature will continue
to send series of shortwaves through the southwest Alaska region
through the next several days. This will bring numerous showers
with brief periods of dry conditions between shortwaves as they
move through the area. This weather pattern will result in a
showery precipitation regime heading into the midweek with
continued onshore flow.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
satellite imagery is showing the occluded low pressure system
parked near 60n 177e with the associated weather front moving into
the northern Bering Sea and extending down just east of the
Pribilof Islands. There is a large swath of moisture over the
eastern Aleutians extending northward through the Bristol Bay and
the Kuskokwim Bay. While...on the backside of the low there is
wrap around moisture being pulled into the western
Aleutians/Bering Sea with stratus and fog accompanied by gusty
westerly winds. The rest of the Bering Sea has for the most part
southerly winds and marine layer stratus with patchy fog this
morning. This low pressure system will remain almost stationary as
we head into the midweek with continued showery precipitation.
Long term forecast (days 3 through 7)...
A persistent large area of low pressure will remain centered over
the Bering through at least next weekend while upper level ridging remains
weak over the eastern interior of Alaska and western Canada. The
net result will put the southern Alaska Mainland into a
persistent cool and showery pattern...with no strong fronts
anticipated moving inland from the Pacific and Gulf...and a
surface ridge along the eastern Gulf and north Gulf Coast
supporting low level onshore flow. Shortwave troughs will
routinely rotate up from the western Gulf and eastern Bering
keeping skies mostly cloudy and bringing scattered
showers...periods of rain and near normal temperatures to much of
southwest and south central Alaska. 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.
The synoptic pattern through at least the next 7 days and
potentially well into next week will continue to be dictated by
dynamic forcing from intense tropical convection over the
equatorial Pacific...which is in a fairly static state right now.
As far as impacts...the milder and cooler conditions mean fire
weather conditions will become more persistently and consistently
favorable for wildfire suppression efforts.
Synopsis and model discussion...jer
southwest Alaska/Bering Sea/Aleutians...period