Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
548 PM EDT Wed Aug 26 2020
Valid 12Z Sun Aug 30 2020 - 12Z Thu Sep 03 2020
...Overview...Preferences...and Weather Highlights/Threats...
...A major cyclone is forecast over the weekend into early next
week to impact the Aleutians to southwest Alaska...
Guidance continues to advertise a significant large-scale pattern
change toward a mean upper low over the Bering Sea and downstream
ridge aloft building over the Northeast Pacific and eastern
mainland/western Canada. There is good agreement that a strong
North Pacific storm will develop and cross the Aleutians and move
into southwest Alaska, and then weaken as it drifts inland into
northern Alaska by midweek.
The latest model guidance showed above average agreement for the
end of August to early September. The agreement led to a
consensus-based forecast, with a blend of the 00-12Z ECMWF/12z
CMC/00z ECMWF Ensemble Mean/06z GEFS Mean used in composing the
fronts and adjusted sea level pressures.
Overall, there is good continuity in the forecast. The 12z ECMWF
trended a bit faster than its prior progs and the other 12z
models. The blend mitigated the timing difference by taking a
consensus approach.
Expect a broad area of precipitation and stronger winds (gales
likely to locally storm conditions possible) to accompany the
storm forecast to track from the North Pacific across the
Aleutians into the southern Bering Sea and southwest Alaska.
Showers with a lead wave and associated warm/moisture advection
stream onshore into the Alaska Peninsula and then extend across
into southeast Alaska/panhandle by the end of the weekend.
Significant precip totals will be possible this weekend into early
next week along the southern coast and over favored terrain inland
from the coast, and this includes areas down stream, all the way
to the panhandle of AK. The peak 850 mb moisture fluxes cross the
upper Aleutians late in the weekend into southwest Alaska as the
system approaches, adding support to heavy rain potential.
Clusters of showers are also expected in advance of the upper
trough across the upper Aleutians and western Alaska into early
next week before gradually decreasing on coverage by mid week as
the low decays as it heads inland.
Below normal highs will develop over western Alaska as the cyclone
arrives. This,combined with the clouds and showers, should
promote below normal temperatures early to middle portions of next
week over western Alaska. Cool temperatures are forecast to
persist north of a stationary front in northern Alaska. Warm
advection ahead of the Aleutians/Bering Sea/southwest AK storm
should spread warmer temperatures across much of southern AK
during the weekend and early next week.
Petersen
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
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