Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Fri Apr 26 2019
Valid 12Z Tue Apr 30 2019 - 12Z Sat May 04 2019
Recent runs of model and ensemble guidance are increasing
confidence in the idea that an upper high over the northern
mainland at the start of the period early Tue will rapidly lift
northward into the Arctic. As this occurs a weakness aloft may
develop over the mainland and/or Gulf of Alaska. The weakness
would consist of some combination of energy from within/around a
Canadian upper low and/or Bering Sea-Aleutians energy approaching
from the west. This type of evolution has low predictability so
confidence regarding this developing weakness is much lower than
for the upper high. Some signals toward North Pacific flow
possibly becoming more progressive late in the period suggest low
predictability as well. At the surface, the evolution aloft
should transfer high pressure emphasis from the mainland into the
Arctic and northern Canada while promoting lower pressures over
the southern mainland and Gulf of Alaska.
Regarding the upper high, today's clustering early in the period
is much better than seen yesterday. After Wed there is still
spread/run-to-run variability, such as 00Z ECMWF mean runs pulling
the high farther north than 12Z runs and the new 12Z GFS
positioning the high eastward of most other models/means. At the
same time there is a wide variety of possibilities for what will
become of the Canadian upper low and surrounding flow.
Historically the most aggressive solutions for energy retrograding
south of a mid-high latitude upper high tend to be
overdone--favoring less weight of the 00Z ECMWF/CMC with time. On
the other side of the spectrum the 12Z GFS is among the least
retrogressive solutions. An intermediate forecast looks most
reasonable at this time as the majority position of the upper high
should promote at least some weakness to develop to the south--and
push a cold front into the Panhandle/eastern mainland.
Evolution across the Bering Sea and North Atlantic is also quite
uncertain by mid-late period within/ahead of general mean
troughing aloft. Thus far there has been better consensus toward
the core of energy aloft remaining over the Bering versus the 12Z
GFS that forms an upper low that slowly tracks south of the
Aleutians. Another alternative is a North Pacific wave lifting
into the Bering. Solutions are so diverse that a conservative
approach closer to the ensemble means would appear best until
better clustering emerges.
In order to reflect consensus early in the period and then
increasingly downplay uncertain specifics farther out in time, the
forecast blend started with the 00Z ECMWF and 06-12Z GFS runs on
Tue, followed by gradually increasing 06Z GEFS/00Z ECMWF mean
input (and replacing the less favored 12Z GFS with the 00Z run
after Wed) so that the means reached 70-80 percent total weight by
days 7-8 Fri-Sat.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart can be found at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- High winds across portions of the Aleutians, Mon, Apr 29.
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