Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
737 PM EDT Thu Sep 23 2021
Valid 12Z Mon Sep 27 2021 - 12Z Fri Oct 01 2021
...Overview and Guidance/Predictability Assessment...
Active pattern will continue across the high latitudes as upper
ridging remains mostly in place between the Hawaiian islands and
the Aleutians. This favors continued/renewed troughing into the
Gulf of Alaska via the North Pacific as well as the Arctic (out of
northeastern Russia). With a rather steady pace and a Goldilocks
state in between fast/unsteady zonal flow and amplified/blocked
flow, the 12Z models/ensembles showed good agreement to start the
forecast period Mon-Wed next week. System in the southern Bering
will track eastward into the Gulf as its attendant fronts push
into the Panhandle and British Columbia. At the same time, an
upper low will drop southeastward out of northeastern Russia/East
Siberian Sea toward the Bering Strait and eventually the Gulf, per
the ensembles. As it exits Russia, the deterministic models
diverge in speed/track of the upper and surface lows, complicated
by energy streaming eastward out of Kamchatka into the southern
portion of the upper low. Given the lead time and several features
involved, opted to rely increasingly on the ensemble means (GEFS
and ECMWF ensembles) as both were close to each other through the
period.
Farther upstream, the JTWC 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 implications for the
downstream North Pacific flow beyond the medium range period
depending on its interaction with the upper trough.
...Weather Highlights...
Light to modest rain will spread across the eastern Aleutians
Monday in advance of the Bering system, with breezy conditions
around the low center. Exiting system near Haida Gwaii will
support some showers for the Panhandle to start the week. By
Tuesday into Wednesday, the Bering system will move into the Gulf,
allowing a southwesterly push of moisture to funnel into the
Panhandle where rainfall amounts may be locally heavy in the
valleys (and snow at the higher elevations). Next system out of
the northern Bering will bring additional rain/snow to western
areas of the mainland. By late next week, as the system slips into
the Gulf, another round of rain/snow will be possible for the
Panhandle and coastal areas of Southcentral. Temperatures will
generally be below normal for the period due to the continued
troughing. Readings may be 5-15 degrees below normal, especially
over the Alaska Range, with max temps in the 20s over the North
Slope/Brooks Range, 30s/40s over the Interior, and near 50 closer
to the coast.
Fracasso
Hazards:
- Heavy precipitation across much of the Alaska Panhandle,
Wed-Thu, Sep 29-Sep 30.
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