Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Sat Jan 27 2018

Valid 12Z Wed Jan 31 2018 - 12Z Sun Feb 04 2018

At least for some aspects of the forecast, guidance has come into
much better agreement.  This is particularly the case for the
evolution of energy initially southeast of the strong eastern
Siberia upper high and position of the lingering weakness near the
southern coast of the mainland, between the upper high and
northeast Pacific ridge that builds in response to mean troughing
that becomes established over the central Pacific.  00z/06z models
have held onto the majority cluster that emerged from the 12z/26
cycle, showing an upper low retrograding along the Aleutians. 
This has been hinted at by the ensemble means even while
operational models were very divergent.  At the same time there is
still meaningful uncertainty regarding interaction of this energy
with central Pacific energy as the overall feature moves westward.
 To the east of the upper low, the weakness aloft near the
southern coast of the mainland has ended up fairly close to the
preferred compromise from continuity. 

Meanwhile significant spread persists regarding the northeastward
extent of a front and associated precip over/near the Gulf of
Alaska.  This reflects differences in position of the northeast
Pacific ridge aloft.  Although consistent with its 12z/26 run, the
00z ECMWF is farther west with the upper ridge axis than other
guidance including its ensemble mean.  On the other hand the 06z
GFS has backed up somewhat versus its 00z run.  The current array
of guidance and similarity/intermediate position of the 06z
GEFS/00z ECMWF means at the surface and aloft recommend a solution
closest to the means for this aspect of the forecast.

By the latter half of the period the ensemble means have been
fairly consistent in showing a gradual elongation of mean
troughing, eventually extending from eastern Asia across the
western half of the northern Pacific and south over the central
Pacific.  Eastern/northeastern Pacific ridging persists while the
eastern Siberia upper high continues a slow clockwise loop toward
northwest mainland Alaska.  Most of the latest models show similar
ideas, the exception being the 06z/12z GFS runs that collapse the
upper high southeastward and potentially over-emphasize the
northeast Pacific ridge. 

Even with the uncertainty in ridge specifics, there is a decent
guidance signal that this pattern should lead to a period of
enhanced moisture flow into the eastern Aleutians/southwest Alaska
Peninsula by next weekend, with one or more surface waves tracking
just west of the gradient.  A blend of the 06z GEFS/00z ECMWF
means along with minority input from the 00z ECMWF provides a good
representation of the most likely evolution.  Some ensembles go
along with sporadic model runs that have been showing surface low
tracks well into the Bering Sea, but so far in sufficiently low
quantity to keep the means along or just south of the Aleutians. 
Recent trends suggesting that the means have been a little too
eager to weaken the upper high favor maintaining some ECMWF weight
in the forecast.  Throughout the period the means have been
consistent with an axis of surface high pressure extending from
the Arctic through the northern mainland into northwest Canada. 
Any breaks in this mean surface ridge will depend on low
confidence shortwave details aloft. 

Based on better agreement early in the period, today's forecast
started with a 06z GFS/00z ECMWF/00z CMC blend from early day 4
Wed into day 5 Thu to provide better definition than the means. 
Increasing detail uncertainty plus lower confidence in various
aspects of 06z GFS/00z CMC solutions after Thu led to trending the
forecast to a blend among the 06z GEFS/00z ECMWF means and 00z
ECMWF by late Fri through Sun.

Rausch