Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
549 PM EST Fri Dec 13 2019
Valid 12Z Tue Dec 17 2019 - 12Z Sat Dec 21 2019
Models were relatively agreeable during the first portion of the
extended forecast period, and a blend of the ECMWF/GFS was used as
a forecast starting point during days 4-5 (Tue-Wed). These
solutions showed relatively small timing and structural
differences with a low pressure system passing near the eastern
Aleutians and the Alaska Peninsula during that time, and the
described blend should resolve these differences favorably. From
day 6 (Thu) onward, this system is forecast to move eastward into
the Gulf of Alaska. Timing differences among the guidance increase
from Thu onward, and a weighting of ECENS/GEFS ensemble means in
the forecast was increased to account for this increased spread.
Models/ensembles show some broad agreement that another relatively
deep low pressure system should enter the western Bering Sea by
day 6 (Thu), with guidance showing continuing to show some
relatively large timing/structural differences that are best
resolved at this point in time by going heavily toward the
ensemble means. Farther north, persistent upper level ridging
across the Arctic Ocean is forecast by ensembles to gradually
shift toward Siberia, allowing lower heights (potentially an upper
low) to spread into the North Slope and the Alaska interior by the
middle of next week. Models/ensemble agree on this general
scenario but show variability on the specific timing of individual
upper waves as well as the possible consolidation of energy into
an upper low. Further warranting increased use of ensemble means
through time. Some continued use (20-40%) of deterministic
solutions was warranted through the end of the forecast period,
however, to add a bit of definition to features beyond that shown
by the ensemble means.
This pattern should initially favor wet conditions across southern
mainland Alaska early in the extended period, the axis of best
moisture should shift east into the Panhandle/Southeast Alaska
from the middle of next week onward. Farther north, initially
above average temperatures across much of the Interior will be
replaced by below average temperatures as upper heights fall by
early next week. Dry conditions should prevail across most of the
Interior, however.
Ryan
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
-Heavy precipitation across portions of the Alaska Panhandle,
Mon-Tue, Dec 16-Dec 17.
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