Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Tue Oct 05 2021
Valid 12Z Sat Oct 09 2021 - 12Z Wed Oct 13 2021
...Active pattern continues with periods of gusty winds and
precipitation across southern Alaska...
...Pattern Overview...
Progressive and energetic flow aloft will lead to multiple surface
low pressure systems affecting portions of Alaska through the
period. The first system in the Gulf of Alaska should be winding
down at the start of the medium range period Saturday, but with
lingering precipitation over the Panhandle and gusty winds over
the Alaska Peninsula. Another low is then forecast to track into
the Bering Sea Sun-Mon, and guidance has become more agreeable for
a triple point low to form and deepen in the Gulf of Alaska around
Tuesday. A third system could also emerge over the Bering Sea by
Tue-Wed. Areas of enhanced precipitation and/or strong winds are
likely with each system.
...Guidance/Predictability Assessment...
Model guidance is fortunately in fairly good agreement throughout
the period with the pattern described above. Deterministic and
ensemble guidance show low pressure lingering along the
Southcentral to southeastern Alaska coast over the weekend, which
maintains continuity from previous forecasts. As the next
mid-upper shortwave comes into the Bering Sea and produces a
surface low early next week, only fairly minor timing and
amplitude differences were seen through Monday. The main exception
is the 12Z UKMET, which had a faster tracking shortwave that went
against consensus, so it was not used today. 12Z deterministic
models generally all showed a mid-upper low closing off
temporarily in the Gulf of Alaska around early Tue, strengthening
a surface low that was initially the Bering surface low pressure
system's triple point. This stronger development was the main
change to the forecast since yesterday, with a deeper surface low.
Then the next system coming into the western Bering Sea Tue and
tracking eastward Wed continues to show better than average
agreement compared to typical late-period forecasts. Overall, the
WPC forecast began with a 12Z ECMWF/GFS/CMC blend at the beginning
of the period, adding some 12Z GEFS/00Z ECens mean to the blend as
the period progressed, but only ended up with 30% means by the day
8 period since the deterministic guidance was fairly agreeable.
...Weather/Hazard Highlights...
Precipitation is expected to continue over the Panhandle Saturday,
as the initial surface low persists but weakens, while gusty winds
could continue in the Alaska Peninsula in the trailing pressure
gradient. Then it is becoming more likely that the next system in
the series will spread precipitation and enhanced winds to the
southwestern mainland early next week, with precipitation likely
beginning Monday along the Gulf Coast as the low strengthens in
the Gulf. The exact placement of the highest winds and heaviest
precipitation will depend on the exact surface low track, however.
Then yet another system tracking across the Bering Sea Tuesday
into Wednesday could spread some precipitation and brisk winds to
the Aleutians and the western mainland.
Temperatures should generally be cooler than normal through the
period for much of the southern part of the mainland and the
Panhandle, particularly in terms of highs. For the northern
two-thirds of the state, near average highs are forecast with a
mixture of slightly below and slightly above normal readings
through the first part of next week. Low temperatures are expected
to generally be above normal for western and northern parts of
Alaska.
Tate
Hazards:
- Heavy precipitation across portions of mainland Alaska, Fri, Oct
8.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Fri-Sat, Oct 8-Oct 9.
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Sat, Oct 8-Oct 9.
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