Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
702 PM EDT Fri Oct 15 2021
Valid 12Z Tue Oct 19 2021 - 12Z Sat Oct 23 2021
...Stormy Pattern Continues from the Aleutians through Southern
and Southeast Alaska...
... Guidance and Predictability Assessment...
The extended range starts Tuesday with a low pressure system in
the Gulf of Alaska and another one moving into the Bering Sea.
Both systems should weaken by midweek as the next shortwave drops
in across the Aleutians and into the Gulf. This in part may help
to reinvigorate whatever might be left of T.S. Namtheun (currently
in the West Pac) by that time, eventually resulting in a deep
surface low (current guidance suggests upper 960s to low 970mb)
well south of Alaska by Thursday/day 6. The latest 12z models
actually show fairly good agreement on this pattern, so a general
model compromise worked as a good starting point for the WPC progs
days 4-6. After that, the models begin to diverge some with
respect to timing and placement of this low as it slowly lifts
northward towards the Panhandle/Gulf of Alaska. on bringing the
low more north towards the Panhandle (ECMWF/CMC) or keeping it
south (GFS). There is also timing/placement spread across the
Bering Sea at the same time as another surface low tries to move
across the eastern Aleutians/AK Pen region day 7-8. Given the time
frame, the models actually show fairly good agreement on the large
scale, but it's the details which remain in question. To help
mitigate these late period differences, WPC trended towards the
ensemble means days 7-8, along with smaller contributions from the
12z ECMWF and GFS. This approach allowed for good continuity from
yesterdays forecast, as well as with downstream across the CONUS.
...Pattern Overview and Weather/Hazard Highlights...
The mean low pressure pattern over the Aleutians and Gulf will
keep cloudy/unsettled conditions through the extended period.
Gusty winds with scattered to widespread precipitation (locally
heavy) will continue into the early and middle part of next week
across parts of the southern Coast and the Panhandle region ahead
of the weakening initial low pressure system in the Gulf. Snow is
also possible in the higher elevations of southern and southeast
Alaska.
At the same time, upstream flow should drive a deep storm into the
Bering Sea by the start of the period with some light
precipitation possible across far western mainland Alaska along
with a period of stronger wind. This system should weaken as
southern stream energy spins up a surface low south of the
Aleutians into the Gulf and yet another deep system enters the
western Bering/Aleutians. This likely brings another round of
gusty winds and moderate precipitation to parts of the Aleutians
Wed-Fri before the low departs into the Gulf and the pressure
gradient weakens. The attendant cold front pushes eastward as we
move into next weekend which should bring a round of enhanced and
potentially heavy precipitation to parts of the Panhandle at the
very end of the period.
Temperatures from the Aleutians to the Panhandle should remain
below normal through the period as a parade of storms moves
through these regions. Into interior and northern Alaska, a
warming trend is expected by the early to middle part of next week
as upper level ridging tries to work over the state.
Santorelli
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Thu, Oct 19-Oct 21.
- Heavy precipitation across portions of the Alaska Panhandle,
Fri, Oct 22.
- High winds across portions of mainland Alaska, Tue, Oct 19.
- High winds across portions of mainland Alaska and the Aleutians,
Wed-Fri, Oct 20-Oct 22.
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