Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
649 PM EST Sun Dec 22 2019
Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019
...Below average temperatures persist over much of Mainland
Alaska...
...Stormy pattern over the Gulf of Alaska and a couple strong
storms tracking south of the Aleutians...
The most pronounced trend in a majority of today's guidance arises
mid-late period as it suggests stronger upper-level ridging over
and south of eastern Siberia. The combination of this ridge and a
deep upper low settling into the North Pacific would lead to a Rex
block configuration which should promote a longer duration of deep
upper troughing over the mainland and extreme northern Gulf of
Alaska--along with persistence of well below normal temperatures
across a large portion of the mainland. This troughing will
encourage mean low pressure to prevail at the surface over the
Gulf (by way of multiple low centers expected to reach the
Gulf/Panhandle during the period) with a precipitation focus over
the southeastern coast and Panhandle. Strong winds may accompany
one or more systems as well. Meanwhile the gradient between
Siberia high pressure and North Pacific low pressure systems will
be fairly tight at times, leading to a potential for strong winds
over the Aleutians.
For specifics of the forecast the starting blend used the 12Z
ECMWF/GFS/UKMET for day 4 Thu and then transitioned toward a
combination of 12Z and 00Z ECMWF runs along with ensemble input
placing more emphasis on the 00Z ECMWF mean than 12Z GEFS mean.
The total ensemble weight reached no higher than 50 percent in
order to maintain a reasonable degree of detail while downplaying
questionable specific ideas from one operational run or the other.
Over the Gulf of Alaska and vicinity early-mid period, there is
decent clustering for the first (and potentially strongest) storm
at the start of the period early day 4 Thu. Then low pressure
should weaken in place just off the southern coast into Fri. By
late Fri/early Sat multiple ECMWF runs and the 12Z CMC are fairly
similar in bringing another system into the Gulf/Panhandle. The
UKMET is slower/southward The latest ECMWF means only provide a
faint hint of this system though. GFS runs have been more erratic
and 06Z/12Z runs are westward of other solutions.
Farther west recent GFS runs have been on the northern side of the
spread with initial low pressure south of the Aleutians. The 12Z
GFS at least trended away from the 06Z run that actually tracked
the low to the Aleutians but remains farther northwest and then
much farther west than other models and ensemble means (including
the GEFS). By the latter half of the period the operational GFS
runs appear too fast with progression of the second and
potentially very deep North Pacific storm in light of remaining
solutions trending toward a well-defined Rex block configuration
aloft. On the other hand the 12Z ECMWF holds the deep storm well
westward of the last few ECMWF runs or ECMWF/GEFS/CMC ensemble
means and 12Z CMC--hence the greater emphasis on the 00Z
ECMWF/ECMWF mean in the initial blend for later in the period.
The 12Z ECMWF mean maintains a similar longitude. Recent trends
for the track have been southward in accordance with the stronger
trend for upper ridging to the north. The specifics are quite
hazy at this point but one or two more systems could reach the
Gulf/Panhandle by Sun-Mon, one that may be supported by energy
from or ahead of the first system to the south of the Aleutians
and another by some combination of energy ahead of the second
North Pacific storm and/or within the southern periphery of the
mainland upper trough.
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
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