Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
435 PM EDT Sat Sep 25 2021
Valid 12Z Wed Sep 29 2021 - 12Z Sun Oct 03 2021
...Three Major Storms for Alaska...
...Guidance/Predictability Assessment and Pattern/Hazard
Highlights...
The models continue to provide a reasonable handling of the medium
range pattern including the overall development of major storms
next week, bolstering forecast confidence. I again suggest a blend
of the 12 UTC GFS/ECMWF/UKMET/Canadian days 4-6 (Wed-Fri) along
with the 19 UTC National Blend of Models and would keep the well
clustered GFS/ECMWF in the mix along with the best compatible
ECMWF ensemble mean for days 7/8 (next weekend). The ECMWF
ensemble mean is slightly more amplified upstream than the 12 UTC
GEFS at these longer time frames and that seems more reasonable
considering expected approach toward the SW Bering Sea of the deep
extratropical low associated with current west Pacific Typhoon
Mindulle. WPC product continuity was well maintained in this
manner along with some manual surface system modifications to
ensure sufficient offshore storm depth/intensity consistent with a
highly supportive and amplified upper flow evolution as a series
of powerful closed lows/troughs work through the Bering Sea and
into the Gulf of Alaska.
A deep storm system through midweek will bring a threat of high
winds/waves across the Gulf of Alaska, allowing a southwesterly
push of moisture to funnel especially into the Panhandle where
rainfall amounts may be locally heavy in the valleys (and snow at
the higher elevations). The next dangerous storm system out of the
northern Bering will robustly dig to bring high winds/waves along
with an enhanced rain/snow to western then southwestern areas of
the mainland. Into later next week as the system slams into the
highly unsettled Gulf, another round of potentially heavy
rain/snow are expected for the Panhandle and coastal areas of
south-central Alaska. Temperatures will overall generally be below
normal for the period due to the continued troughing over much of
the state. Farther upstream, powerhouse typhoon Mindulle will
start to recurve into the westerlies from the south of Japan by
midweek. After that, the ensemble consensus takes the system to
the northeast into/toward the trough to its north/northeast. A
deep extratropical low associated with Mindulle is slated to track
toward the southwest Bering Sea next weekend as a maritime and
Aleutians heavy seas/wind/rainfall threat.
Schichtel
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