Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
645 PM EST Tue Mar 09 2021
Valid 12Z Sat Mar 13 2021 - 12Z Wed Mar 17 2021
...Overview and Guidance/Uncertainty Assessment...
Today's guidance provides added trends toward a more blocky
pattern with time. Expect a sharpening upper ridge reaching the
mainland by the start of the weekend to split off a closed high
that remains in the Arctic for a while next week. At the same
time an elongated closed low over the northeastern Pacific will
likely approach the Panhandle and then lift northward before
either departing or shearing out underneath the upper high.
Meanwhile an upper trough that crosses the Bering Sea and
Aleutians during the weekend should elongate across the eastern
Bering/Alaska Peninsula by early Monday, possibly forming a closed
low soon thereafter. Then digging flow on the western side of the
overall trough should pull the upper low farther south into the
Pacific by the middle of next week.
From the weekend into the start of next week, the 12Z GFS/CMC and
12Z/00Z ECMWF provided the best clustering in principle for
meaningful large scale features. Confidence is below average for
the specifics of surface low pressure that may track near or into
the southern Panhandle ahead of the Northeast Pacific upper low,
given significant detail spread in latest model runs and the
relatively small scale of the system. This favors an intermediate
approach for now. Farther west there is decent agreement among
most models/means that a strong system over the far northwestern
Bering Sea will anchor a front that crosses the eastern Bering,
with a triple point wave likely breaking off near the Alaska
Peninsula around Sunday into early Monday.
By days 7-8 Tuesday-Wednesday the ensemble means have been fairly
consistent in advertising some troughing from the mainland into
the Pacific but with ambiguity in the specifics. Operational
GFS/ECMWF runs fit within that general template though
unsurprisingly with greater spread for various aspects of one or
more streams. The 12Z model/mean cycle suggests a more coherent
trend/clustering toward having an upper low drop south or
southeast of the Alaska Peninsula by midweek while a weakness
persists over the mainland. Individual models still vary with the
ultimate evolution/track of the wave that forms near the Alaska
Peninsula as well as one or more other surface lows. Thus by this
time frame the forecast blend incorporated some 12Z GEFS/00Z ECMWF
mean input. As for operational solutions, both ECMWF runs were
reasonable blend components through the end of the period. The
12Z GFS became flatter than consensus with North Pacific flow by
next Wednesday, favoring introduction of the parallel GFS to some
degree. Finally the blend phased out the CMC late due to northern
stream details beginning to differ from consensus.
...Weather/Hazards Highlights...
The developing system that may track into or near the southern
Panhandle this weekend may bring some moderate to heavy
precipitation to locations in its vicinity. At this time
confidence is lower than desired for exact details of this system.
Meanwhile a front crossing the eastern Bering Sea and Aleutians
early in the weekend will be accompanied by brisk to strong winds
and a precipitation shield that should extend into the far western
mainland. Then a triple point wave near the Alaska Peninsula
should bring more precipitation focus to the northern Peninsula
and southwestern mainland while the leading front will likely
extend moisture into the southeastern coast and Panhandle. Some
precipitation could linger along the southern coast through the
first half of the week depending on the specifics of the
aforementioned wave and one or more other lows. Expect unsettled
conditions to persist over the Aleutians for a number of days.
The North Slope will see above to well above normal temperatures
through the period with highest anomalies most likely around
Sunday-Monday. Farther south the weekend will start out on the
cold side with below to well below normal readings except for
perhaps far western locations. After Saturday there will be a
warming trend from west to east as high pressure departs into
Canada. By Monday most below normal temperatures should be
confined to parts of Southcentral as well as the Panhandle which
should be colder than average through the forecast period.
Rausch
Hazards:
- Heavy snow across portions of the Alaska Panhandle, Fri-Sat, Mar
12-Mar 13.
- High winds across portions of the Aleutians, Sat, Mar 13.
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