Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
602 PM EDT Mon Sep 07 2020
Valid 12Z Fri Sep 11 2020 - 12Z Tue Sep 15 2020
...Potent Weekend Storm for the Aleutians then Southwest to
Southern Alaska early next week...
...Weather/Threats Highlights and Guidance/Predictability
Assessment...
A potent weekend storm is slated reach the southern Bering
Sea/Aleutians 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.
Models and ensembles offer a reasonably similar larger scale
pattern evolution aloft, but still differ with embedded system
details and continuity, especially at the complex surface, for the
storm as well as for a separate Arctic stream flow. The Arctic
flow is more benign, but does feature some cooling frontal push to
an unsettled North Slope/Interior this week underneath an
amplified Arctic closed low/trough.
The WPC Alaskan medium range product suite was primarily derived
from a composite blend of the 12 UTC GFS/ECMWF and GEFS/ECMWF
ensemble means days 4-8. The GFS/ECMWF were highly weighted in
this blend only Day4/Friday, transitioning preferences quickly in
favor of the more consistent/compatible ensemble means and the 19
UTC National Blend of Models thereafter in a period of growing
forecast spread. Manually adjustments to the blend were applied to
ensure sufficient storm depth consistent with the tropical history
of the system and expected favorable upper support.
Schichtel
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 for parts of the Alaska Panhandle, mainland Alaska,
and 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