Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Thu Jul 01 2021
Valid 12Z Mon Jul 05 2021 - 12Z Fri Jul 09 2021
...Significant rainfall possible over parts of the state but with
uncertainty over the details...
...Overview and Guidance/Predictability Evaluation...
There is better than average agreement in the models and means for
the large scale pattern. North Pacific/western Aleutians energy
will likely merge with a northern Bering upper low early in the
week, followed by the upper low eventually dropping into or near
the Gulf of Alaska. Meanwhile flow around the periphery of a deep
Arctic low should begin to lower heights aloft over the
northern/western mainland later in the week. This overall pattern
evolution will favor broad coverage of precipitation next week but
significant model spread for embedded details causes difficulty in
resolving where and when the highest totals will occur.
Already early in the period, guidance has been showing a lot of
spread and run-to-run variability for low pressure expected to be
near the western Aleutians as of early day 4 Monday. Albeit in
weaker form, the latest ensemble means generally support the 12Z
ECMWF/CMC in principle with a northeastward track into the Bering
Sea followed by a separate wave developing near the southern coast
by midweek. Alternative scenarios have included a much
weaker/suppressed wave with perhaps a separate weak northern low,
or a somewhat stronger but still suppressed Aleutians to Gulf
wave. These latter options cannot be ruled out but for the
purposes of a single deterministic forecast the preference went to
a slightly tempered version of the ECMWF/CMC due to support of the
ensemble means (which made up 20 percent of the early-week blend).
Leading energy crossing the Gulf early in the week is another
uncertainty. 12Z ECMWF/CMC preference for the upstream system
yields at least a modest upper low but confidence here is even
lower.
By the latter half of the period the ensemble means remain in very
good agreement, both for the upper low that settles into the Gulf
and the cyclonic flow that reaches the northwestern mainland.
Individual models show typical spread for specifics of these
features as well as for shortwave details of the northwesterly
flow across the Bering and vicinity. The last two ECMWF runs and
12Z GEFS/00Z ECMWF means clustered the best in principle so those
solutions provided the primary basis of the forecast, while some
aspects of the CMC were close enough to allow for minor inclusion
as well. The new 12Z ECMWF mean maintained support for this
approach.
...Weather/Threats Highlights...
The general pattern will support a broad area of moisture
overspreading the state with many locations seeing multiple days
of precipitation. Intensity and duration of precipitation over
some areas will be sensitive to the uncertain evolution of low
pressure expected to track out of the western Aleutians early in
the week and the mid-late week evolution over the Gulf. The
initial Aleutians/Bering system could also produce some brisk to
strong winds. Upstream flow could bring another surface system
and precip area into the western mainland late in the week. At
this time expect the best potential for highest five-day totals to
be over portions of the southern coast/mainland into the
Panhandle. At least localized significant totals will be possible
elsewhere though. The clouds and rainfall will keep high
temperatures below normal over most of the state with the coolest
anomalies tending to be over the southern mainland. Min
temperatures may be above normal over northern/eastern areas but
otherwise should be modestly below normal.
Rausch
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 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