Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
330 PM EDT Thu Aug 08 2019
Valid 12Z Mon Aug 12 2019 - 12Z Fri Aug 16 2019
Guidance still depicts an amplified/blocky pattern with latest
GEFS/ECMWF/CMC ensemble means fairly similar and consistent in
principle through days 4-8/Mon-Fri. The ensemble mean cluster
shows one closed low just north/northeast of mainland Alaska while
an initial Bering Sea closed low is replaced by an upstream
western Pacific system that tracks into the western Bering late in
the week. Meanwhile upper ridging should rebuild for a time over
the northeast Pacific and late in the period an eastern
Bering/western mainland ridge should strengthen ahead of the
western Pacific/Bering storm.
In spite of the relative agreement among the means, the Arctic
upper low has exhibited a lot of spread and run-to-run variability
in operational model runs. ECMWF runs have been somewhat less
erratic than GFS runs over the past couple days. The spread in
the guidance keeps confidence lower than desired for resolving
exactly how far southward cooler air will push in response to
shortwave energy within/around the upper low. The current array
of guidance suggests a compromise upper low path leaning a little
more toward (but not completely to) the western ECMWF versus the
eastern GFS. A moderate eastward adjustment in the new 12Z ECMWF
supports this idea.
Farther southwest there is lingering uncertainty over exactly what
will happen with energy from the initial Bering Sea system. At
the very least the 00Z CMC becomes one of the less probable
solutions in bringing nearly all of the upper low energy across
the mainland/Gulf of Alaska by Wed-Thu. However there is some
degree of consensus that flow ahead of this system, as well as the
front pushing southward into the northern mainland, may support
areas of enhanced precipitation.
In contrast to the detail uncertainty from the eastern Bering
through the mainland and parts of the Arctic, thus far the
models/means have been more agreeable and consistent regarding the
system expected to deepen over the western Pacific and eventually
track into the western Aleutians/Bering Sea. This relative
agreement allows for maintaining some inclusion of operational
guidance through the end of the period in order to provide some
enhancement of the means. The new 12Z ECMWF adds a question mark
though, splitting the system into two separate waves. The
gradient ahead of this storm may produce an area of strong winds
over parts of the Aleutians/Bering Sea. Downstream the guidance
agrees on a strengthening upper ridge over the eastern Bering and
western mainland but differs on strength/position. Some recent
GFS runs including the 12Z version have been on the strong and/or
eastern side of the spread for the ridge. The 06Z GFS is also a
bit on the stronger side but with an axis aligned closer to
consensus.
Based on the guidance evaluation today's forecast started with an
operational model blend (ECMWF/GFS/UKMET/CMC) for day 4 Mon and
then gradually increased ensemble mean input so that the means
provided 60 percent weight by days 7-8 Thu-Fri. The 12Z run was
reasonable for the GFS component during the first couple days but
the 06Z version compared better to remaining guidance later in the
week, requiring a mid-period transition for GFS input.
Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
- High winds across portions of the Aleutians, Wed-Thu, Aug 14-Aug
15.
- High significant wave heights for coastal portions of the
Aleutians, Wed-Thu, Aug 14-Aug 15.
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html