Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Wed Sep 12 2018

Valid 12Z Sun Sep 16 2018 - 12Z Thu Sep 20 2018

Looking first at the ensemble means, there is reasonable agreement
with respect to gradual erosion of the ridge aloft initially over
Mainland Alaska as upper low/trough energy to the west approaches.
 There may still be some eventual rebuilding of a flat ridge in
response to an upstream trough anchored by an upper low forecast
to track over or near Kamchatka around the middle of next week. 
However the means differ regarding the relative prominence of the
mainland ridge versus flow around the western periphery of a
northern Canada upper low.

With respect to the mainland ridge versus Canada/Arctic cyclonic
flow discrepancy, the 00Z-06Z GFS/06Z GEFS mean show the greatest
northeastward extent of the ridge while the 00Z ECMWF eventually
shows the most Arctic cyclonic flow extending into the northern
mainland.  The stronger/northeastward ridge scenario is a fairly
new idea while recent ECMWF means show good continuity and earlier
GEFS means had some northwesterly flow extending into the
northeastern mainland as well.  The 12Z GFS run has considerably
moderated the ridge to yield a solution closer to a preferred
compromise.

The other significant difference in today's guidance involves the
ultimate track of the upper low initially over eastern Siberia.  A
number of ECMWF runs have displayed a track fairly far southward
into the mainland while the consensus of other operational models
and GEFS/ECMWF/CMC means would the feature farther northward.  It
remains to be seen if it loses its definition as quickly as seen
in the means after Tue though.

Meanwhile ensemble clustering has improved over the past 24 hours
for weak low pressure forecast to track away from Kodiak Island. 
The primary trend for this low is for a farther southward path
across the northeastern Pacific.  On the other side of the
forecast domain there is decent agreement (among ensemble means
and GFS/ECMWF runs) that low pressure associated with the
late-period Kamchatka upper low will push a front into the Bering
Sea/Aleutians by Wed-Thu. There are still alternative solutions
that are faster (06Z parallel GFS) or slower/more suppressed (00Z
CMC).

The early part of the forecast started primarily with the 12Z
GFS/00Z ECMWF to reflect the best definition for features of
interest and preferred compromise for flow over the northeastern
mainland and vicinity.  From late day 5 Mon onward the forecast
held onto modest 12Z GFS weight while otherwise going toward the
ensemble means with somewhat more of a tilt toward the 00Z ECMWF
mean relative to the 00Z GEFS mean.  Teleconnections relative to
the core of positive anomalies near the Aleutians in most D+8
multi-day mean charts provide support for the ensembles which show
moderate progression versus the 00Z ECMWF's slow moving upper low
over the mainland by late in the period.

Rausch


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