Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Sat Aug 07 2021
Valid 12Z Wed Aug 11 2021 - 12Z Sun Aug 15 2021
...Multi-day Heavy Precipitation Threat for portions of southern
Alaska next week...
...Overview and Guidance/Predictability Evaluation...
Model and ensemble solutions overall seem reasonably well
clustered Wednesday into Friday and a composite blend of the 12
UTC GFS/ECMWF/Canadian was used along with the 19 UTC National
Blend of Models to derive the WPC Alaskan product suite in a
pattern with above normal predictability. However, steadily
increasingly forecast spread and run to run continuity issues
become increasingly evident in the models heading into next
weekend. Accordingly relied more significantly at these longer
time frames on the still compatible 12 UTC GEFS/ECMWF ensemble
means as per collaboration with local WFO's, albeit with manual
adjustments to maintain lost system organization/detail/comtinuity
despite lowered predictability.
A cooling upper level trough will be in place over much of the
state and the Bering Sea for much of next week. An upper level
ridge over British Columbia and the Pacific Northwest will tend to
limit the eastward progress of the trough, resulting in a
multi-day heavy rainfall threat from south-central into southeast
Alaska. This will especially be from the Kenai Peninsula to the
northern Southeast Panhandle as fueled by a deep layered and long
fetch plume of tropical sourced moisture with extratropical lows
associated with Nida, 91W, and Mirinae from the west Pacific. By
next weekend, a storm system over the North Pacific may affect the
Aleutians. This system would be an extratropical low associated
with Lupit. Meanwhile, a closed low over the Arctic Ocean may
force a front southward through an unsettled North Slope and
Brooks Range mid-later next week. The vigorous closed low may
eject next weekend with approach of upstream system energies.
...Weather/Threats Highlights...
Multiple rounds of heavy precipitation are expected from
south-central Alaska to northern portions of the Southeast
Panhandle as strong and deep layered southwesterly flow advects
copious moisture inland. This moisture feed will be periodically
enhanced with approach of extratropical systems associated with
west-Pacific tropical systems Nida, 91W and Mirinae. This could
result in some locations potentially getting 3 to 8+ inches of
rainfall across topographically favored areas in a long fetch
atmospheric river episode. This multi-day event will focus
Wednesday-Friday, but uncertainty with moisture/extratropical low
track associated with Mirinae opens the door for a continued
threat into next weekend. WPC surface progs take a stab at a
possible track into the Gulf of Alaska that is not well depicted
in guidance, but has decent upstream continuity. This pattern will
likely lead to some instances of flooding and enhanced run-off.
Meanwhile, upper level troughing over the state will also tend to
keep temperatures below normal across most of the state for this
forecast period. These anomalies will likely be even greater for
an unsettled Brooks Range and points northward of an Arctic front
sinking into the Interior mid-later next week, moving through in
response to a potent closed low north of the Arctic Coast. The
closed Arctic low may eject next weekend in response to approach
of upstream upper trough energies that may allow a new low/system
work into the Chukchi Sea, Bering Strait, and northern Bering Sea.
Schichtel
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html