Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Tue Sep 07 2021
Valid 12Z Sat Sep 11 2021 - 12Z Wed Sep 15 2021
...Heavy rainfall likely late weekend into early next week from
the Peninsula region to the Panhandle with accumulating snow
possible in the higher elevations...
...Pattern Overview and Model Guidance...
The best clustering of guidance continues to show that a mean
upper low drifting from the northern Bering Sea toward western
Mainland Alaska will be the dominant feature during the period.
Surface systems supported by this low and surrounding shortwave
energy will produce an active pattern with heaviest precipitation
forecast to extend from parts of the Alaska Peninsula across the
southern coast/far southern mainland and into the Panhandle.
There is some persistent spread as well as variability among
consecutive runs for details that will require additional time for
fine-tuning.
Among the latest solutions, the 12Z ECMWF/GFS/UKMET offered the
most coherent grouping for a blend that would provide improved
detail over the means during the first half of the period. There
are differences in specifics even among these but as a whole they
support two defined features, a developing southern Bering wave
that tracks toward and then lingers near the western coast of the
mainland along with a North Pacific wave that should track toward
the southern coast. The front anchored by this wave may reach the
Panhandle by early next week. Recent trends have been somewhat
slower with this front. The 12Z CMC is faster than the majority
for best-defined low pressure nearing the western coast of the
mainland and farther south with the North Pacific wave. Also note
that the blend downplays the deeper/southward GFS for the system
initially near the Panhandle.
By mid-late period the 12Z GFS strayed to the south of the
dominant cluster for the upper low near the western mainland while
the 12Z ECMWF remained closer to the latest ensemble means.
Meanwhile there is still a decent signal for a system that may
reach the Aleutians/Bering Sea by next Tuesday-Wednesday but with
a lot of timing/track spread. Latest GFS runs have tended to be
on the slower side of the envelope. The 12Z ECMWF adjusted
considerably slower after the 00Z run was on the fast side. The
12Z CMC is also fast. Guidance has been erratic for details over
the Arctic. The latest consensus blend suggests that some degree
of mean ridging aloft should extend from northwestern Canada to
the northern coast of the mainland by next Monday-Wednesday.
Model/mean comparisons and detail uncertainty led to a rapid
increase of ensemble mean weight after early Monday while the
forecast incorporated more lingering operational ECMWF input
relative to the GFS. The 12Z CMC mean was a bit deeper than other
means with the Aleutians wave late in the period and was
reasonable elsewhere so it was included as well.
...Sensible Weather...
A strong Northeast Pacific system late this week should be
weakening near the Panhandle as of early Saturday. Expect
associated moderate to locally heavy precipitation to continue
into Saturday over parts of the southern Panhandle. Farther
upstream one system is likely to track from the southern Bering
Sea northeast toward the western coast of the mainland while a
North Pacific system tracks near the Alaska Peninsula and southern
coast. These features should produce another significant
precipitation event spreading from west to east during the latter
half of the weekend into early next week. Best potential for
highest totals will extend from parts of the
Peninsula/southwestern mainland through the southern coast and
Panhandle. Favored terrain over the southern mainland may also
see enhanced totals. In addition lowering snow levels may bring
accumulating snowfall to some of the mountains across southern
Alaska. Moisture may reach farther northward across the mainland
but with precipitation tending to be lighter. Another system
could bring rain and brisk winds into the Aleutians by midweek but
with low confidence for the specifics.
Much of the southern mainland and Panhandle will see below normal
high temperatures for most of the period. As the Bering Sea upper
low approaches, some of the cool air will gradually spread
northeastward. Thus the above normal readings across the
northeast during the weekend should decline to normal or below
normal for next week. Clouds and moisture will tend to favor
above normal low temperatures over most locations except for parts
of the southwest.
Rausch
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Fri, Sep 10 and Sun-Mon, Sep 12-Sep 13.
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