Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 PM EDT Mon Aug 13 2018

Valid 12Z Fri Aug 17 2018 - 12Z Tue Aug 21 2018

Some improvement in model/ensemble consensus was noted compared to
yesterday, during days 4-5 (Fri-Sat), and model/ensemble consensus
was sufficient to warrant use of a majority deterministic
multi-model/ensemble blend (including the GFS/ECMWF/ECENS/GEFS) as
a basis for the forecast. These solutions showed general agreement
on the low pressure system approaching the eastern Aleutians and
Alaska Peninsula from the south during that time frame. A trend
was noted over the past day toward greater amplification of the
flow in general, likely in response to the upstream extratropical
transition/absorption of the remnants of Tropical Storm Hector.

From day 6 (Sun) onward, uncertainty continues to quickly
increase, although some very broad consensus does seem to be
emerging. The amplified flow pattern will support a northward or
perhaps even slight west of north track of the surface low toward
the Alaska Peninsula through Mon as the system occludes. Farther
west, an additional amplifying trough/upper low exiting eastern
Asia are expected to absorb the remnants of Tropical Storm Hector
as it recurves across the North Pacific (triggering continued
broad flow amplification, as described above). This system should
enter the Bering Sea on day 6 and then move relatively slowly
eastward through day 8. Deterministic solutions are not quite as
intense with this low as they were yesterday, but confidence in
this portion of the forecast remains quite low given the complex
nature of the interactions occurring.

Precipitation looks fairly widespread across the North Slope and
Brooks Range Fri-Sat as the Arctic shortwave energy and a frontal
boundary cross the region. Farther south, onshore flow will
initially support scattered showers across southern mainland and
Southeast Alaska. The low pressure system approaching the Alaska
Peninsula by the weekend will likely support more somewhat
widespread rains for much of southern Alaska, lingering into early
next week. Finally, the potential low pressure system in the
Bering Sea by next weekend could result in potential rains and
gusty winds 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