Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
537 PM EDT Sun Sep 20 2020
Valid 12Z Thu Sep 24 2020 - 12Z Mon Sep 28 2020
...Overview and Guidance/Predictability Assessment...
An upper low in the Gulf of Alaska to begin the period (Thursday)
will weaken in favor of an amplifying shortwave/deepening surface
low crossing south of the Aleutians and into the Gulf by next
weekend. Meanwhile, another upper low drops through the Bering
Strait mid-late week, eventually establishing mean troughing over
much of Alaska by early next week.
There is decent agreement in the models on the overall large scale
pattern, but differences in the details, especially in regards to
timing/intensity of the shortwave crossing south of the Aleutians
and an eventual surface low into the Gulf. The 12z GFS remains the
fastest of the deterministic models with the shortwave, although
the 12z ECMWF is a little faster than the previous 00z run. The
ensemble means generally follow suit with their deterministic
counterparts, but with slightly better agreement. For days 4-5,
the WPC forecast relied on a majority deterministic model blend
between the 12z GFS/ECMWF, though more weighting on the ECMWF
which is closer to the ensemble means. After day 5, leaned
increasingly heavier on the ensemble means (00z ECENS/12z GEFS) to
mitigate the detail uncertainties. Continued a little weighting
with the deterministic models even through day 8, just for some
added system definition.
...Weather/Threats Highlights...
Heavy rainfall and gusty winds across the Panhandle at the end of
the short range should be waning to begin the medium range period,
though showers may linger through the period across the southern
Mainland/Panhandle region within an otherwise unsettled pattern.
The next surface low should keep the heaviest rain south of the
Aleutians, but heavy rainfall may again ramp up across the
Panhandle/southern Alaska by the weekend/early next week as the
system enters the Gulf. Mean troughing over much of the state,
especially by late period, should keep temperatures near or below
normal.
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
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