Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
625 PM EDT Sun Aug 8 2021
Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 2021
...Atmospheric river expected to hammer parts of southeastern
Alaska with very heavy rain late in the week...
...Overview and Guidance/Predictability Evaluation...
A well developed Arctic low and trailing upper level trough over
the Bering and Aleutians is expected to be in place at the
beginning of the forecast period on Thursday, along with a rather
strong upper ridge over British Columbia and the Pacific
Northwest. Shortwave energy exiting the Aleutians trough should
emerge over the northern Gulf and attempt to displace the ridge
axis to the east, and this will tend to focus strong onshore flow
over southeastern Alaska and the panhandle region. Farther
upstream will be the eventual remnants of Tropical Storm Lupit,
which should pass south of the Aleutians.
Decent synoptic scale agreement exists among the 12Z guidance
suite on Thursday, with differences becoming more apparent by
Friday with the progression of the trough across the Gulf region.
The 12Z becomes a faster and much stronger solution by Saturday
over the Gulf, whereas the CMC is depicting nearly zonal flow
across this same general area. There is better support for the
ECMWF/UKMET idea through Saturday, along with the 12Z GEFS mean.
By later in the forecast period, the CMC is stronger with building
a ridge axis across the state and this does not have much in the
way of ensemble support, so more of a ECMWF/GEFS/ECENS approach
seems prudent by the end of the forecast period.
...Weather/Threats Highlights...
The main weather headline will be the potential for extremely
heavy rainfall for portions of southeast Alaska, particularly for
windward terrain from near Mount St. Elias to Juneau. There is
now a good signal in the CMC/ECMWF/UKMET/GFS for an enhanced
atmospheric river event for the end of the week that will be
directly nearly perpendicular to the coast, and rainfall totals on
the order of 4 to 8 inches are within the realm of possibility for
these areas. This could result in mudslides and serious rises on
creeks and rivers. The axis of the atmospheric river then moves
eastward and gradually weakens going into the weekend across
remaining portions of the southeast panhandle region, with 2 to 5
inches of rainfall possible for the greater Juneau area and points
southeastward.
Elsewhere, upper level troughing over much of Alaska will tend to
keep temperatures generally below normal across most of the state
for this forecast period. These anomalies will likely be even
greater for the northern third of the states as a cold front and a
polar upper low govern the weather pattern. The closed Arctic low
may eject eastward next weekend in response to an approaching
upstream upper trough from eastern Siberia.
Hamrick
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