Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Thu Apr 05 2018

Valid 12Z Mon Apr 09 2018 - 12Z Fri Apr 13 2018

From the mainland into the Arctic, some aspects of the forecast
have declined in confidence with recent model/ensemble runs
showing more spread and run-run/day-day variability.  The most
pronounced trend of consensus versus yesterday's majority cluster
is toward the Arctic ridge/upper high hanging on farther eastward.
 This seemed to be hinted at initially by the 12z/04 GFS and
06z/04 GEFS mean.  Among model/ensemble mean solutions through the
06z cycle, the 06z GEFS mean is the weak extreme for the Arctic
ridge so forecast preferences lean away from that solution
especially early-mid period.  The persistence of the Arctic ridge
will delay/confine farther northeastward any cyclonic flow that
had been advertised.  By the latter half of the period the
operational models show a variety of possible evolutions aloft
over the mainland--such as the 12z GFS merging the Arctic high and
a northern offshoot of the western Canada mean ridge by Wed-Thu
versus the 00z ECMWF that has an east-west then
northeast-southwest weakness.  Ensemble means generally suggest
somewhat of a col region over the mainland, between
north-northeast Pacific/Canadian archipelago mean lows and western
Canada/Arctic ridges.

While detail differences continue across the North Pacific stream,
a model/ensemble consensus maintains reasonably good continuity. 
Early in the period an operational model emphasis refines the
strength of the system initially over the Bering Sea on day 4 Mon.
 Some operational solutions are suggesting that a northwestward
tracking area of low pressure to the east of this system may track
just south of Kodiak Island, bringing a period of enhanced
wind/rainfall to that area and parts of the southern coast.  For
that feature the 00z ECMWF is on the fast side of the ensemble
spread and otherwise there is still sufficiently low confidence to
go too aggressively to an operational solution, with a model blend
yielding a forecast somewhat closer to the means (with some
northward hedge from continuity).  There is still a good signal
for the next system in the series, likely to track along or a
little north of 50N latitude day 6 Wed onward.  Compared to
yesterday the 00z ECMWF has improved its definition of the system
while the ECMWF mean still looks too diffuse until
reaching/passing the central Aleutians.  Overall the GFS/CMC and
their means have been stronger and more consistent with this
system so far.  The eastern periphery of the next system may begin
to influence the western Aleutians by day 8 Fri.  Currently
models/means show better than average agreement for this system.

As was the case yesterday, the forecast emphasizes operational
guidance early in the period (06z-12z GFS/00z ECMWF/00z CMC) with
only modest ensemble input, followed by a trend toward a more
balanced model/mean blend that includes the aforementioned models
and varying amounts of the 06z GEFS mean and 00z NAEFS/ECMWF means
with minor manual enhancement where appropriate.  This helps to
strengthen the means a bit for North Pacific systems while
disparate model solutions aloft over/near the mainland cancel out
to keep the forecast closer to the means.

Rausch