Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Thu Sep 13 2018
Valid 12Z Mon Sep 17 2018 - 12Z Fri Sep 21 2018
From the large scale perspective the most agreeable features
during the period appear to be the broad upper low covering
northern Canada, and farther west an upper trough whose embedded
upper low is most likely to pass over or near Kamchatka mid-late
week along with the leading downstream ridge. Between these
features the forecast is more ambiguous with respect to eastern
Siberia/northern Bering Sea upper low/trough energy as of day 4
Mon, leading to greater uncertainty for specifics across Mainland
Alaska and into the Gulf of Alaska.
With somewhat more of a split in guidance today, solutions for the
initial eastern Siberia upper low range from keeping most energy
fairly far north over/near the northern mainland (06Z GFS/00Z CMC)
to bringing the upper low southeastward across the mainland toward
or across the Gulf of Alaska depending on timing (00Z GFS/ECMWF).
There are few enough ECMWF ensemble members showing the latter
scenario that the corresponding mean shows no indication of such
potential, while the latest GEFS means are at least starting to
hint at it. It is worth noting that both ensemble means have
trended stronger with the upper ridge that builds into the Bering
Sea early-mid week, and there may be a fine line with respect to
whether the ridge may sharpen just enough to allow for
southeastward progression of energy across the mainland
toward/into the Gulf. With the 12Z GFS reverting to a 00Z
ECMWF-like solution, relative potential for one side of the spread
or the other is at least closer to balanced versus 24 hours ago.
Thus prefer an initial blend that brings at least a modest amount
of shortwave energy southeastward but remains conservative with
potential Gulf of Alaska low pressure late in the period.
The models and ensembles have generally come into better agreement
for Arctic/northwestern Canada flow that affects the northern
mainland, not far from the previously favored compromise approach.
At the moment the 00Z/12Z GFS runs differ the most from consensus
as they have the Arctic ridge drifting farther east than most
Overall the guidance seems to show more spread than yesterday with
surface low/frontal details over the Bering Sea/Aleutians. At
some valid times there is somewhat of a slower trend overall in
due to the stronger trend with the downstream ridge aloft.
Individual models/ensemble members vary considerably with leading
low pressure tracking out of the western Pacific and initially
pushing the mean frontal boundary into the Bering/Aleutians, and
beyond that some solutions like the 12Z GFS/00Z ECMWF develop a
trailing wave that becomes rather strong as it tracks through the
Aleutians and into the Bering late in the forecast. Given this
spread the ensemble means provide a more appealing starting point
at this time. Preference leans more toward the ECMWF mean as
recent runs have been either more stable or at least a 12-hourly
cycle ahead of the GEFS in reflecting latest trends across this
part of the forecast domain.
Based on the composite of above preferences the forecast blend
started with mostly 06Z GFS and 00Z ECMWF/CMC with modest 06Z
GEFS/00Z ECMWF mean input early in the period. Then the ensemble
mean weight steadily increased from mid-late period, with day 8
Fri consisting of a 70/30 ratio of ECMWF mean to GEFS mean.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at: