Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
709 PM EDT Thu Aug 26 2021
Valid 12Z Mon Aug 30 2021 - 12Z Fri Sep 03 2021
...Heavy rainfall possible for portions of the southern Coast
region during the middle part of next week...
...Pattern Overview and Model Guidance...
A large scale upper trough will progress eastward across the
Aleutians into the Gulf of Alaska next week as an upper low
settles across the Bering Sea. A couple of shortwaves moving
through the base of the trough should send a couple of surface low
pressure systems across the Aleutians into the Alaska
Peninsula/southern Coast. Meanwhile, upper level ridging should
build across the mainland Alaska the first half of next week
before slowly shifting east as lower heights move into western
Alaska. To the north, a strong polar vortex and associated
mid-level westerly flow will be in place across the Arctic and
North Slope early in the week before heights begin to rise again
across the region and the Polar Vortex retreats to the north.
Models and ensembles continue to show fairly good agreement on the
synoptic pattern through much of next week, with typical
uncertainties regarding the timing and details of individual
systems later in the period. A general model compromise worked
well as a starting point for days 4-5, leaning more towards the
ensemble means days 6-8 to help mitigate the smaller scale
differences. Minor contributions from the ECMWF/GFS were continued
through day 8 though, just to add some definition to the washed
out means. This approach maintained reasonable continuity with
yesterdays WPC forecast as well as the forecast downstream across
western North America.
...Sensible Weather...
Much of mainland Alaska should be dry to start next week with the
exception of some light showers along a frontal boundary draped
across northern Alaska. The main weather maker will be a low
pressure system approaching the Alaska Peninsula, lifting into the
eastern Bering Sea, as it's attendant cold front drifts eastward
through the Gulf. This will bring the potential for moderate to
heavy rainfall and gusty winds from the Peninsula to the southern
Coast. Models indicate 2 to 3 inches of rain is possible,
especially for the southern coastal parts of the upper Peninsula,
Kodiak Island, Kenai, and the Cook Inlet Thursday into Friday.
Heavy rainfall may also extend eastward into far southeast Alaska
and parts of the Panhandle next Friday and the weekend.
Temperatures across the northern and central parts of Alaska
should begin below to well below normal on Monday, before a
warming trend kicks in the rest of the week. By Friday, much of
the state could be near or above normal underneath of amplified
upper level ridging.
Santorelli
Hazards:
- Heavy rain across portions of mainland Alaska, Thu, Sep 2.
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