Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
602 PM EDT Tue Sep 08 2020
Valid 12Z Sat Sep 12 2020 - 12Z Wed Sep 16 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 to then
spread into southwest/AKpen and southern Alaska into early next
week.
The models and ensembles continue to offer a reasonably similar
larger scale pattern evolution aloft, with the typical smaller
scale embedded system detail differences, especially after day 4.
To the north, separate Arctic stream flow should bring a cooling
frontal push to an unsettled North Slope/Interior this week,
underneath an amplified Arctic closed low/trough. The ensemble
means along with the GFS/CMC support this overall scenario better
than that of the 12z ECMWF. The 12z ECMWF stayed consistent with
it's 00z run showing additional energy undercutting the bigger
upper low in the Bering Sea day 5-6, which brings a surface low
towards the Eastern Aleutians, a solution not shown by the other
deterministic models or means. It's also farther south/west with
the Arctic upper low which supports stronger northerly flow for
longer over the mainland late in the period.
The WPC Alaskan medium range product suite was primarily derived
from a composite blend of the 12z GFS/CMC/ECMWF for day 4,
transitioning to heavier weighting of the ensemble means (00z
ECENS/12z NAEFS) beyond that time, with decreasing contributions
from the 12z GFS/CMC days 5-8. Given the explanation above, the
ECMWF was not included in the blend past day 5. This approach
provides good continuity with yesterday's WPC forecast as well.
Manual adjustments to the blend were also applied to the big
weekend storm to ensure sufficient storm depth consistent with the
tropical history of the system and expected 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.
- High winds across portions of mainland Alaska and the Aleutians,
Sun-Mon, Sep 13-Sep 14.
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