Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EDT Sun Aug 12 2018

Valid 12Z Thu Aug 16 2018 - 12Z Mon Aug 20 2018

A modest degree of improvement in model/ensemble consensus was
noted compared to yesterday, but a number of complexities and
unanswered questions remain in the extended period. During days
4-5 (Thu-Fri) model/ensemble consensus was sufficient to warrant
use of a multi-model/ensemble blend (including the
GFS/ECMWF/CMC/ECENS/NAEFS) as a basis for the forecast. These
solutions showed general agreement on an upper low lingering near
the Alaska Peninsula during this time with additional energy
approaching from the west (although with fairly substantial timing
differences). Consensus has also continued to increase the some
element of Arctic shortwave energy will dig southward into the
North Slope/Brooks Range regions on Fri, but the degree of height
falls remains uncertain.

From day 6 (Sat) onward, uncertainty continues to quickly
increase, although some very broad consensus does seem to be
emerging. A wavy baroclinic zone across the North Pacific looks to
become increasingly active, with one wave of low pressure
developing south of the eastern Aleutians and likely to move
northeastward toward the Gulf of Alaska. Farther west, an
additional amplifying trough/upper low should exit eastern Asia
and encounter the remnants of Hurricane Hector which should be
recurving across the North Pacific by that time. While this
process is likely to trigger further flow amplification,
uncertainties with respect to timing result in a very large range
of model solutions for the resultant cyclone by days 7-8. A number
of deterministic solutions show a deepening surface cyclone either
entering the Bering Sea by next Sat night/Sun (GFS/ECMWF/CMC), or
perhaps sliding south of the Aleutians (FV3). Ensemble means seem
to suggest that something in the Bering Sea is more likely. Given
the spread, ensemble means comprise a majority of the forecast
from day 6 onward, with deterministic guidance eliminated entirely
by day 8. This forecast does depict the two systems, one entering
the Gulf by Sun and the second crossing the Bering Sea Sun-Mon.
Given the spread, pressures were not lowered significantly this
forecast cycle, but knowing that the flow may be becoming
increasingly amplified by this time, the potential exists for
either of these cyclones to be quite a bit deeper than is shown at
the current time.

Precipitation looks fairly widespread across the North Slope and
Brooks Range Thu-Sat as the Arctic shortwave energy and a frontal
boundary cross the region. Temperatures will be sufficiently cold
at the higher elevations to support snow. As heights across this
area lower further toward the weekend, any remaining precipitation
could mix with snow even north of the Brooks Range at the lower
elevations. Farther south, onshore flow will initially support
scattered showers across southern mainland and Southeast Alaska.
The approaching low pressure system in the Gulf by the weekend
will likely support more widespread rains for these areas
lingering into early next week. Finally, the potential low
pressure system in the Bering Sea by next weekend could result in
potential rain/wind impacts for portions of the Aleutians and
perhaps even western mainland, although confidence is much too low
at this time to discuss specific impacts or locations.


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