Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Sat Sep 01 2018

Valid 12Z Wed Sep 05 2018 - 12Z Sun Sep 09 2018


The deterministic models diverge in details but each shows a
highly amplified or cutoff pattern as an upper low stalls over the
northeast Pacific, which then backs up the pattern with the high
amplitude ridge slow to move east across the Bering Sea and
Aleutians.   In turn the next low is delayed by several hours if
the overall trend is correct.

Further north in northern AK and the arctic ocean, northerly
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.

The ECMWF and GFS showed relatively good consensus with the
ensemble means initially on day 4 (Wed), with the spread gradually
growing afterward.
The clusters fell into 2 separate camps, with the 12z GFS and 06z
GEFS Mean
slower to progress systems across the north Pacific.

Usually less weighting of the gfs is used at longer ranges but
given its slower movement matches well with the overall high
amplitude pattern with potential for closed highs and lows to
decrease forward motion of systems, the gfs and GEFS were equally
weighted with the 00z ECMWF and the ECMWF ensemble mean.

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