Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
642 PM EST Tue Dec 15 2020
Valid 12Z Sat Dec 19 2020 - 12Z Wed Dec 23 2020
...Guidance and Uncertainty Assessment...
The anticipated transition of the large scale pattern as a deep
storm moves into the Aleutians and Bering Sea remains on track.
This system will likely 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.
Through the weekend the pattern will feature the recently
persistent lower predictability regime of progressive North
Pacific mean flow and a Bering Sea through northern
mainland/northern Canada weakness aloft. There is decent
continuity in principle for some 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 primary forecast
issue during the weekend appears to be on Sat when the 12Z
GFS/ECMWF, and 12Z CMC with different details, suggest a
mid-latitude Pacific wave may track to or near the southern
Panhandle. This is in contrast to the UKMET that tracks the wave
much farther northwest. Ensemble means are generally more
consolidated over the Gulf but some hint at the idea of a wave
merging with lingering low pressure over the northern Gulf. With
below average confidence preferred a 12Z GFS/ECMWF blend to
reflect detail specifics for this evolution. By Sun this solution
represented consensus fairly well for important features at the
surface and aloft, including the storm approaching the western
Aleutians.
From Mon through Wed the guidance consensus is consistent in
principle, bringing the Aleutians storm into the Bering Sea with
the upper low possibly opening up somewhat by midweek. At the
same time upper ridging will build into the mainland and western
Canada. However within the agreeable mean pattern the operational
models have been differing for details of one or more waves that
could develop along the front extending out from the parent
low--at some point possibly becoming more dominant than the
original low. Late in the period there are also differences in
where the best-defined surface low may track. For day 6 Mon a
blend of the 12Z/00Z ECMWF runs and 12Z GFS/CMC represented
consensus well for the Aleutians storm. Thereafter the forecast
transitioned to a half model/half mean blend to balance the
synoptic agreement of the means and some of the signals for wave
development in the operational models. Ensemble weight tilted
more to the 12Z GEFS run since it had a deeper low than the 00Z
ECMWF mean. The new 12Z ECMWF mean has trended a little deeper
than the 00Z run.
...Weather/Hazards Highlights...
Precipitation along the southern coast and Panhandle during the
weekend could be locally moderate to heavy but should exhibit a
general lighter trend with time. Most remaining parts of the
mainland should be fairly dry through the weekend. The deep storm
moving into the Aleutians and Bering Sea will likely bring its
leading significant precipitation and brisk to strong winds into
the western Aleutians on Sun, then continuing into the eastern
Aleutians and Alaska Peninsula by Mon and the southern/western
mainland by Tue. Moisture could extend as far east as the
Panhandle by midweek. One or more frontal waves could provide
some added focus for winds and precipitation between the eastern
Aleutians and southern coast of the mainland.
The axis of very low heights aloft centered over the northern half
of the mainland during the weekend will promote a large area of
below to much below normal high temperatures over a majority of
the mainland. The far southeast corner of the mainland and parts
of the Panhandle may see some above normal highs. Low
temperatures should not be as extreme in most cases and there may
be more pockets of modestly above normal readings. After Mon
expect a rapid and pronounced warming trend as upper ridging
builds over the mainland/western Canada ahead of the
Aleutians/Bering storm. By next Wed expect many areas to see
above to well above normal temperatures.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Fri-Sat, Dec 18-Dec 19.
- High winds across portions of mainland Alaska, Fri, Dec 18 and
Mon-Tue, Dec 21-Dec 22.
- Much below normal temperatures across portions of mainland
Alaska, Fri-Tue, Dec 18-Dec 22.
- Heavy precipitation across portions of the Eastern Aleutians and
mainland Alaska, Mon-Tue, Dec 21-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