Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Thu Oct 07 2021
Valid 12Z Mon Oct 11 2021 - 12Z Fri Oct 15 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. A surface low in the Bering Sea near the southwestern
mainland Monday is expected to form a triple point low in the Gulf
of Alaska/northeast Pacific by Tuesday. Then Tuesday into
Wednesday, another low should move across the Bering Sea and
possibly cross into the Gulf over the Alaska Peninsula, and this
low pressure in the Gulf is likely to linger through the end of
the workweek. Areas of enhanced precipitation and/or strong winds
are likely with each system.
...Guidance/Predictability Assessment...
Model agreement was fairly good for the initial part of the
forecast, and though there have been run-to-run differences, most
model guidance currently shows a mid-upper low closing off
temporarily in the Gulf of Alaska around Tuesday, which also has
implications for a stronger triple point surface low. The main
outlier was the 12Z CMC, which takes the main shortwave northward
rather than eastward Monday into Tuesday, which is not supported
by other guidance. Thus used a deterministic model blend of the
12Z GFS, ECMWF, and some UKMET for the early part of the period.
The next system tracking quickly across the Bering Sea Tuesday and
into the Gulf has fairly good agreement with the surface low
between the GFS and ECMWF and their ensemble means, despite some
timing and amplitude differences with shortwaves aloft. Thus the
model blend for the latter part of the period was comprised of a
deterministic GFS/EC and the GEFS and EC ensemble mean blend for
the most part. But did observe that the 12Z ECMWF looked like an
outlier with upper-level and surface lows near the Bering Strait
around next Thursday into Friday, arising from model differences
over Russia earlier on. Leaned away from that solution given the
model consensus is for low amplitude ridging over the Bering by
then.
...Weather/Hazard Highlights...
The initial low pressure system in the Bering Sea will spread
light to moderate precipitation to the Aleutians, Alaska
Panhandle, and the southwestern mainland early in the week. Then
with low development in the Gulf of Alaska, Southcentral Alaska
can expect enhanced precipitation beginning Monday with some
threat of heavy amounts. Gusty winds are also possible, but will
depend on the exact strength and track of the surface low. The low
will then cause potentially heavy precipitation into the Panhandle
beginning Tuesday as it tracks eastward. Then the next low
pressure system should spread precipitation and brisk winds to the
Aleutians and the western mainland Tuesday into midweek, while
persistent low pressure in the Gulf keeps the moist inflow into
the Panhandle and thus precipitation chances there through late
week.
Temperatures should generally be cooler than normal through next
week 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. Low
temperatures are expected to be generally above normal for western
and northern parts of Alaska.
Tate
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Wed, Oct 11-Oct 13.
- Heavy precipitation across portions of the Alaska Panhandle,
Tue-Wed, Oct 12-Oct 13.
- Heavy rain across portions of the Alaska Panhandle, Tue-Wed, Oct
12-Oct 13.
- High winds across portions of mainland Alaska and the Aleutians,
Sun-Tue, Oct 10-Oct 12.
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