Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Sat Aug 10 2019
Valid 12Z Wed Aug 14 2019 - 12Z Sun Aug 18 2019
Today's guidance continues to show an increasingly
amplified/blocky pattern taking shape after midweek. Most
models/ensemble means have furthered recent trends toward a
stronger North Pacific ridge aloft building into the
eastern/northern Bering Sea. This ridge should support a
deepening upper trough over northwestern Canada into the
northeastern Pacific while keeping low pressure to the southwest
of the Aleutians (in contrast to earlier days when the system was
expected to reach the western Aleutians or even the Bering Sea).
Confidence remains low regarding specifics of short range Arctic
energy. There is relative consensus among the 00-06Z models and
12Z GFS that a compact feature should be just off the northwest
coast of the mainland as of 12Z Wed. However soon after that time
solutions diverge for timing and exact path as it feeds into the
developing mean trough. At the time of forecast preparation a
modest majority and recent trends favor leaning partially in the
slower 00Z ECMWF/CMC direction versus faster GFS runs (06Z run a
bit slower than the 00/12Z versions). Not surprisingly the
remaining 12Z models have made some adjustments, primarily with
slightly faster timing to or toward the GFS. Trends also show
more amplified troughing to the southwest of the feature as it
continues inland.
In the wake of this low/trough there are further question marks
with upstream Arctic flow as well as path of the upper low that
starts the period near the western Canadian Archipelago (00Z
ECMWF/CMC both dropping the upper low into the Yukon versus GFS
runs keeping it well northward). The ensemble means support an
intermediate solution, to which the 12Z ECMWF/CMC have adjusted.
Interestingly, even with the various detail differences the 06Z
GFS and 00Z ECMWF/CMC end up with fairly similar solutions over
the mainland and northeastern Pacific around day 7 Sat.
A blend of the 00Z ECMWF/06Z GFS/00Z CMC provided a good starting
point to represent detail and forecast preferences early in the
period, days 4-5 Wed-Thu. For the rest of the forecast the
various detail discrepancies in operational runs warranted a
gradual increase of 06Z GEFS and 00Z NAEFS/ECMWF mean input--but
only up to about 50 percent total weight by day 8 Sun as the more
agreeable aspects of the models offered some detail enhancement
relative to the means alone.
Separate frontal systems affecting the northern and southern
mainland may produce some areas of enhanced precipitation during
the first half of the period. Exact duration will depend on
timing of shortwave energy dropping southeastward across the
mainland. The system over the western Pacific should still extend
an area of fairly strong winds and one or more bands of
significant rainfall into the Aleutians but with less eastward
extent than forecast in prior days.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart 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