Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
313 PM EDT Mon Sep 03 2018

Valid 12Z Fri Sep 07 2018 - 12Z Tue Sep 11 2018


The deterministic models show a highly amplified  cutoff pattern
as an omega block forms over the northern Pacific with the
northern jet stream going over the top of the Bering Sea ridge in
the Arctic Ocean.

The models cluster well with the formation of a reinforcing upper
low over the Gulf of AK that drops south into the northern Pacific
Fri into Sat 08 Sep. 
The GFS allows the omega block to persist a  little longer than
the ECMWF/ECMWF ensemble mean, and this solutions was given more
weight.
Consequently the forecasts mainly consisted of blending the 06-12z
GFS and 06-12z GEFS Mean, with less weight on the 00z-12z ECMWF
and ECMWF Ensemble Mean. The 12 Canadian was also slower moving
the closed low towards Hadai Gwaii and British Columbia.

The upstream high amplitude closed high over the Bering Sea is
slow to move and acts as a block to upstream energy ejecting from
northeast Asia.
The northern stream on day 6 Sun shows lower heights over
northeast Russia/Siberia extending to Kamchatka and the Bering
Straits, supporting formation of a frontal system that continues
through the end of the forecast period.

In the Arctic Ocean and northern AK, a mid/upper-level Arctic
shortwave energy progresses steadily east, allowing a cold front
to form and progress across northeast AK into the Yukon.  Typical
timing and phasing differences with the upper trough exist, with
enough clustering to resolve those differences with a
multi-model/ensemble mean consensus.


Petersen


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