Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Mon Apr 29 2019
Valid 12Z Fri May 03 2019 - 12Z Tue May 07 2019
The large scale pattern across Alaska will remain relatively
unchanged through the extended forecast period. A mean trough will
slowly progress eastward across the Aleutians through the period,
with somewhat transient upper ridging centered across the Gulf of
Alaska, and a more persistent ridge of high pressure farther north
in the Arctic Ocean. This flow pattern results in a constant
stream of shortwave energy crossing near/south of the Aleutians
and moving into mainland Alaska, with additional shortwave energy
rotating around the southern periphery of the arctic ridge
affecting the North Slope. One significant question that remains
is whether shortwave energy will be able to periodically break
down the northern extent of the Gulf ridge (as shown by the
ECMWF/CMC and to a lesser extent the FV3), or whether the ridge
will hold strong (as shown by the GFS). Teleconnections associated
with some of the stronger hemispheric 500 hPa height anomalies do
not show strong support for the Gulf ridge, leading to the
conclusion that at least a temporary/periodic weakening of the
ridge northern extent seems feasible. The 00Z ECMWF and 06Z FV3
also showed similar evolution/progression of the upper low slowly
moving east across the Aleutians and into the southern Bering Sea
from Sat onward, and the associated surface low pressure system.
Thus, the forecast during days 4-5 (Fri-Sat) was based heavily on
a blend of the ECMWF and FV3 (more weight placed toward the
ECMWF), with gradually increasing weight placed on ECENS/NAEFS
ensemble means through time, and majority ensemble weighting after
day 6 (Sun).
Ryan
Bullet points from the WPC Days 3-7 Hazards chart can be found 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