Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
725 PM EDT Fri Sep 24 2021
Valid 12Z Tue Sep 28 2021 - 12Z Sat Oct 02 2021
...Two More Major Storms for Alaska Next Week...
...Guidance/Predictability Assessment and Pattern/Hazard
Highlights...
Models and ensembles offer better than normal agreement with the
stormy pattern set to continue to affect portions of Alaska into
next week, bolstering forecast confidence. Accordingly, a
composite of the 12 UTC GFS/ECMWF/UKMET/Canadian and the 19 UTC
National Blend of Models was used as the basis for the WPC
forecast days 4-6 (Tue-Thu) whose blend acts to mitigate lingering
smaller scale system differences. These embedded system
differences then seem best addressed by additional inclusion of
larger scale compatible 12 UTC GEFS and ECMWF ensemble mean inputs
for days 7 (next Fri) and especially day 8 amid faster growing
forecast spread. 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.
By Tuesday into Wednesday, a deep storm system will bring a threat
of high winds/waves moves into the Gulf of Alaska from the Bering
Sea, 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 Gulf, another round
of potentially heavy rain/snow are expected for the Panhandle and
coastal areas of Southcentral. Temperatures will overall generally
be below normal for the period due to the continued troughing over
much of the state.
Farther upstream, the JTWC still forecasts Tropical Storm 20W
(Mindulle) to strengthen into a strong hurricane and start to
recurve northward south of Japan by midweek. After that, the
ensemble consensus takes the system to the northeast into/toward
the trough to its north/northeast. This may have amplitude
implications for the downstream North Pacific flow beyond the
medium range period.
Schichtel
Hazards:
- Heavy precipitation across much of the Alaska Panhandle,
Wed-Fri, Sep 29-Oct 1.
- Heavy precipitation along the southern coastal section and
higher terrain of Mainland Alaska, Thu-Fri, Sep 30-Oct 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