Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Apr 14 2019
Valid 12Z Thu Apr 18 2019 - 12Z Mon Apr 22 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, especially during the second
half of the forecast period.
Model consensus has gradually improved that a relatively deep low
pressure system should move north into the Gulf of Alaska by Thu.
For several runs models had shown some variability between taking
the low more farther east closer to the Alaska Panhandle, or
farther west toward southern mainland Alaska. Solutions seem to be
settling more toward the western solutions, with the last holdout
(GEM global) coming on board with the 12Z run. This system appears
likely to produce an episode of widespread and potentially heavy
precipitation, along with gusty winds, for portions of the Alaska
Panhandle on Thu. In the wake of this low pressure system,
predictability gradually decreases through time. Model consensus
has improved a bit around an arctic shortwave progged to dive
south across the eastern Bering Sea late Fri into Sat, with
potential interactions with energy crossing the north Pacific
during that time. A second arctic shortwave following a similar
track late Sat into Sun shows increased spread over the leading
wave. 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.
While spread has perhaps reduced a bit compared to 24 hours ago,
models still vary widely on the timing and intensity of this
system. Given these considerations, the forecast during days 4-5
(Thu-Fri) was based heavily on a blend of the 00Z ECMWF/12Z GFS.
After Fri, weighting of ECENS/GEFS ensemble means was gradually
increased through time to account for increasing spread/decreasing
predictability.
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