Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EST Mon Jan 18 2021
Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021
...Overview and Guidance/Uncertainty Assessment...
The majority of today's guidance shows a mean ridge aloft
prevailing over the northeastern Pacific and Mainland Alaska with
a couple shortwaves most likely splitting as they head into the
mean ridge. Models and means continue to refine/adjust their
forecasts for individual features but the continuity for the
overall pattern is generally somewhat better than was the case
during the latter half of the period 24 hours ago.
The first system of interest will track northeastward from the
eastern Aleutians/southeastern Bering Sea early Fri onward. An
operational model consensus continues to lead the ensemble means
for this system. Among the means the GEFS was earliest to improve
its definition while it has taken until the new 12Z run of the
ECMWF mean to start reflecting this feature. Even among the
operational models there are detail uncertainties that will likely
take into the short range to be resolved, given the importance of
small scale shortwave details that have low predictability until a
shorter time frame. Latest solutions hint at the system splitting
into two waves, one that lifts quickly northward and a second
slower/southward one that tracks toward the southwestern mainland.
The 12Z GFS is a deep extreme with this southern wave. Recent
trends for a stronger downstream ridge aloft lead to a slower
adjustment for the supporting dynamics aloft and somewhat later
development and/or departure of a southern coast/Panhandle wave
during the weekend.
Farther upstream the majority of guidance has trended noticeably
stronger and somewhat slower (the latter trend continuing from
yesterday) for the system forecast to track into the western
Aleutians and then the Bering Sea during the weekend and early
next week. Recent ECMWF/CMC runs have displayed a more coherent
evolution and better comparison to the means versus the 06Z/12Z
GFS runs. Not surprisingly, the stronger trend for this storm
supports a somewhat stronger/sharper downstream ridge but the
ridge is still not as strong as most guidance was showing two days
ago. There is decent continuity with the leading front reaching
the eastern Bering/Aleutians by early Mon and then dissipating
(though with weak waviness still possible across the Alaska
Peninsula later Mon) as the upper support weakens/splits to the
north and southeast.
Finally, there has been a signal in the guidance for another storm
to emerge from the western Pacific during the first half of next
week but with significant differences in track/timing. The 00Z
ECMWF, 12Z CMC, and 06Z/12Z GFS runs cluster well for a day 8
forecast with arrival into the Aleutians/Bering Sea by next Tue.
Some ensemble means have been hinting at this system as well but
have been eager to merge it with the preceding system. Latest
GEFS means are the closest in principle to the aforementioned
operational cluster. After being ill-defined in the 00Z run, the
new 12Z ECMWF mean has trended toward this idea as well. The 12Z
ECMWF is slower than other solutions.
During the first half of the period a 12Z operational model blend
reflects the most common ideas of guidance while downplaying less
confident details of any particular solution. Then the forecast
adjusts to a model/mean blend that maintains some CMC input, split
ECMWF input between the last two runs and eventually more toward
the old 00Z version, and uses a little more 12Z GEFS versus 00Z
ECMWF mean that was the latest run available at preparation time.
...Weather/Hazards Highlights...
The wave(s) tracking northeastward from the eastern
Aleutians/southeastern Bering Sea late this week will bring
meaningful precipitation to locations from the Alaska Peninsula
through the Kenai Peninsula and northward over the western
mainland. Expect some locally moderate to heavy amounts over
favored terrain. Then a wave forecast to develop along the
southern coast and track into the Panhandle during the weekend
should extend precipitation into the southeastern coast/Panhandle.
There is a fair amount of spread for precip intensity with this
wave but at least some moderate totals appear likely. Meanwhile
precipitation should gradually become lighter and more scattered
with time over the mainland. The Aleutians should see an episode
of precipitation/stronger winds during the weekend as low pressure
tracks into the western Aleutians/Bering Sea and the leading front
extends out to the east. Some moisture from this system may reach
into the southwest corner of the mainland/Alaska Peninsula by
early next week but likely not extend any farther eastward.
Another system forecast to track into the Aleutians/Bering Sea by
next Tue may bring another period of enhanced precipitation/winds.
The majority of the state will see above to much above normal
temperatures late this week/early weekend, with the most extreme
anomalies likely to be over northern and western areas. Then
expect a cooling trend from late weekend into next week. The
Panhandle may see some below normal readings as early as late this
week/weekend while parts of Southcentral could drop below normal
by Sun onward. Other parts of the eastern mainland could decline
to below normal readings by next Tue.
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
No significant hazards are expected over Alaska during this
forecast period.
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