Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
643 PM EDT Sun Aug 01 2021
Valid 12Z Thu Aug 05 2021 - 12Z Mon Aug 09 2021
...Overview and Guidance/Predictability Evaluation...
An amplified and blocky ridge in place over mainland Alaska should
break down early in the period as a pattern shift gets underway
during the extended range. As a stagnant upper level low in the
southeast Gulf of Alaska finally shifts east into Vancouver,
Canada, and the ridge weakens, this should allow for troughing
associated with a deep upper level low across the Bering Sea to
drive a cold front into western Alaska. The Bering Sea low will
meander in the region through the entire period while a couple of
shortwaves/surface lows rotate around the base of the trough into
the Aleutians.
The guidance shows above average agreement on the overall large
scale pattern through the period, with differences mostly related
to the details of perturbations within the Bering Sea trough. The
WPC forecast relied heavily on a blend of the 12z GFS and ECMWF
through day 6, which were closer with their solutions/placement of
systems to the ensemble means. To help mitigate the less uncertain
details late period, the forecast leaned on the ensemble means,
although modest amounts of the deterministic models were still
included out through day 8 to help maintain some definition to the
individual systems which tend to get washed out by the means.
...Weather/Threats Highlights...
Moderate to heavy rain across the Panhandle should lessen after
day 4/Thursday as the low shifts into Canada and weak ridging
moves into the region. Widespread showers are also expected across
western Alaska late this week as moisture surges northward ahead
of the Bering Sea upper low. By Friday and into the weekend, the
moisture should begin overspreading the southern and central
portions of Alaska as a cold front shifts inland. Periods of
showers will continue to affect the Aleutians through next weekend
associated with a couple of weaker embedded surface lows.
Temperatures across the state should be rather warm and 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 by Friday, temperatures should quickly trend cooler
into the weekend and early next week. 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