Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
619 PM EST Thu Mar 05 2020
Valid 12Z Mon Mar 09 2020 - 12Z Fri Mar 13 2020
...Overview and Guidance/Predictability Assessment...
Model and ensemble guidance shows a moderate progression of
features during the period. During the Mon-Wed time frame expect
a couple streams of shortwave trough energy to promote surface low
pressure over the Gulf of Alaska, followed by a southeastward
departure as the dynamics aloft continue to the east. A weakening
wave/frontal system will likely cross the Bering Sea around
midweek. Then a strong and broad storm system should reach a
position just northwest of the mainland by early next Fri after
tracking over or west of Japan and Kamchatka earlier in the week.
Regarding forecast specifics, one question mark involves the exact
details of low pressure from south of the Alaska Peninsula into
the Gulf during the first half of the week. Based on 12Z guidance
there is a split between two general scenarios: 1. a developing
system tracking south of the Peninsula and replacing a leading
wave over the northern Gulf as of early Mon (UKMET/CMC); 2. a more
subtle representation of the western wave initially and either
farther east development in the Gulf or reinforcement of the
initial Gulf low (GFS/ECMWF). Perhaps not surprisingly given that
the means are averaging many solutions, they depict only one Gulf
low through this time frame. The fine-scale nature of important
shortwave details that will influence surface evolution suggests
it may take into the short-range period to resolve. Therefore
current preference is an intermediate approach with a 60 percent
tilt to the GFS/ECMWF relative to the UKMET/CMC. Also this
approach represents consensus well for the weakening wave tracking
over the western Bering into midweek and associated frontal system.
Overall guidance agreement is better than average in principle for
the strong and broad northeastward-moving system expected to reach
northwest of the mainland by early Fri. There is typical spread
and run-to-run variability with strength and track/timing,
favoring a transition of the forecast to a model/ensemble mean
blend by Thu-Fri. Latest GFS runs are a bit on the faster/eastern
side of the full guidance spread but not to an extreme degree for
the extended nature of the forecast with the GEFS mean also
moderately faster than the ECMWF/CMC means. During the past
couple days the ECMWF/ECMWF mean have been more stable with the
general evolution of this system versus the GFS/GEFS mean. Thus
the late-week model/mean blend tilts a bit more to the ECMWF/ECMWF
mean scenario than the GFS/GEFS (with modest CMC weight maintained
as well).
...Weather Highlights/Threats...
Early-mid week Gulf of Alaska low pressure will support enhanced
precipitation along the Alaska Panhandle with somewhat lesser
amounts extending back to the southeastern coast of the mainland.
Expect this activity to trend lighter by the latter half of the
week. Meanwhile cold high pressure building into/across the state
from west to east will bring a period of much below normal
temperatures. Readings will moderate late in the week, again from
west to east, ahead of the system forecast to track across Siberia
and just northwest of the mainland. This system will likely bring
a broad moisture shield into the mainland Thu-Fri with fairly
strong winds helping to enhance precipitation totals over favored
terrain.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon-Tue, Mar 9-Mar 10.
- Much below normal temperatures for the outer islands of western
Alaska, Mon, Mar 9.
- Much below normal temperatures across western Alaska, Mon-Tue,
Mar 9-Mar 10.
- Much below normal temperatures across central to eastern Alaska,
Tue-Thu, Mar 10-Mar 12.
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