Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM EST Fri Mar 02 2018

Valid 12Z Tue Mar 06 2018 - 12Z Sat Mar 10 2018

For the Tue-Sat period latest guidance continues to show a
transition from a progressive pattern of shortwaves rounding a
retrograding Pacific mean ridge toward a slower evolving pattern
that has a fairly deep mean trough settling over the mainland and
Gulf of Alaska with a possible embedded closed low over a portion
of the mainland.  GFS/ECMWF runs and their means have been fairly
agreeable/consistent with the overall pattern evolution but with
meaningful differences for some details.  Other operational models
and individual ensemble members also suggest ongoing potential for
aspects of the forecast to develop in ways other than the most
likely consensus ideas.

Leading shortwave energy over the northeastern Pacific/Gulf of
Alaska should support a weak surface wave offshore the Panhandle
at the start of the forecast 12z Tue, with reasonable continuity
from yesterday.  After that time solutions diverge for shortwave
timing (00z-06z GFS/GEFS slowest, 12z GFS nudging a bit faster),
with little change in the relative spread of guidance over the
past day favoring an intermediate approach.

The next shortwave in the series, reaching the eastern Bering Sea
and vicinity as of early Tue, still exhibits a lot of spread in
the guidance.  The past couple ECMWF runs have trended much
weaker/more progressive than the 00z/01 run--in the direction of
recent GFS runs through the 06z cycle.  However the 00z
CMC/UKMET/CMC still show strong digging of this energy into the
northeastern Pacific with a more pronounced surface wave than
GFS/ECMWF runs through 06z.  A significant number of CMC ensembles
and some ECMWF members show a more amplified shortwave than the
GFS/ECMWF.  Thus the 00z ECMWF/NAEFS means hedge a bit stronger
than the GFS/ECMWF.  Indeed the new 12z GFS has switched to a more
strongly amplifying shortwave and northeast Pacific surface low. 
Based on this full array of guidance the preference went to a 12z
GFS/00z ECMWF blend with a little ECMWF/NAEFS mean input to yield
an evolution similar to continuity from yesterday aside from
slightly slower timing.

At least in terms of depth the models/means appear to have
converged to some degree for the upper low/troughing heading
toward and into the mainland during the latter half of the week. 
00z/06z GFS runs are strongest with energy aloft reaching the Gulf
of Alaska by early day 7 Fri leading to the deepest low pressure
near the southern coast.  The 12z GFS has adjusted some details
aloft so its southern coast surface low is not as deep as previous
runs.  Meanwhile at that time the ensemble means are a little
deeper than yesterday with the western mainland upper low which
supports somewhat longer persistence of a separate surface low in
its vicinity--not far from yesterday's model/mean blend.  By next
Sat model details diverge enough to recommend considerable
emphasis on the GEFS/ECMWF means which keep low pressure over the
northern Gulf.  Note that the 00z CMC is offset well southwest of
consensus for the late period pattern while the CMC mean hedges a
bit south/southwest of the other means.

Upstream uncertainty comes into the picture over the western
Aleutians/Bering around day 8 Sat with energy departing from
western Asia earlier in the period.  Not a lot of 00z ensemble
members depict the fast/strong system depicted in 12z GFS/00z
ECMWF runs, so the ensemble means provide the best option for now.

Over/near the northern mainland from Tue into late week there
appears to be a moderate northward adjustment from continuity at
some valid times for the wavy front near the northern coast. 
Lesser predictability of finer surface details relative to flow
aloft--and some lingering uncertainty with shortwave evolution as
well--suggest further adjustments may be in store.

Rausch