Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Thu Sep 02 2021
Valid 12Z Mon Sep 06 2021 - 12Z Fri Sep 10 2021
...Pattern Overview and Model Guidance...
The main large-scale upper-level feature affecting Alaska through
the period will be a broad upper low spinning west of the state,
possibly meandering a bit southward into the Bering Sea as the
workweek progresses. While this feature's existence has good
agreement among model/ensemble guidance, shortwave troughs and
thus surface low pressure systems moving around this broad gyre
are less consistent. First, the shortwave lifting northward
through the western part of the state on Mon/Tue has some
timing/strength differences. But another shortwave tracking across
the southern side of the upper low could cause even more forecast
issues in terms of the surface pattern as well. The shortwave is
generally expected to move eastward from the northwestern Pacific
somewhere near the Aleutians and the southwestern mainland, but
with ample differences in timing and the degree of interaction
with the main upper low from model to model and run to run,
lessening confidence. Surface low tracks vary quite a bit with
possibilities for lows north of the Aleutians into the Bering Sea
as well as south into the Gulf of Alaska. The 12Z GFS and ECMWF
came in farther north with this surface low compared to previous
runs, which aligns better with the ensemble mean solutions. Thus
the WPC forecast leaned toward a forecast into the
Bering/southwestern mainland for now, but changes in future
forecast issuances are quite possible.
Additionally, for the early part of the workweek, a potent
shortwave will push a surface low through the northeastern Pacific
toward the Panhandle. The current 12Z model cycle and the previous
00Z model cycle have both trended much stronger with the surface
low among basically all reliable guidance, and deepening this low
is the main change to the previous WPC forecast. The low is
forecast to fill/weaken midweek as it approaches the Panhandle.
The WPC forecast utilized a mainly operational model blend early
in the forecast period, trending toward the GEFS and EC means
later in the period. The 12Z CMC was excluded after Day 4/Mon, as
its pattern especially in the northwest Pacific pushed the
aforementioned shortwave eastward more quickly than consensus as
it phased more with the persistent northern low.
...Sensible Weather...
A wet pattern is likely across the Panhandle for the first part of
the workweek given moist inflow ahead of the potent surface low
moving toward the region, and rainfall totals could be modest to
locally heavy there. Precipitation is also expected in the
northwest part of the state early in the week with the initial
shortwave, with higher totals in terrain enhanced areas, some of
which could fall as snow. Then, despite the uncertainty in the
exact track of the southern shortwave and low, enhanced rainfall
seems possible for the Aleutians and then likely across the Alaska
Peninsula and eastward toward the Kenai Peninsula for the latter
part of the week.
With general southerly moist flow across the state, cloudy
conditions are likely. High temperatures are expected to be around
or below normal across all but the eastern part of the mainland
especially for the early part of the week. Meanwhile, low
temperatures are likely to be around or slightly above average for
most of Alaska other than the western mainland.
Tate
No significant hazards are expected over Alaska during this
forecast period.
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