Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
645 PM EDT Sat Oct 03 2020
Valid 12Z Wed Oct 07 2020 - 12Z Sun Oct 11 2020
...Overview and Guidance/Predictability Assessment...
Most of the latest models and ensemble means are
similar/consistent with the idea of progressive North Pacific flow
helping to develop an elongated upper trough over the northeastern
Pacific and possibly into the Gulf of Alaska, ahead of a deep
north-central Bering Sea storm that will weaken very slowly as it
likely tracks toward the coast of the southwestern mainland. By
early day 5 Thu consensus still depicts a frontal wave forming
near the Alaska Peninsula but after that time agreement remains
poor regarding details of this wave and a possible
northward-tracking wave ahead of it. Important details are small
in scale, leading to continued low predictability. There is a
general signal that whatever low pressure exists over the
northeastern Pacific into Fri-Sat should trend north/northwest
into the Gulf--favored by evolution of troughing aloft and the
approach of the Bering Sea upper low. Meanwhile compared to
yesterday a number of solutions have changed the depiction of the
upper ridge that crosses the mainland mid-late week, forming a
closed high that ultimately settles over the Arctic in contrast to
a more open ridge from western Canada northwestward. The
potential for energy to slide underneath the upper high provides
added complexity to the forecast next weekend.
A 12Z operational model blend provided good continuity/detail into
early day 5 Thu. After that time the forecast began to
incorporate 12Z GEFS/00Z ECMWF mean input (reaching 50-60 percent
by days 7-8 Sat-Sun) as confidence rapidly decreased for surface
details over the Northeast Pacific/Gulf of Alaska. The forecast
excluded the 12Z CMC after Thu due to not bringing the Alaska
Peninsula wave into the northeastern Pacific and straying south
with the Bering Sea upper low. Meanwhile ECMWF input also
included some of the old 00Z run mid-late period to downplay the
12Z run that trended faster than consensus with the Bering Sea
system. The new 12Z ECMWF mean is quicker to close off an Arctic
upper high, supporting trends in the 12Z GEFS/CMC means and recent
ECMWF runs/12Z GFS.
...Weather/Threats Highlights...
Around midweek the strong Bering storm will be spreading a broad
area of precipitation and strong winds across the Aleutians and
Bering Sea with some of the leading moisture and brisk winds
extending into the far southwestern mainland. From Thu into the
weekend there will be potential for one or more areas of focused
precipitation within an area from the Alaska Peninsula/Kodiak
Island through the southern coast and Panhandle. However
predictability and confidence are well below average for important
details of one or more surface low centers over the northeastern
Pacific/Gulf of Alaska during this time. Therefore it will take
additional time to determine the specifics of coverage, duration,
and intensity of precipitation. The Aleutians should see
unsettled weather with the strong midweek winds trending gradually
weaker with time, though still remaining on the brisk side in most
cases.
Much of the state will see above normal temperatures from Wed
through the weekend. The Panhandle is the one likely exception
with mostly below normal readings, while some pockets of below
normal highs may exist over the southern mainland as well.
Anomalies for min temperatures will generally be warmer than those
for highs across a majority of the state.
Rausch
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
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