Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
759 PM EDT Tue Sep 22 2020
Valid 12Z Sat Sep 26 2020 - 12Z Wed Sep 30 2020
...Overview and Guidance/Predictability Assessment...
One upper low rotating south of the region will drive a surface
low into the Gulf and northward towards the southern coast next
weekend into early next week. There will be another upper-level
low that will drop through the Bering Strait on Friday that will
help establish a mean trough over the region by early next week.
By the tail-end of the extended period a deepening surface low and
associated frontal system will approach to the south of the
western Aleutians.
Run-to-run consistency has improved somewhat, especially for the
first couple of days. The preferred blend for this forecast was
varying weights of 12Z CMC/ECWMF/GFS with light amounts of the
ECWMF/GEFS ensemble means. While there is increasing spread in
timing and placement with the low near the Aleutians/Gulf by the
end of the forecast, utilizing the means wash out some of the
strength/details with this system- particularly the surface winds.
This low will likely have gale-force winds associated with it,
therefore, using more of the deterministic for the latter half of
the extended helped maintain at least small-craft wind speeds.
Winds will be an element of concern going forward with future
forecasts.
...Weather/Threats Highlights...
Parts of the southern coastal areas may have heavy rain associated
with a surface low entering the Gulf of Alaska and hanging around
for a few days. There may be a mix of snow, particularly in the
higher elevations. As the low weakens so does the coverage of
precipitation. However, another low is expected to lift northward
through the Gulf which may result in locally moderate to heavy
precipitation by early/mid-week across the Southeast and parts of
the southern Mainland. The western Aleutians will have
increasingly unsettled and rainy conditions by early next week as
another low pressure system approaches. With mean troughing in
place over much of the state, daily temperatures will largely be
near or below normal for late September.
Campbell
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 precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue, Sep 29.
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