Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Mon Dec 05 2022
Valid 12Z Fri Dec 09 2022 - 12Z Tue Dec 13 2022
...Overview...
The ensemble means are fairly agreeable in depicting a sequence of
upper features that have moderate amplitude and progression.
However individual models and ensemble members have been showing a
lot of spread as well as run-to-run variability and/or trending
for embedded features. This is particularly the case for one or
more systems that could affect areas from the Aleutians through
the western/southern mainland, but also applies to specifics
within a general signal for increased storminess over and possibly
east/north of the northwestern Pacific by early next week. A
somewhat more confident aspect of the forecast is that surface
high pressure building across most of the state late week into the
weekend should ultimately settle over at least eastern areas and
into western Canada, leading to a much colder trend over
southern/eastern locations.
...Guidance/Predictability Assessment and Preferences...
Initial smaller-medium scale systems as of early Friday involve a
wave/frontal system over the Bering Sea/Aleutians, low pressure
that may track along the northwest coast of the Bering Sea, and
low pressure off the southeastern coast/Panhandle. Among the
latest guidance the 12Z ECMWF was in the minority for washing out
the Bering Sea system as well as for a farther eastward extent of
flow ahead of the system tracking to the north. The 12Z GFS was
also questionable in combining this system and the one tracking to
the north, eventually bringing the surface low into the western
mainland. Interestingly the new 18Z GFS has trended toward the
12Z ECMWF for the northern system, but other solutions still
suggest a little more westward deflection. Finally,
low-predictability shortwave details will have a say in the
details of low pressure initially over or near the Gulf of Alaska.
The most significant large-scale forecast problem involves a
system that should reach south of the Aleutians by early day 5
Saturday. Differences in evolution and timing of supporting
dynamics have led to a very rapid divergence of solutions among
each other and from continuity. Sunday's guidance did not even
suggest a threat from such a system, and before arrival of 12Z
solutions the main question was whether the system's northern
periphery could affect the Aleutians as it tracked eastward into
the northeastern Pacific (with possibly some influence on the
southern Panhandle). The 12Z ECMWF and CMC then jumped northward
into the Bering Sea by Sunday due to a slower closed low aloft,
with the 12Z ECMWF surface track then continuing northeast into
the mainland and the CMC tracking somewhat farther south
(weakening in place along the southwestern coast of the mainland).
The 12Z GEFS/CMCens means generally reflect an ambiguous
combination of Bering Sea/North Pacific low pressure that
ultimately consolidates into a system tracking just south of the
Alaska Peninsula on Sunday on its way into the northeastern
Pacific. The 00Z ECMWF mean was less defined and southward with
the low but the new 12Z version has adjusted closer to the other
means. Given the rapid trending and broad spread, preference at
this time was to follow a conservative approach with a moderately
southern track closer to the 12Z GEFS/CMC means and 12Z GFS/00Z
ECMWF. The 18Z GFS has furthered the northward trend in that
model, so confirmation from the next cycle of the ensemble means
may favor additional adjustment from the manual forecast.
For early next week the guidance has offered a general signal for
one or more significant surface lows within an area from the
northwestern Pacific into the Aleutians and western Bering Sea.
12Z model trends have been toward slower overall progress of
strongest low pressure relative to the means, while the GEFS mean
is noticeably weaker than the other means.
Today's forecast started with a 12Z model blend (splitting ECMWF
input between the 12Z and old 00Z runs) early in the period. The
significant decrease in forecast confidence thereafter led to a
rapid increase in the total weight of the ensemble means so that
they made up 60-75 percent of the blend by days 7-8
Monday-Tuesday. The 12Z GFS/00Z ECMWF provided lingering
operational input. 12Z ECMWF mean/18Z GFS runs arriving after
forecast preparation suggest more adjustments are likely in the
future.
...Weather/Hazards Highlights...
The most confident part of the forecast appears to be with the
establishment of well below normal temperatures over
southern/eastern areas as surface high pressure settles over
portions of the mainland into western Canada. Meanwhile above
normal readings should prevail over approximately the northwestern
half of the state, likely with a gradual moderation of anomalies
with time. The gradient between the aforementioned high pressure
and mean low pressure over the Gulf/northeastern Pacific should
result in some locally strong gap winds from the weekend onward
along the southern coast and Panhandle. Guidance is still
resolving important details for North Pacific low pressure that
may affect the Aleutians and potentially parts of the
western/southern mainland during the weekend into early next week.
Just 12-24 hours ago guidance was suggesting a non-existent or
very suppressed system, but latest trends are increasing the
potential for some areas to see meaningful precipitation and some
brisk to strong winds. Some moisture could eventually reach as
far east as the Panhandle. There are also question marks about
the eastward influence of a system that may track well west of the
northwestern mainland, with some potential (though for now in the
minority) for a brief period of increased precipitation and wind.
Finally, another system may affect some areas over the Aleutians
and Bering Sea early next week with enhanced precipitation and
wind but again the specifics are very uncertain at this time.
Rausch
Hazards:
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Fri-Mon, Dec 9-Dec 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