Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
629 PM EDT Sat Jul 31 2021
Valid 12Z Wed Aug 04 2021 - 12Z Sun Aug 08 2021
...Overview and Guidance/Predictability Evaluation...
An amplified upper level pattern should be in place to begin the
extended range on Sunday with a blocky ridge across mainland
Alaska keeping deep troughing in both the Gulf of Alaska and the
Bering Sea. The ridge will limit the overall eastward progression
of the Bering Sea low, and should force shortwave perturbations
northward across the eastern Aleutians and western Alaska. The
ridge over the mainland should begin to break down by day 6/Friday
which would allow upper troughing to move into Alaska, while the
Bering Sea low rotates slightly eastward next weekend.
The guidance shows above average agreement on the overall large
scale pattern through the period, with differences mostly related
to a couple of shortwaves/surface lows rounding the base of the
Bering Sea trough. The WPC forecast for days 4-6 was based on a
majority deterministic model blend between the 12z GFS, ECMWF, and
CMC. After that, the forecast trends towards the ECENS and GEFS
ensemble means to help mitigate the less uncertain details.
Although, at least half of the deterministic guidance was still
included even at day 8 which helped maintain some definition to
the individual systems which tend to get washed out by the means.
...Weather/Threats Highlights...
The best chance for rain for the first half of the period should
be across portions of the eastern Aleutians/Alaska Peninsula into
parts of the west coast as moisture continues to surge northward
ahead of the Bering Sea upper low. Moderate to locally heavy rain
is also expected across parts of the Panhandle region Wednesday
and Thursday as a second upper low meanders in the area before
eventually shifting into Vancouver, Canada. By Friday and into the
weekend, expect moisture to begin overspreading the southern half
of the mainland as a cold front shifts inland, with showers also
continuing across the Aleutians associated with a couple weaker
embedded surface lows.
Temperatures across the state should be above to well above
average for most of the region underneath of the strong upper
level ridge. As the ridge begins to break down and troughing moves
into the state, temperatures should trend cooler into the weekend.
Portions of the North Slope region could see daytime highs 10-15
degrees below normal as a shortwave drops southward from the
Beaufort Sea.
Santorelli
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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