Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
629 PM EST Tue Dec 17 2019
Valid 12Z Sat Dec 21 2019 - 12Z Wed Dec 25 2019
...Below average temperatures expected across the Alaska Interior
through at least the weekend...
...Two potentially strong storms may track near the Aleutians,
bringing precipitation and strong winds...
Guidance depicts an active North Pacific and Aleutians/southern
Bering pattern with two systems that operational models and to a
lesser extent ensembles suggest could be quite strong. The first
should reach the Aleutians and vicinity during the weekend
followed by another around the middle of next week/Christmas Day.
While the signal for each system is fairly good, spread and
run-to-run variability with track/timing persist so confidence is
somewhat lower for precise impacts in terms of precipitation and
winds at any specific location. At higher latitudes the latest
operational runs in particular show a tendency for a modest upper
ridge to build into eastern Siberia by the start of the next week,
resulting in troughing and possible persistence of below normal
temperatures over parts of the mainland. The 12Z GEFS mean hints
at this idea but overall the ensembles are more ill-defined with
the pattern at these latitudes. Even farther north/northeast
there is initially good consensus regarding an upper low over/near
the Canadian Archipelago with some moderate divergence in
location/orientation by mid-late period.
During the early-middle part of the forecast preferences leaned to
a blend of the 12Z ECMWF/GFS with somewhat more weight on the
ECMWF. This solution represented consensus for the first
Aleutians/southern Bering storm with many operational solutions
depicting a mid-upper 960's mb low as of 12Z Sun. Consistent with
some other storms over recent months the GFS was deepest. It
could have partial merit but may be overdone given overwhelming
clustering among the other models. Track has also been an issue
with the 12Z GFS/ECMWF southern Bering path providing a good
compromise between some earlier southward ECMWF runs (as well as
WPC continuity to some degree) and more northward 12Z GEFS
mean/CMC. One piece of the uncertainty puzzle is some fast-moving
upstream energy that comes in underneath the system.
Guidance spread for the second Aleutians storm required a
transition in operational runs, as both the 12Z GFS and ECMWF
strayed faster than their previous runs or the 12Z GEFS/00Z ECMWF
means and 12Z CMC. This favored adjusting the operational part of
the blend toward the 06Z GFS and 00Z ECMWF later in the period.
The new 12Z ECMWF mean has trended faster though. The desire to
maintain a reasonable amount of detail given operational model
signals led to ensemble mean guidance reaching only 30-40 percent
total weight by mid-late period.
Elsewhere, multiple low pressure centers may exist over the Gulf
of Alaska during the period--first supported by leading shortwave
energy and then dynamics ahead of the first Aleutians/southern
Bering storm generating a frontal wave. Depending on how upper
troughing may evolve over the mainland (with a model/mean blend
helping to navigate the detail uncertainty), some of the embedded
energy could influence Gulf low pressure as well. Evolution over
the Gulf should favor a precipitation focus over the southeastern
coast and Panhandle. Meanwhile the cyclonic flow aloft over the
Arctic may bring a cold front to the northern coast of the
mainland during the first half of next week. The 12Z ECMWF
ultimately wobbled toward the western side of the spread with the
Arctic upper low next week, providing another reason to lean away
from that solution late in the period.
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:
- High winds across portions of the Aleutians, Sun-Mon, Dec 22-Dec
23.
- Much below normal temperatures across portions of mainland
Alaska, Fri-Mon, Dec 20-Dec 23.
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