Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Sat Apr 13 2019
Valid 12Z Wed Apr 17 2019 - 12Z Sun Apr 21 2019
Models/ensembles show broad consensus that upper ridging should
persist/expand across eastern Russia and the western Bering Sea
during the extended forecast period. A strong jet across the north
Pacific will undercut the ridge with a stream of shortwave energy
feeding into and reinforcing a mean upper-level trough extending
from mainland Alaska south into the Gulf of Alaska. Shortwave
energy spilling over the crest of the ridge will also move south
across the eastern Bering Sea throughout the period, interacting
with the mean trough as well as energy from the north Pacific,
introducing an additional degree of complexity/chaos to the
forecast and reducing predictability, particularly by the second
half the forecast period.
Models show some consensus that a potentially deep low pressure
system should move north into the Gulf of Alaska Wed-Thu, but
solutions differ on the exact track that the system will take. The
00Z ECMWF and 06Z/12Z runs of the GFS showed relatively good
consistency in taking the low to near the Kenai Peninsula by late
Thu, but the 12Z ECMWF shifted farther east, keeping the low much
closer to the Alaska Panhandle. Ensemble spread has generally
supported an idea closer to the GFS or previous ECMWF, but
confidence remain a bit below average given the continued
variability of solutions. Regardless, this system appears likely
to produce a period of widespread and potentially heavy
precipitation for portions of the Alaska Panhandle late Wed into
Thu. In the wake of this low pressure system, predictability
decreases quickly through time. Models vary significantly with how
to handle arctic shortwave energy diving south across the eastern
Bering Sea by Fri-Sat, with potential interactions with energy
crossing the north Pacific during that time. Models have varied
substantially with a potential low pressure system moving well
south of the Aleutians on Sat and its eventual track/evolution as
it perhaps approaches the Gulf by Sun. Given these considerations,
the forecast during days 4-5 (Wed-Thu) was based heavily on a
blend of the 00Z ECMWF and 06Z/12Z GFS runs. After Thu, weighting
of ECENS/GEFS ensemble means was boosted substantially given the
quickly increasing uncertainty.
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