Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Tue Aug 31 2021
Valid 12Z Sat Sep 04 2021 - 12Z Wed Sep 08 2021
...Pattern Overview and Model Guidance...
Today's guidance offers some similarity in the large scale
evolution but a number of meaningful embedded uncertainties. A
leading northeastern Pacific system may track near Haida Gwaii
early in the weekend. Farther west initial southern Bering
Sea/Aleutians energy will likely evolve as an upper low/trough and
eventually bring a system into the northeastern Pacific while some
height falls should extend into the mainland to the southeast of a
mean upper low that becomes established to the north of eastern
Siberia. Additional energy emerging from Asia may generate an
Aleutians/Bering Sea system by days 7-8 Tuesday-Wednesday.
A composite of 12Z operational models early in the period followed
by a trend toward more 12Z GEFS/00Z ECMWF mean input provided the
best balance of detail while reflecting only the somewhat more
confident aspects of the large scale evolution. Of particular
note, the blend shows less aggressive trough amplification into
the mainland during the weekend into early next week compared to
the GFS--thus toning down somewhat the precipitation amounts over
and east of the eastern Kenai Peninsula. There is another
vigorous system that the 12Z GFS rotates through the northwestern
coast of the mainland around Tuesday and confidence for that is
fairly low given the full range of other guidance. By late in the
period the preferred blend provides a compromise for the possible
northeastern Pacific system, between the slower ECMWF/CMC and fast
GFS (ensemble means generally between the two). There is also
considerable uncertainty over the character of upper dynamics that
will support low pressure expected to reach into the
Aleutians/Bering Sea late in the period. Thus confidence is low
for the strength and track of the surface low.
...Sensible Weather...
The forecast pattern will promote a fairly unsettled period with
multiple areas of precipitation from the weekend into the middle
of next week. A system tracking near Haida Gwaii early in the
weekend may bring some moisture to the southern mainland.
Lingering low pressure/surface troughing could help to focus areas
of precipitation along the southern coast during the weekend into
the start of next week, with some uncertainty for amounts but more
likely with totals lower than forecast by latest GFS runs. Also
expect precipitation to accompany a front that pushes into the
northwestern mainland during the weekend and then stalls before
lifting back to the north. Toward Tuesday and/or Wednesday low
pressure reaching the northeastern Pacific could increase
precipitation over the Panhandle while an Aleutians/Bering Sea
system may extend its accompanying moisture into the western
mainland.
Expect a colder trend toward below normal high temperatures to
progress from west to east across the mainland while areas over
and near the Panhandle likely see below normal highs through the
period. Min temperatures should still be above normal through the
weekend followed by some areas of below normal mins reaching
portions of western and interior areas.
Rausch
Hazards:
- Heavy rain across portions of mainland Alaska, Fri, Sep 3.
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