Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EST Wed Dec 16 2020
Valid 12Z Sun Dec 20 2020 - 12Z Thu Dec 24 2020
...Guidance and Uncertainty Assessment...
In general, model guidance continues to produce a low confidence
forecast for much of the extend period; however, there is
consensus for high pressure setting out over the Arctic and a mean
trough with one or more waves to move through the Bering Sea and
Aleutians during this time. The upper-level jet will be very
strong as it tracks over the North Pacific. A deep storm is
progged to move into the Aleutians and Bering Sea, likely to
generate a broad area of enhanced precipitation and strong winds,
spreading from the western Aleutians on Sun and continuing
eastward toward/into the southern and western mainland during the
first half of next week. There has been persistent height falls to
retrograde into the mainland during the weekend around the
periphery of a slowly retreating upper ridge/high. Uncertainties
within the weakness aloft and for impulses in Pacific flow keep
confidence in the lower half of the range for individual Bering
Sea and Gulf of Alaska/northeastern Pacific surface lows.
The CMC/GFS/ECWMF had deeper surface lows with this feature and
were fairly clustered, at least initially, and was the starting
point for the extended forecast. As the storm progressed through
the weekend and into early next week, the spread greatly increased
and the means had increasing weighting-- more than 50% by midweek.
...Weather/Hazards Highlights...
Areas along the southern coast and the Panhandle could have
moderate to heavy precipitation through the weekend before letting
up some early next week. Locations to the north/Interior will
mainly be dry this weekend prior to the arrival of a deep storm
system. This system will move through the Aleutians and Bering
Sea; likely bringing its leading significant precipitation and
brisk to strong winds into the western Aleutians Sunday- advancing
through the eastern Aleutians and Alaska Peninsula by Monday,
southern/western mainland by Tuesday with moisture possibly as far
east as the Panhandle by Wednesday. One or more frontal waves
could provide some added focus for winds and precipitation between
the eastern Aleutians and southern coast of the mainland.
Maximum temperatures across portions of the Mainland are expected
to fall below/much below average this weekend; whereas, Southeast
Alaska and extreme southeast portions of the Mainland will likely
climb above seasonal averages for later December. There wills be
an abrupt warming trend next week with much of the region
observing above/well above normal temperatures.
Campbell
Hazards:
- Heavy precipitation across the Alaska Peninsula, Mon, Dec 21.
- Heavy precipitation across portions of southern Alaska, Tue-Wed,
Dec 22-Dec 23.
- Heavy precipitation across portions of the Alaska Panhandle into
southern Alaska, Wed, Dec 23.
- Heavy snow across the eastern portion of the Alaska Panhandle,
Sat, Dec 19.
- High winds across the Alaska Peninsula into southern Alaska,
Mon-Tue, Dec 21-Dec 22.
- Much below normal temperatures across much of the lower
elevations of eastern Alaska, Sat-Tue, Dec 19-Dec 22.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards 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