Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
604 PM EDT Thu Sep 10 2020
Valid 12Z Mon Sep 14 2020 - 12Z Fri Sep 18 2020
...Potent storm with impacts for the Aleutians and into
Southwest/parts of southern Alaska this weekend into early next
week...
...Weather/Threats Highlights and Guidance/Predictability
Assessment...
By the beginning of the Alaska medium range period (Monday, Sept
14), a potent storm is slated to reach the Bering Sea as the
attendant cold front swings through the Aleutians. This low will
have ample energy from a northern stream closed upper trough/low
as well as tropical influences from former Super Typhoon Haishen.
Expect a threat for high winds/waves and heavy rainfall impacting
the Bering Sea/Aleutians this weekend, moving into southwest AK by
early next week. There is more uncertainty regarding the evolution
of this low past about mid week and thus severity of impacts
across Southern Alaska and the Gulf Coast. Regardless, some
precipitation and gusty winds are likely as the front weakens as
it moves eastward next week. Elsewhere, a separate Arctic stream
flow to the north should bring a cooling frontal push into the
North Slope region next week, while models trend stronger with
ridging over much of the Interior/Eastern Alaska.
The models and ensembles continue to come into better agreement at
least for the first day or two of the period with regards to the
large low entering the Bering Sea. The 12z GFS is quicker to erode
the ridge across mainland Alaska through next week, also with a
quicker progression of the main upper low. The trend seems to be
for a slower progression of the low, lingering across the Bering
longer and with stronger ridging inland. This is best represented
by the 12z deterministic ECMWF and the ensemble means. The WPC
forecast blend for today used a majority deterministic blend, more
weighted towards the ECMWF than the GFS, for days 4-5. After that,
differences become much more apparent especially with respect to
additional energies undercutting the main upper low/trough sending
various surface lows towards the Aleutians/Gulf region. To help
mitigate these differences, the WPC forecast leaned heavily
towards the ensemble means (ECENS/GEFS) for days 6-8.
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, Mon-Wed, Sep
14-Sep 16.
- Heavy rain across portions of 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