Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
601 PM EDT Fri Sep 25 2020
Valid 12Z Tue Sep 29 2020 - 12Z Sat Oct 03 2020
...Overview and Guidance/Predictability Assessment...
Model/ensemble guidance is agreeable and consistent in showing a
transition to a very amplified upper pattern during the medium
range, featuring a strengthening ridge over western Canada and a
mean trough (with a couple of embedded lows) over and extending
well south from the western mainland. This configuration should
lead to a very wet period from portions of the Peninsula region to
the Southern Coast and Panhandle. Guidance behavior thus far
suggests well above average confidence in the mean pattern but
models continue to have considerable difficulty with the specifics
for individual systems.
Two main forecast concerns for the period across Alaska. The first
is a deep surface low lifting northward towards the Peninsula
region days 4-5 (Tuesday-Wednesday). The latest 12z operational
GFS is the slowest of the solutions, although the 12z ECMWF did
trend slower from its 00z run. Despite the minor timing
differences, a compromise of the two provided a good starting
point. After day 5, operational models continue to struggle with a
couple of shortwaves rounding the base of the mean trough, and
especially with extra tropical remnants of cyclone Dolphin. This
low enters the domain by day 5 and should stay well south of the
Aleutians as it translates east, and then begins to shift towards
the Alaska Peninsula by days 6/7 steered by the blocky ridge over
Western Canada. The 12z ECMWF is faster with the surface low,
while the positioning of the 12z GFS is much closer to that of the
ensemble means (which do show very good agreement). Given the time
range and continued uncertainty, the WPC blend leaned more towards
the ensemble means by days 7 and 8, with some small contributions
of the 12z GFS just for some added pattern definition.
...Weather/Threats Highlights...
The general pattern evolution will favor increasing potential for
heavy precipitation from the Peninsula to the northern Panhandle.
The details of timing, magnitude, and coverage are far from
resolved though. The first surface low tracking northward towards
Kodiak Island, should pull significant moisture into the Gulf of
Alaska and eventually the southern Coast/northern Panhandle early
next week, with gusty, gale force, winds likely to accompany.
Trailing cold front may provide localized enhancement of
precipitation beyond midweek as well. The next system (remnants of
Dolphin) will probably bring another round of moderate to heavy
rainfall late next week, but the timing and intensity details
remain in question. Elsewhere, expect periods of mostly light
precipitation over the Aleutians with some scattered activity over
the mainland away from the main focus in the south.
Below normal high temperatures will prevail over the North Slope
(though gradually moderating with time) and pockets of the
mainland. Expect widespread coverage of above normal lows
prevailing across much of the Alaska mainland.
Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of mainland Alaska, Tue-Fri,
Sep 29-Oct 2.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Fri, Sep 28-Oct 2.
- High winds across portions of mainland Alaska, Tue-Wed, Sep
29-Sep 30.
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