Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
759 PM EDT Wed Oct 06 2021
Valid 12Z Sun Oct 10 2021 - 12Z Thu Oct 14 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 is forecast to track into the Bering Sea
into southwest Alaska Sunday into Monday, then form a triple point
low deepening in the Gulf of Alaska around Tuesday. Then Tuesday
into Thursday, another low should move across the Bering Sea and
possibly across or near the Aleutians, leading to additional low
pressure in the Gulf. Areas of enhanced precipitation and/or
strong winds are likely with each system.
...Guidance/Predictability Assessment...
Model guidance is in reasonably good agreement compared to
typical, but with somewhat more model spread than yesterday's
forecast. There were minor timing and amplitude differences
initially with the mid-upper shortwave coming into the Bering Sea
Sunday into Monday, with GFS runs a bit weaker/slower, the 12Z
ECMWF/CMC a bit stronger and faster, and the 12Z UKMET sort of a
compromise with a slower but deeper solution. Overall a blend of
the 12Z deterministic guidance worked well for the beginning of
the forecast period to average out these differences. The low
pressure system trended faster compared to the previous forecast
per the latest guidance.
By around Tuesday, less guidance is showing a mid-upper low
closing off temporarily in the Gulf of Alaska compared to
yesterday's agreeable guidance showing such; the 12Z ECMWF is the
main model still doing so. So there is lower confidence with a
trough or closed low given the day-to-day model variability. This
also has implications for the strength of the surface
low--generally weaker than yesterday's forecast, and WPC trended
this direction. The 12Z CMC does appear to be a southern outlier
with the surface low and it was excluded from the blend after the
beginning of the period.
The next low pressure system has fairly good agreement that it
should track into the Bering Sea Tuesday, but there are timing and
track differences as it moves east (north, over, or south of the
Aleutians), but will likely reinforce low pressure in the Gulf by
Thursday. By the latter part of the forecast period, used a 50/50
blend of deterministic GFS/EC and the GEFS and EC ensemble means.
...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 spread 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.
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 the Alaska Panhandle and
mainland Alaska, Mon-Wed, Oct 11-Oct 13.
- Heavy precipitation across portions of the Alaska Panhandle,
Sat, Oct 9 and 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,
Sat, 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