Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Sun Jul 08 2018

Valid 12Z Thu Jul 12 2018 - 12Z Mon Jul 16 2018

Model consensus has improved substantially over the past day with
respect to the evolution/interaction of two upper lows in the
vicinity of the Aleutians by later this week. The consensus has
emerged along the lines of the idea shown by the ECMWF/CMC
yesterday, in which the two shortwaves/upper lows merge by Thu
night/Fri across the southern Bering Sea, with the
combined/deepened upper low moving into southern mainland Alaska
by Fri night/Sat, and into the northern Gulf by Sat night/Sun. The
ECMWF/GFS/FV3/CMC all show this general idea, with some fairly
minor differences in timing Thu-Fri. Thus, a majority
deterministic blend including the ECMWF and GFS served as a
starting point for the forecast during days 4-5.

After that, model differences increase once again, with the
ECMWF/GFS/FV3 consensus taking the system east into the northern
Gulf, although the CMC persists in holding the low farther west
across the eastern Bering Sea. The amplified trough/upper low
across mainland Alaska should allow for downstream upper ridging
to persist across Southeast Alaska through Sat, before heights
begin to fall in advance of the approaching trough. Farther north,
the ECMWF/GFS both suggest the potential for Arctic shortwave
energy to dig into the North Slope and northern portions of the
interior by Sat night-Sun. Ensembles suggest that
spread/uncertainty are a bit higher than the seeming agreement
among the GFS/ECMWF would indicate, thus ECENS/GEFS ensemble mean
weighting was increased gradually after day 5, with a majority of
the forecast comprised of ensemble means by days 7-8.

In terms of sensible weather impacts, this pattern will result in
a period of near to below average temperatures along with frequent
precipitation for much of the state. Rain could be heavy at times
across southern mainland Alaska Fri into Sat due favorable upper
dynamics in association with the upper trough. By late Sat into
Sunday, precipitation may become more widespread across the North
Slope as the Arctic shortwave and associated surface cold front
approach.

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