Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
740 PM EDT Sat Aug 14 2021
Valid 12Z Wed Aug 18 2021 - 12Z Sun Aug 22 2021
...Overview...
Guidance as a whole advertises low-amplitude mean flow aloft
between an upper low over the Arctic Ocean and a ridge over the
eastern Pacific. Embedded shortwaves should move along in
multiple streams, with considerable uncertainty over the amplitude
and progression of most of these features as well as for what
interaction may occur between streams at any particular time. The
average of guidance shows upper heights declining some over the
Gulf of Alaska and vicinity by next weekend. This pattern should
promote a cool and unsettled period over much of the state but
with less confidence in the specifics.
...Guidance/Predictability Evaluation...
The medium to smaller scale of individual shortwaves along with
the possibility for some stream interaction lead to fairly low
predictability for details during the period. The most agreeable
feature in principle (at least early in the period) is a shortwave
aloft and associated surface wave likely to emerge over the Bering
Sea by around midweek and then continue toward/into the mainland.
GFS runs stray somewhat to the faster side of the guidance spread
as is the model's occasional tendency, though not to such an
extreme extent as to preclude at least minority inclusion in the
forecast. Meanwhile by day 6 Friday the 12Z CMC/UKMET both become
more amplified with the shortwave versus other models and the
ensemble means. Through the latter half of the period solutions
increasingly diverge among each other and over consecutive runs
for features within the North Pacific, Asia/Bering, and Arctic
streams. The 12Z CMC is generally on its own with its upper low
that reaches the northwestern mainland by next Saturday.
Differences in the North Pacific stream lead to a full range of
possibilities between meaningful precipitation reaching areas from
the eastern Aleutians into the southern coast/Panhandle and fairly
dry conditions.
Starting with a 12Z operational model blend early in the period
reflected the most agreeable detail for the initial Bering Sea
system tracking toward the mainland. Then transitioning toward
more 12Z GEFS/00Z ECMWF mean weight (reaching 70 percent by day 8
Sunday) with lingering input from the past two ECMWF runs and 12Z
GFS provided a more conservative approach given the low
predictability of individual features.
...Weather/Threats Highlights...
The Bering Sea system tracking toward/into the mainland as well as
a leading front already draped over the mainland will likely help
to focus organized precipitation over portions of the central and
southern mainland during mid-late week. Some pockets of enhanced
activity are possible. The magnitude of totals from the eastern
side of the Kenai Peninsula into the Panhandle will be sensitive
to the upper shortwave's exact amplitude which is yet to be
determined with much confidence. A more amplified shortwave that
would lead to higher totals currently appears to be less probable
but such a scenario is still worth monitoring. One or more
upstream features in the Arctic-Bering streams and/or North
Pacific stream may produce other areas of organized precipitation
during the late week into weekend time frame but again with
minimal confidence in the details. The unsettled pattern should
keep much of the state below normal for high temperatures during
the period. Lows will generally be more moderate with best
potential for above normal readings over the southeastern half of
the state.
Rausch
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