Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
329 PM EDT Thu Aug 09 2018

Valid 12Z Mon Aug 13 2018 - 12Z Fri Aug 17 2018

Models show general agreement on the large scale flow pattern in
the vicinity of Alaska during the medium range, but differ
substantially with respect to the details. Broad and persistent
upper ridging across eastern Asia will continue to promote
upper-level troughing from the Bering Sea across mainland Alaska
and the Gulf. Initially on day 4 (Mon) models and ensembles showed
generally good agreement that a low pressure system will be moving
from the Bering Sea into western mainland Alaska, before
continuing to move northeastward/inland on Tue as it weakens,
meanwhile the surface front is expected to sweep east across the
Gulf and Southeast Alaska. The 00Z ECMWF/CMC solutions seemed to
show the greatest degree of run-to-run consistency as well as
clustering with the ensemble means, and a blend of these solutions
served as a forecast starting point during days 4-5. After day 5,
model spread quickly escalates and deterministic solutions become
nearly unusable by the end of the forecast period. Greatest points
of contention are the timing/amplitude of additional upper energy
approaching/crossing the Bering Sea, as well as the southward
excursion (or lack thereof) of Arctic energy into the Northern
Slope and northern interior. Given the rapidly increasing spread
after day 5, ensemble means (00Z ECENS/NAEFS) served as a majority
of the forecast, with rapidly diminishing influence of
deterministic solutions through the remainder of the forecast
period.

The cyclone moving from the Bering Sea into mainland Alaska, and
the associated front crossing the Gulf Mon-Tue will result in
widespread and potentially heavy precipitation, especially across
areas along the Gulf. Farther inland, scattered to numerous
showers will focus locally heavy precipitation amounts in favored
terrain areas. Even in the wake of this system, lingering
mid/upper-level energy across the interior will keep areas of
precipitation fairly common. The next low pressure system should
also begin to spread precipitation across the Aleutians by mid to
late next week, although confidence is relatively low in the
timing. Additionally, the potential exists for Arctic low pressure
to bring precipitation to the Northern slope and northern interior
by late next week, although confidence is very low in this aspect
of the forecast.


Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html