Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Mon Jan 29 2018

Valid 12Z Fri Feb 02 2018 - 12Z Tue Feb 06 2018

Forecast uncertainties and approach today remain similar to recent
days.  Guidance is still having difficulty in resolving the
evolution/longevity of an upper high meandering near eastern
Siberia and the balance between this high and eastern Pacific
ridging.  To the south and west there is ongoing uncertainty for
the details of individual surface low centers supported by central
Pacific trough energy.

Overall preferences maintain a solution that is more heavily
weighted toward the 00z ECMWF/ECMWF mean with minority
contribution from other solutions that are most comparable to this
scenario.  These include the 00z GFS/CMC and 00z-06z GEFS means. 
ECMWF/GEFS means have been the most consistent over multiple runs,
keeping an even balance between a persistent eastern Siberian
upper high and eastern Pacific ridge.  ECMWF runs have had some
continuity changes but as a whole have been much closer to the
ensemble means in principle.  GFS runs have been very
erratic--some progressing/collapsing the upper high (as in the
current 12z run), others retrograding it (as in the 06z run). 
Either way leads to those runs extending systems/moisture farther
north through the Gulf of Alaska than the means or ECMWF runs. 
The occasional GFS run is closer to the means with the 00z run
falling in this category.  Toward the end of the period there are
signals (somewhat more from operational runs than ensemble means)
toward more cyclonic flow/lower thicknesses over at least the
eastern mainland as the upper high weakens somewhat. 
Teleconnections relative to an agreeable positive height anomaly
center near 70N/170E suggest upper troughing could be as far south
as the Gulf of Alaska but eastern Pacific ridging may keep any
trough axis somewhat farther north.

Across the North Pacific there are still some detail differences
involving energy initially retrograding across the Aleutians as
well as nearby central Pacific energy.  While the 12z GFS is not
favored for its handling of the upper high, early in the period it
has trended faster than 00z/06z runs with the Aleutians energy--a
nod toward the 00z ECMWF/CMC.  Also there is a decent model
cluster showing a well defined system reaching the eastern
Aleutians by around late 5 Sat.  So while the ECMWF mean has been
very consistent with its northern Pacific/Aleutians forecast in
recent runs, operational model enhancement from the
ECMWF/GFS/CMC/UKMET looks reasonable early in the period. 
Multi-day trends and a trimming of the northern side of ensemble
low spread from prior days offer support for keeping surface low
tracks generally along or south of the Aleutians.  Current
expectation is that the early period combination of energy aloft
should should retrograde while mid-latitude flow emerges across
the western Pacific, ultimately leading to an east-west mean
trough just south of the Aleutians by the end of the period.

Farther east over the Pacific, in the days 6-7 Sun-Mon time frame
there is some agreement in principle for a shortwave rounding the
eastern Pacific ridge aloft and supporting a surface system that
most likely tracks just south of the Panhandle.  Details are still
sensitive to issues described earlier.  Toward day 8 Tue the
forecast reflects model/ensemble ideas for another system reaching
a position south/southwest of the Alaska Peninsula but with fairly
low confidence.

Rausch