Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EST Wed Dec 30 2020
Valid 12Z Sun Jan 03 2021 - 12Z Thu Jan 07 2021
...Overview and Guidance/Uncertainty Assessment...
Latest/recent models and ensemble means hold onto fairly similar
ideas for the general mean pattern but indicate ongoing
uncertainties for embedded specifics. Consensus still shows some
combination of retrograding northern mainland energy, a
southward-moving Arctic feature to the northwest, and lingering
cyclonic flow over the Bering leading to a mean low whose center
wobbles back to near eastern Siberia toward mid-late next week.
Meanwhile guidance is consistent in showing that the northern
fringe of progressive North Pacific flow will pull the initial
deep low over the southern Bering Sea eastward on a track near the
Aleutians early next week. The forecast also maintains mean
troughing aloft over the northeastern Pacific/Gulf of Alaska with
shortwaves carried by the Pacific stream supporting one or more
surface lows whose details are yet to be confidently resolved.
Primary emphasis on the 12Z/00Z ECMWF runs and 12Z GFS/CMC for a
majority of the period represented the most common elements of
guidance across the forecast domain, with somewhat more detail
than the means while also downplaying less confident aspects of
any particular model run. The blend included a small minority of
12Z GEFS/00Z ECMWF mean input by day 7 Wed, and then increased the
means to 40-50 percent weight by day 7 Thu as some differences
become more pronounced.
After recent trends for the deep low initially over the southern
Bering, the past day of guidance has held into the theme of
eastward progression early-mid period before likely becoming
absorbed within the broader cyclonic flow over the northeastern
Pacific/Gulf. After being split for timing yesterday, the new 12Z
ECMWF is more on its own with a farther eastward solution versus
its prior run or other 12Z models. ECMWF runs do converge for
longitude (though with a latitude difference) by early day 6 Tue.
Over the northeastern Pacific/Gulf, confidence is reasonably high
for the initial low expected over the Gulf as the forecast starts
early Sun. Then detail confidence declines rapidly for trailing
lows, though there is a general signal for one to reach the area
by Mon with a potentially stronger development reaching south of
the Gulf by early Tue and then lifting northward. The 12Z GFS/CMC
and their means cluster fairly well for this Tue-midweek low while
the 12Z ECMWF's strongest development is farther east. The new
12Z ECMWF mean is showing somewhat of a trend toward the GFS
cluster in principle. Most models/means signal another
development to the south of this system by day 8 Thu.
Late in the period guidance has been showing potential for another
strong system emerging over the western Pacific, with some
potential effect on the Aleutians after midweek. Thus far the
ECMWF mean has been the most consistent and intermediate guidance
source, while various operational model runs have differed
considerably for timing and track.
The complexity of multiple individual features will likely temper
confidence for a while regarding the mainland/Bering/Siberia
evolution aloft. CMC weight in the blend is sufficiently low to
discount that model's eventual emphasis for lowest heights aloft
over the southeastern Bering (not seen in other guidance), though
there is agreement on more modest troughing over the southeast
Bering/southwestern coast of the mainland by Wed-Thu.
...Weather/Hazards Highlights...
The potential for multiple Northeast Pacific-Gulf of Alaska
systems will support periods of rain/snow over the Panhandle and
southern coast during the period. Specifics of each system will
determine precipitation intensity--which could be locally moderate
to heavy at times--as well as coverage. One or more of these
systems could be sufficiently strong to generate an episode of
brisk to strong winds as well. Gradually weakening southern
Bering low pressure should track near the Aleutians early in the
week, accompanied by some precipitation and locally enhanced
winds. The pattern will likely maintain periods of precip over
the Aleutians for the rest of the period. The next system
emerging into the western Pacific will be worth monitoring after
midweek. Current solutions range from a far enough south track to
have minimal effects on the Aleutians, to more pronounced winds
and precipitation.
A majority of the northwestern three-fourths of the mainland
should see below normal temperatures--especially for highs--to
start next week as upper level energy retrogrades over the area.
Readings should moderate toward midweek as heights aloft rise
somewhat with north-central locations seeing the best chance for
above normal readings. Meanwhile near to above normal
temperatures (perhaps locally well above normal for lows) are
should prevail over the Panhandle and Southcentral.
Rausch
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Sat-Sun, Jan 2-Jan 3.
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