Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Sun Mar 04 2018

Valid 12Z Thu Mar 08 2018 - 12Z Mon Mar 12 2018

Today's guidance maintains decent continuity with the large scale
evolution though with some typical but locally important spread
and day to day adjustments for specifics.  The most notable trend
in guidance regards the track of the upper low heading toward the
mainland from the extreme northern Bering Sea.  Heading into
yesterday the guidance had started showing a farther west and then
south track going along with a stronger western Canada into
eastern mainland Alaska ridge.  More recent solutions have now
reversed that trend, bringing the upper low at least through the
southwestern mainland in a manner closer to that of two days ago.

The forecast remains on track for the overall amplification of the
pattern as a ridge building into the west-central Pacific extends
into the Bering while troughing over the eastern half of the
Pacific deepens and sharpens.  Guidance is also consistent with a
weakness aloft remaining over much of the western mainland through
the weekend into the start of next week.  During the latter half
of the period GFS runs in varying ways are still tending to
flatten the Bering ridge aloft to a greater extent than other
solutions, allowing for faster surface wave/front progression
across the Bering and shifting the eastern Pacific upper trough
farther eastward.  Among 00z ensemble members such a scenario
remains in the minority though the operational 00z ECMWF/CMC do
bring a front into the Bering by days 7-8 Sun-Mon (but still
slower than the GFS runs).  The one other uncertainty to monitor
late in the period involves the potential for energy on the
eastern side of the Pacific trough aloft to generate surface low
pressure that tracks into the Gulf of Alaska, affecting the
southern coast and/or Panhandle.  Over both areas of interest the
system scale lends itself to low predictability 7-8 days out in
time.

During the first half of the period an operational model blend
incorporating aspects of the 00z/04-12z/03 ECMWF, 12z GFS, and 00z
CMC helped to resolve ongoing detail differences at the surface
and aloft.  The 00z GFS started out quite progressive versus other
models/means for low pressure areas with the 06z GFS adjusting
partially back and then the 12z run completing that trend.  There
is still a fair amount of uncertainty on the exact track of parent
low pressure nearing the western mainland, plus important
differences on the evolution/track of a potential triple point
and/or other frontal wave development over the northern Gulf of
Alaska Thu-Fri.  Solutions are very sensitive to details aloft and
will take further time to be resolved satisfactorily.  For both
features the ensemble means have been much steadier than
operational runs, holding the parent low farther west than 06z/00z
GFS runs from Thu onward and showing a low center near the Kenai
Peninsula as of 12z Fri.  For this latter low today's ensemble
means are noticeably deeper than in previous days.  This trend
seems to support including some components of the 12z GFS and
12z/03 ECMWF in the forecast, but ongoing uncertainty favors not
going too much deeper than the 06z GEFS mean which is deepest
among the means.  A farther southeast track as per the 00z ECMWF
or UKMET is possible but less likely based on ensembles thus far.

Upper low energy that drifts across the southwestern mainland
should lead to persistence or redevelopment of low pressure
near/east of Kodiak Island by early day 6 Sat.  At that time the
aforementioned model blend ended up being very close to the
ensemble means so that day served as the perfect transition to an
ensemble mean blend (half 00z ECMWF mean and the rest 06z GEFS/00z
NAEFS) for days 7-8 Sun-Mon.  This solution maintains a broad area
of low pressure over the northeastern Pacific while awaiting more
clarity on specific of individual features.  Also it depicts a
mere frontal zone over the Bering (possible wave over the western
Bering) given that signals are still fairly modest for passage of
a well defined system.  However an early look at the new 12z ECMWF
reveals a switch to the GFS scenario so what confidence existed is
lower than before.

Rausch