Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
146 PM EST Tue Feb 13 2018

Valid 12Z Sat Feb 17 2018 - 12Z Wed Feb 21 2018

Today's guidance maintains a strong mean ridge aloft extending
from the eastern Pacific northward through mainland Alaska.  There
will be periodic intrusions of shortwave energy, in a continuation
of some aspects of the pattern in the short range, but lowest
heights aloft will remain well to the west thus keeping
significant low pressure systems over the Bering Sea.

For the Sat-Sun time frame guidance seems to be still in the
process of trying to resolve the details a wave expected to track
into the Bering from south of the eastern Aleutians, along with
the evolution of supporting shortwave energy.  Among recent
guidance the 00z ECMWF is on the fast side of the spread, while
the 06z/12z GFS are slowest due to interaction with and/or greater
emphasis on a feature just to the west.  By Mon the differences
extend into the Gulf of Alaska with model/ensemble solutions quite
varied at the surface and aloft.  In fact the 12z GFS evolution
has nearly all shortwave energy going around the western side of
the ridge.  While certainly possible it represents a departure
from continuity, majority of other guidance, and the established
pattern that has been allowing shortwaves to pass through the mean
ridge.  Confidence in any particular solution is quite low given
this spread and the typical difficulty with forecasting energy
moving into/through a mean ridge aloft.  For this part of the
forecast prefer a compromise among the 00z operational models into
day 5 Sun to provide somewhat more definition than the means and
compromise timing, followed by some ensemble mean inclusion as
model specifics diverge to a greater extent.  This approach leads
to a conservative/weak depiction at the surface over and south of
the Gulf of Alaska by day 6 Mon.

Upstream the models/ensembles provide a decent signal for a fairly
strong western Pacific storm tracking into the Bering by early
next week.  ECMWF/GFS runs from the past day are fairly similar in
principle with timing closest to the 00z ECMWF mean, though
00z/06z GFS runs to begin to stray on the fast/northern side of
the spread especially after late Mon.  The 12z GFS has adjusted
slower.  GEFS/CMC ensemble means hold the low farther back to the
southwest. 

As of next Tue-Wed there is a fair amount of spread regarding to
what degree height falls dampen the mainland ridge aloft.  In
general most guidance holds onto somewhat higher heights than the
00z ECMWF/ECMWF mean.  The GFS has varied widely, ranging from
keeping a very strong ridge in the 00z run to having a 00z
ECMWF-like solution in the 12z run.   An ensemble mean compromise
best balances competing considerations, guidance tendency to
weaken ridges too quickly versus the established pattern allowing
occasional shortwave passage. 

By the end of day 8 Wed the current and recent ensemble means are
remarkably similar in showing another significant system whose
eastern gradient should begin to affect the western Aleutians.  A
number of operational model runs are in the general neighborhood
of the means as well.

Based on preferences over the course of the period, today's blend
included the 00z GFS/ECMWF/UKMET/CMC from day 4 Sat into day 5 Sun
and then gradually increased 06z GEFS and 00z NAEFS/ECMWF mean
weighting so that day 8 Wed consisted of half 00z ECMWF mean and
the rest GEFS/NAEFS.  Around mid-period the GFS contribution was
split between the 06z and 00z runs due to the former being a
little less extreme with progression of the system tracking into
the northern Bering at that time.

Rausch