Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
555 PM EDT Thu Sep 24 2020
Valid 12Z Mon Sep 28 2020 - 12Z Fri Oct 02 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 strong ridge over western Canada and a mean
trough (with one or more 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 (Monday-Tuesday), eventually rotating north and
westward towards western AK/eastern Bering Sea. The latest 12z
operational GFS and ECMWF show enough agreement both regarding
strength and timing of this low that a majority weighting of these
in the blend works well. After day 5, operational models show
increased chaos with a couple of shortwaves rounding the base of
the mean trough, and especially with extra tropical remnants of
cyclone Dolphin (currently located east of Japan). This low enters
the picture by day 5 and should stay well south of the Aleutians
as it translates east, and then begin to shift northward into the
Gulf by days 7/8 aided by the blocky ridge over Western Canada.
The 12z GFS is faster and stronger with the surface low, however
the 12z ECMWF is a hair faster/stronger than its previous 00z run.
However, given the time range and continued uncertainty, the WPC
blend leaned more towards the ensemble means by days 7 and 8, with
some inclusion of the operational models just to maintain some
definition to the system.
...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 winds likely to accompany. Trailing cold
front could contain one or more waves for localized enhancement
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 southern
mainland/Panhandle with some pockets of warmer than average highs
could exist over the south. Other locations should see above
normal highs during the period. Expect widespread coverage of
above normal lows with greatest anomalies tending to be over some
central/northern areas. Any modestly below normal lows should be
localized and limited to the extreme western/southwestern
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 the Alaska Panhandle and
mainland Alaska, Sun, Sep 27.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Sep 29-Sep 30.
- High winds across portions of the Alaska Panhandle and mainland
Alaska, Sun, Sep 27.
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