Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
645 PM EST Sun Jan 17 2021
Valid 12Z Thu Jan 21 2021 - 12Z Mon Jan 25 2021
...Overview and Guidance/Uncertainty Assessment...
Guidance still appears to be in a state of flux regarding various
aspects of the forecast. In terms of the large scale flow the
most pronounced change over the past day is a trend toward
weaker/less amplified mean ridging that builds in behind a
shortwave that passes through the longer-term mean ridge around
Fri-Sat. This trend results in significantly more progression
across the Bering Sea and Aleutians by next Sun-Mon versus what
most models and means were showing yesterday. Earlier in the
forecast period, models are becoming better defined and with a
farther north track for a low pressure center now expected to
track from the North Pacific through the southeastern Bering Sea
and into the western mainland from Thu into early Sat.
Due to dataflow problems preventing use of the 12Z ECMWF, today's
forecast used input from the 00Z ECMWF along with the 12Z
GFS/CMC/UKMET and 12Z GEFS/00Z ECMWF means. For the concentrated
low pressure initially moving northward over the Bering Sea there
is still some divergence for track. Latest GFS runs have pulled
back toward the means while other models to varying degrees are
still farther east. The UKMET was a fast extreme. Meanwhile the
operational models are trending steadily better defined and
clustered for the North Pacific into western mainland system. The
00Z ECMWF was the fast extreme with this feature. Based on
guidance comparisons for these two features the first half of the
period emphasized the 12Z GFS/CMC somewhat more than the 00Z
ECMWF/12Z UKMET. The 12Z ECMWF changed only modestly for the
Bering low but trended favorably slower for the second system.
There is better continuity for a wave evolving near the southern
coast by early Sat but consensus has slowed its southeastward
departure by perhaps half a day. Agreement/continuity are fairly
good in principle for low pressure emerging into the western
Bering Sea during the weekend. The average of guidance seems to
be taking slightly longer to develop it though (12Z ECMWF slower
than model/mean consensus). The less amplified trend for flow
aloft now allows the surface low and its associated front to
progress eastward across the Bering and Aleutians from the weekend
into the start of next week. There are signals that a frontal
wave could reach or develop near the eastern Aleutians/Alaska
Peninsula by next Mon but of course this has very low confidence
in light of recent variability with the larger scale flow. The
latter half of the forecast used increasing weight of the ensemble
means, reaching 60-70 percent total by day 8 Mon.
...Weather/Hazards Highlights...
After midweek the combination of flow ahead of the initial system
moving northward over the Bering Sea and then the North Pacific
system currently expected to track northeastward into the western
mainland Thu-Sat will likely spread a broad area of precipitation
from the Alaska Peninsula through the Kenai Peninsula and
northward across the western mainland. Locally moderate to heavy
amounts will be possible over favored terrain. A modest wave
forming near the southern coast and then tracking southeast should
also focus precipitation (some at least moderate) over the
southeastern coast and Panhandle, most likely around Fri-Sat.
Expect lighter and more scattered precipitation to progress across
much of the mainland during the weekend as shortwave energy aloft
passes through. The system forecast to reach the western Bering
Sea by this weekend should bring an increase of precipitation and
winds to the western Aleutians and then continued eastward
progression across the Bering Sea/Aleutians into the start of next
week. The leading edge of this moisture may reach the western
mainland by next Mon but with low confidence.
Late this week anticipate above to much above normal temperatures
over most of the state, with the most extreme anomalies likely to
be over the western half of the mainland as southerly or
southeasterly low level flow prevails. Passage of upper dynamics
during the weekend should promote a gradual cooling trend, with
Southcentral possibly seeing some areas of below normal readings
from the weekend into early next week and portions of the
Panhandle trending below normal as well. The northwestern half of
the mainland should remain above normal but with less extreme
anomalies.
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