Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Fri Aug 06 2021
Valid 12Z Tue Aug 10 2021 - 12Z Sat Aug 14 2021
...Heavy rainfall expected for portions of southern Alaska next
week...
...Overview and Guidance/Predictability Evaluation...
A broad upper level trough is expected to be in place across much
of the state and extending across much of the Bering Sea and the
Aleutians through much of next week. An upper level ridge axis
situated over British Columbia and the Pacific Northwest will tend
to limit the eastward progress of the trough, resulting in rather
unsettled weather across much of the southern third of the state.
This will especially hold true from the Kenai Peninsula to the
southeast Panhandle region. Meanwhile, a closed low over the
Arctic will likely send a cold front southward across the North
Slope and Brooks Range, and a storm system over the North Pacific
may begin affecting the Aleutians by next weekend.
On the synoptic scale, the deterministic guidance is in relatively
good agreement through the middle of the week. By Thursday and
into Friday, the CMC is indicating more upper level ridging
building in across the Aleutians, whilst the ensemble means are
more supportive of a trough over this region. By the end of the
forecast period, considerable model spread is evident with
shortwave perturbations crossing the North Pacific. The WPC
fronts/pressures forecast was primarily derived from a 12Z
GFS/ECMWF/GEFS and 00Z ECENS blend, along with some previous WPC
continuity, and then more of the EC and GEFS means for the end of
next week and next Saturday.
...Weather/Threats Highlights...
The upper level troughing over the state will tend to keep
temperatures below early August averages across most of the state
next week. These anomalies will likely be even greater for the
Brooks Range and points northward with the arctic front moving
through in response to the closed low north of the Arctic Coast.
In terms of precipitation, multiple rounds of heavy rain are
expected from the Alaska Peninsula to the southeast panhandle
region as strong southwesterly flow at the low and mid levels
advect copious moisture towards the coast at a nearly
perpendicular angle. This could result in some locations
potentially getting 3 to 6+ inches of rainfall across
topographically favored areas as a couple of atmospheric river
events are appearing increasingly likely. This will likely lead
to some instances of flooding and enhanced run-off.
Hamrick
Hazards:
- Heavy rain across portions of the Alaska Panhandle and
south-central mainland Alaska, Mon-Fri, Aug 9-Aug 13.
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