Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
204 PM EST Thu Mar 01 2018

Valid 12Z Mon Mar 05 2018 - 12Z Fri Mar 09 2018

Today's guidance shows better than average agreement in principle
with the sequence of systems expected to track across the northern
Bering Sea toward/into mainland Alaska and the Gulf of Alaska. 
These systems are rounding the northern periphery of a Pacific
mean ridge that is likely to retrograde with time.  Within the
agreeable pattern there are some trends and model/ensemble spread
that suggest some details will take more time to resolve. 

During the early to middle part of the forecast period (day 4 Mon
into day 6 Wed) a blend of the 00z ECMWF and 00z/06z GFS along
with a little 00z CMC represented consensus well with good support
from the ensemble means.  Leading shortwave energy should reflect
at the surface as low pressure that may brush the northwest coast
of the mainland on Mon while a weak Gulf of Alaska wave forms
along the trailing front.  The next system should reach the
northern Bering by Tue with somewhat more surface low spread in
the guidance as might be expected given the more distant time
frame.  The operational model average falls well within the
ensemble envelope.  At the moment the greatest uncertainty with
the associated upper trough appears to be its amplitude, as
ECMWF/CMC model-mean solutions show a deeper shortwave than
GFS/GEFS solutions.  00z UKMET/NAVGEM offer further detail
differences but are generally a compromise.  The preferred model
blend led to such a compromise approach for both the upper
shortwave and the wave that develops over the Gulf of Alaska by
Wed.

By the latter half of the forecast the dominant feature will be a
deeper system tracking across the Bering Sea toward the mainland,
with the core of the upper system most likely to be over or near
the western mainland by day 8 Fri.  Both GEFS and ECMWF means have
trended deeper with the core of the trough/embedded low aloft
compared to 24 hours ago though the GEFS trend has been more
significant given a weaker starting point yesterday.  Most
operational models are currently deeper than the means.  In light
of the deeper trend of the means aloft, maintaining at least half
weight of the operational guidance--with the rest of the blend
consisting of more 00z ECMWF mean than 06z GFS mean--seems
reasonable into day 8.  At that time there is better ensemble
clustering for surface low pressure along the southern coast near
the Kenai Peninsula or Prince William Sound versus residual Bering
Sea/mainland low pressure.  The 12z GFS trended somewhat
weaker/broader aloft versus the 00z/06z operational models,
leading to faster progression of leading surface low
pressure/front.  While this difference led to maintaining the
preliminary preference incorporating the 00z-06z models and
ensemble means, earlier ECMWF runs had been faster as well so
system timing is a meaningful uncertainty in spite of the relative
similarities of 00z-06z models/ensemble means.

Rausch