Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
659 PM EST Sun Dec 20 2020
Valid 12Z Thu Dec 24 2020 - 12Z Mon Dec 28 2020
...Guidance and Uncertainty Assessment...
Overall, the model guidance is in fairly decent agreement once
again. However, the 06Z/12Z runs of the GFS were running about 6
to 12 hours faster than the clustered deeper 12Z ECWMF/CMC
solutions, especially with the initial system over the eastern
Gulf and the next wave behind it. Given the spread in timing and
amplitude, the evolutions are similar. A general blend of the 06Z
GFS and 12Z GFS/CMC/ECWMF was the preferred starting point while
incorporating the 00Z ECWMF ensembles and GEFS means by mid-period
and increasing weight by the end of the forecast. This compromise
keep a general trough-iness over the Gulf of Alaska with ridging
over Canada. There are decent model signals that a couple of
weaker cyclones will edge closer towards the Alaska Panhandle
during the Christmas holiday, but uncertainty regarding their
speeds of motion and intensities remain high. By next weekend,
global model ensemble means continue to show decent agreement that
the next large-scale cyclone will be moving into the Aleutians.
...Weather/Hazards Highlights...
The southwest and southern portions of the state will be have been
impacted with heavy precipitation and high winds just prior to the
start of the extended period; the heavy precipitation will shift
to the Panhandle Wednesday. The chance of heavy rain should
generally decrease during the Christmas holiday along the entire
stretch of the southern coastal sections, but a couple of smaller
scale cyclones could bring periods of enhanced precipitation near
the Alaska Peninsula and Alaska Panhandle. Another large cyclone
is expected to track across the Aleutians ushering in high winds
and more precipitation - gale to storm force winds and periods of
heavy precipitation appear likely. Under the ridge, milder than
average temperatures will overspread much of the Mainland and
continue through the holiday before cooling down by the weekend.
Campbell
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