Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
709 PM EDT Thu Oct 14 2021
Valid 12Z Mon Oct 18 2021 - 12Z Fri Oct 22 2021
...Stormy Pattern Continues from the Aleutians through Southern
and Southeast Alaska...
... Guidance and Predictability Assessment...
The extended range starts Monday 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 (12z ECMWF shows 950mb central pressure, 12z GFS/CMC are in
the low 970s) well south of Alaska by Thursday/day 7. 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 really diverge
on bringing the low more north towards the Panhandle (ECMWF/CMC)
or keeping it south (GFS). There is also a lot of spread across
the Bering Sea at the same time as another surface low enters the
region. The ECMWF and CMC are much faster with this, and has it
moving across the AK Pen by day 8/Friday, while the GFS keeps it
well north in the Bering. The ensemble means show good agreement
somewhere in the middle. To help mitigate these late period
differences, WPC leaned heavily on the ensemble means days 7-8.
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 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. Northerly flow around the backside of the low is also
likely to lead to gusty winds across parts of the Aleutians as
well, especially into Monday.
At the same time, upstream flow drive a deep storm into the Bering
Sea by the start of the period with some light precipitation
possible across far western mainland Alaska. This system should
weaken as a stronger surface low moves south of the Aleutians.
This shouldn't have much impact on sensible weather across the
Aleutians/southern coast except for a few fairly light and
scattered precipitation. Yet another deep cyclone makes its way
into the Bering late in the period, which likely brings another
round of gusty winds and moderate precipitation to parts of the
Aleutians. There's potential for impacts to spread farther
downstream towards southern Alaska, but this remains highly
dependent on exact placement/track of the low, which is very
uncertain at this time.
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, Mon-Wed, Oct 18-Oct 20.
- Heavy precipitation across portions of mainland Alaska, Sun, Oct
17.
- High winds across portions of mainland Alaska and the Aleutians,
Sun, Oct 17.
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