Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
623 PM EDT Wed Sep 09 2020
Valid 12Z Sun Sep 13 2020 - 12Z Thu Sep 17 2020
...Potent Weekend Storm for the Aleutians then Southwest to
Southern Alaska early next week...
...Weather/Threats Highlights and Guidance/Predictability
Assessment...
A potent storm is slated to reach the southern Bering
Sea/Aleutians by this weekend with ample energy from a northern
stream closed upper trough/low as well as tropical influences.
There is a growing signal for deep low development as enhanced by
an infusion of energy/moisture with the extratropical evolution of
former Super Typhoon Haishen. This leads to a threat of high
winds/waves and heavy rains for the Bering Sea/Aleutians this
weekend to then spread into southwest/AKpen and southern Alaska
into early next week. Some accumulating snows are possible in the
higher elevations of southern Alaska as well. In addition, a
separate Arctic stream flow to the north should bring a cooling
frontal push into an unsettled North Slope/Interior this week.
Models and ensembles have come into better agreement for the first
few days of the period across the board, though some of the more
typical smaller scale embedded flow detail differences remain.
This leads well to a majority deterministic model blend (between
the 12z ECMWF/GFS/CMC) for days 4 and into day 5. After that time,
the forecast begins to get increasingly chaotic, especially at the
surface, with regards to individual pieces of energy rounding the
base of the large upper low sending various surface lows towards
the Aleutians/southern Alaska early to mid next week. While the
12z ECMWF is less of an outlier than in previous days, and more
closely resembles that of the GFS, it seems safe at this point to
lean more on the ensemble means days 6-8 to mitigate the
uncertainties. This approach provides good continuity with
yesterday's WPC forecast, with manual adjustments made to ensure
sufficient storm depth which is consistent with the tropical
history of the system and favorable upper level support.
Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain across portions of mainland Alaska and the Aleutians,
Sun-Mon, Sep 13-Sep 14.
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Tue-Wed, Sep 15-Sep 16.
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