Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Fri Aug 09 2019
Valid 12Z Tue Aug 13 2019 - 12Z Sat Aug 17 2019
Over the past 24 hours the most prominent trend in the guidance is
that the ensemble means show a stronger ridge building into the
eastern Bering Sea and vicinity, leading to increasing amplitude
within an already blocky pattern. This adjustment leads to a
greater northerly component of flow aloft over the mainland and
Gulf of Alaska by the latter half of the period while holding back
strong western Pacific low pressure that had been expected to
track into the western Bering Sea by late week. Individual
operational model runs over the past couple days have hinted at
the potential for a fairly strong ridge but with
differences/variability in longitude. Overall recent ECMWF/ECMWF
ensemble mean runs have been closer to the current clustering than
some GFS/GEFS mean runs that were somewhat farther east. The new
12Z ECMWF has shifted the ridge eastward though. Prefer to update
the forecast with a model/mean blend to reflect the latest
guidance trends, while remaining less aggressive than the
ECMWF/CMC runs that have the strongest ridge. Newest GFS runs are
weaker than the 12Z/08 run so confidence is not quite high enough
to go with the strongest side of the spread yet.
Meanwhile predictability is low for small scale details
within/around the upper low forecast to be northeast of the
mainland. At the moment there is a common signal that energy
associated with a compact upper low initially over the western
Canadian Archipelago will loop around the larger scale circulation
during the short range and track over/near the northern mainland
by next Tue-Wed. A blend emphasizing operational guidance
reflects this detail better than the means but specifics will
likely change to some degree over future runs.
Forecast considerations led to starting out with a blend tilted 80
percent toward the 12Z GFS and 00Z ECMWF/UKMET for day 4 Tue into
day 5 Wed, with the rest consisting of the 06Z GEFS/00Z ECMWF
means. Then the forecast quickly trended to 50-60 percent
ensemble weight while holding onto some GFS/ECMWF input.
During the first half of the period portions of the southern half
of the mainland may see an episode of enhanced precipitation with
a band of moisture associated with shortwave energy aloft and a
wavy cold front crossing the extreme southern mainland/Gulf of
Alaska. The pattern over the northern mainland may also promote
some locally significant precipitation but specifics are more
unclear. The western Pacific storm tracking toward the western
Aleutians is slower than in yesterday's guidance but there is
still reasonable consensus on the potential for the leading
gradient to generate a band of strong winds over parts of the
Aleutians and Bering Sea, with some focused rainfall possible as
well.
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
- Heavy rain across portions of interior Alaska, Tue-Wed, Aug
13-Aug 14.
- 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