Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
627 PM EDT Wed Aug 25 2021
Valid 12Z Sun Aug 29 2021 - 12Z Thu Sep 02 2021
...Pattern Overview and Model Guidance...
A split flow pattern is expected to be in place with a strong
polar vortex and associated mid-level westerly flow over the
Arctic, and a slow moving (almost Omega block) progression of
shortwaves over the North Pacific. The storm system initially
over the eastern Gulf will exit over British Columbia and weaken,
followed by a building ridge over the Gulf and extending into
central Alaska. By the middle of next week, it appears likely a
large scale upper trough and potential closed low could develop
across the Bering and the Aleutians.
In terms of the 12Z model guidance suite, there has been some
improvement in the degree of model spread compared to the past
couple of days. By Tuesday, the GFS becomes stronger with a
shortwave trough south of the Aleutians and its ridge extends
farther north into Alaska compared to the model consensus, and the
CMC is more robust with height falls from the polar vortex over
the North Slope towards the end of the forecast period. However,
these differences are not considered egregious and some of their
solutions were incorporated into the forecast process, along with
a slightly higher component of the 12Z ECMWF and increasing use of
the EC/GEFS means by the second half of the forecast period.
...Sensible Weather...
Improving weather can be expected for much of the state, including
the panhandle region, by Sunday as the Gulf low moves inland and
weakens, with some lingering showers remaining south of Juneau.
The next significant round of rainfall arrives for the Aleutians
and eastward to the Kenai Peninsula for the beginning to middle of
next week in conjunction with the slow moving storm system south
of the region. There is still some significant model spread
regarding the placement of the main QPF axis, so some adjustments
in future forecasts can be expected going forward. Another area
of significant rainfall and mountain snow is also likely for
portions of interior western Alaska for Sunday and Monday.
In terms of temperatures, below average readings are likely across
much of the Interior through the beginning of the week, with a
moderation trend likely through Wednesday as the upper level ridge
tries to build back in. Temperatures should continue to remain on
the cool side for coastal portions of southern Alaska where clouds
and rainfall will be more prevalent.
Hamrick
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat, Aug 28.
- High winds across portions of mainland Alaska, Sat, Aug 28.
- Much below normal temperatures across portions of mainland
Alaska, Sat, Aug 28.
- High significant wave heights for coastal portions of mainland
Alaska, Sat, Aug 28.
- Heavy rain across portions of mainland Alaska, Wed, Sep 1.
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