Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Tue Feb 19 2019

Valid 12Z Sat Feb 23 2019 - 12Z Wed Feb 27 2019

Highly amplified flow pattern expected to develop across Alaska by
the beginning of the extended period, with upper-level ridging
from the Gulf north across Alaska, and a mean trough across the
Bering Sea. Confidence in this general pattern is above average as
models/ensembles show very good consensus at the large scale.
During the day 4-6 time frame (Sat-Mon) models even show
relatively good consensus with respect to the details of a low
pressure system crossing the Aleutians and moving into the western
Bering Sea, and another low following on the heels of the first,
reaching the Aleutians Sun night. Relatively low spread/good
consensus with respect to these features lent to heavy use of
deterministic solutions in the forecast, the GFS and ECMWF in
particular. Additional detail differences emerge after Mon, both
respect to additional low pressure systems traversing the mean
trough and with the exact strength/evolution of the upper ridge.
The GFS and CMC have been consistent in developing a highly
anomalous closed upper ridge across the interior of mainland
Alaska, early next week. The ECMWF, which had shown a weaker ridge
farther south, has now joined the GFS/CMC with the 12Z cycle. This
solution would keep the most significant shortwave energy/westerly
flow west of mainland Alaska, whereas a solution like the 00Z
ECMWF would allow more influence from the North Pacific trough to
reach western mainland AK. Despite the trend with the 12Z
guidance, solutions have waffled on this over the past several
runs, so at this time the best approach is to lean toward ensemble
means (ECENS and GEFS) for the forecast by early next week. This
would keep the mean storm track across the Aleutians/Bering Sea,
with some potential affects in terms of precip/gusty winds across
the Alaska Peninsula and western mainland Alaska.

With 500 hPa height anomalies reaching at least +2 to +3 standard
deviations across Alaska and northwestern Canada by early next
week, highly anomalous temperatures are also likely. The greatest
temperature anomalies will occur across the North Slope, where
temperatures exceeding 40 deg F above average are possible by this
weekend, with some areas perhaps exceeding 50 deg above average by
early next week. High temperatures in Utqiagvik may even
approach/surpass freezing.


Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html