Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
556 PM EST Sun Nov 01 2020
Valid 12Z Thu Nov 05 2020 - 12Z Mon Nov 09 2020
...Guidance/Predictability Assessment...
Today's guidance maintains better than average agreement for the
expected large scale pattern during the period. Expect a mean
ridge to settle over and south of the mainland while a broad upper
trough prevails well upstream, anchored by a couple upper lows
over/south of eastern Siberia. Energy ahead of a leading strong
Kamchatka/Siberia storm will pass through the ridge late this
week. Then a likely sharper shortwave crossing the Bering
Sea/Pacific on Sat should push into/through the mean ridge during
Sun-Mon, supporting a system that may track into the Gulf of
Alaska. Also during that time another strong system reaching the
western part of the domain may spread another broad area of
moisture and strong winds across the Bering Sea/Aleutians.
A 12Z model consensus with more GFS/ECMWF emphasis relative to the
UKMET/CMC represents consensus well for the first half of the
period (Thu-early Sat) when clustering is good for significant
features. Continuity for the overall evolution is favorable but
there has been somewhat of a faster trend over the past couple
days with the front/strong wind field crossing the Bering and
reaching the western mainland, along with improved definition of a
transient Gulf wave around Fri.
After early Sat guidance appears to have trended toward a
compromise between yesterday's extremes for the system that may
develop in response to the shortwave crossing the Bering
Sea/Pacific. ECMWF/ECMWF mean runs have adjusted toward more
stream phasing (pulling its previously suppressed surface system
northward to just south of the Alaska Peninsula) while latest
GFS/GEFS runs have toned down the strength of the northern part of
the shortwave (weakening the surface reflection reaching
along/north of the Peninsula compared to 00Z and earlier runs).
Beyond the specifics of the shortwave energy there is the
continued uncertainty over how much interaction will occur with a
low/mid-latitude Pacific system. 12Z model runs are hinting at
lower potential for the system to be fully picked up and
incorporated into the main northern low. While recent trends have
been faster, the 12Z GFS/CMC may be a little overdone with their
speed (GFS also now on the suppressed side). Hence late-period
preference leans toward the past two ECMWF runs/06Z GFS and latest
ensemble means. Aside from timing differences there is good
agreement on a track into the Gulf.
For the deep system that may reach the western Bering Sea by next
Mon, ECMWF/CMC runs have generally been more coherent with their
surface low(s) than GFS runs over the past day. The typically
more consolidated ensemble means are extremely close through the
end of next Mon. As with the northeastern Pacific/Gulf pattern,
favored model runs compared well enough to the means that at least
2/3 model input relative to the means could be maintained through
day 8 Mon.
...Weather/Threats Highlights...
The broad area of moisture and strong winds crossing the Bering
Sea/Aleutians as of the start of the period early Thu will quickly
reach the western mainland late this week and bring some enhanced
precipitation to favored terrain. Some moisture will likely
extend to other portions of the mainland. A brief period of
mostly light precip may accompany a transient Gulf wave around
Fri. There is still moderately high uncertainty over precise
coverage and intensity of precipitation associated with low
pressure whose dominant center is most likely to track just south
of the Alaska Peninsula and into the Gulf of Alaska. Over the
past day there has been some favorable convergence among solutions
toward a moderate amount of moisture reaching the southwestern
mainland. However there is still enough track spread for the
surface low to yield below average confidence for whether heavier
activity may reach portions of the Peninsula, southern coast, and
Panhandle. The strong storm that may emerge over or near the
western Bering Sea late in the period should spread another area
of precipitation and strong winds across the Bering Sea/Aleutians
next Sun-Mon.
A west-to-east warming trend in progress at the start of the
period should ultimately confine below normal temperatures to
parts of Southcentral Alaska and the Panhandle by the weekend and
early next week. Southcentral may see embedded pockets of
modestly above normal readings late in the period. The North
Slope will see much above normal temperatures through the period
with plus 20F and greater anomalies possible while significant
positive anomalies will also extend southwestward through the
western and parts of the central mainland. Overall expect the min
temperatures to be more extreme versus normal relative to max
temps.
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
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