Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
635 PM EST Sun Feb 02 2020
Valid 12Z Thu Feb 06 2020 - 12Z Mon Feb 10 2020
...Overview...
The best clustering of latest guidance shows an Arctic upper low
settling close to the northwest corner of the mainland late this
week before retrograding into eastern Siberia and then lifting
northward/northeastward in response to upstream flow. Within a
progressive North Pacific/Aleutians stream expect a midweek
Aleutians system to continue across the eastern Aleutians/Alaska
Peninsula late this week and then dissipate, followed by a
stronger system heading into the Aleutians/Bering Sea around
Fri-Sat. Ridging aloft that builds into the mainland and
northwestern Canada ahead of this system should support an
ultimate storm track near the western coast of the mainland. By
late in the period shortwave energy filtering through the upper
ridge may support a wave near the Panhandle by Mon, while
strengthening flow ahead of a developing western Pacific system
may overspread the Aleutians and Bering Sea.
...Guidance Evaluation...
Model/ensemble trends over the past day have yielded much improved
(though not yet ideal) agreement for the Arctic upper low expected
to be near the northwest corner of the mainland for at least the
first half of the period. After some recent erratic runs, the
late-week 12Z ECMWF forecast has arrived at a solution close to
the GFS--which itself has adjusted a bit westward from a number of
prior runs that had been on the eastern side of the spread. The
UKMET is fairly close while the CMC strays westward of the primary
model/ensemble cluster. By the latter half of the period the
overwhelming majority of guidance--including the 12Z ECMWF
mean--supports some retrogression of the upper low in contrast to
the 12Z ECMWF that keeps it over/near the northern mainland. The
relatively better agreement provides some increase in confidence
but the behavior of guidance thus far suggests an ongoing risk for
meaningful adjustment of consensus.
To the south clustering is improving for track of the mid-late
week Aleutians system, with some help from the reduction of spread
for flow to the north. Latest ensemble means depict a track over
Bristol Bay and into the extreme southwest mainland/eastern
Peninsula, most similar to the 12Z ECMWF. Through its 12Z run the
GFS continues to be on the southern side of the envelope, south of
the Peninsula, though the UKMET combines both ideas with a
consensus parent low but also a southern triple point wave. The
new 18Z GFS shows a trend toward consensus.
Guidance continues to offer varying details for North Pacific low
pressure extending into the Aleutians/Bering Sea by Fri-Sat.
Operational runs are still showing better defined development
versus the means for a leading frontal wave relative to the
western parent low, though the means do hint at this evolution.
The more eastern position of the Arctic upper low in the 12Z ECMWF
ultimately leads to a storm track through the central-eastern
mainland in contrast to consensus and continuity that maintain a
farther west track. This requires transitioning the ECMWF
component of the forecast blend from the 12Z run to the prior 00Z
run around mid-period. Also, including some ensemble mean input
mid-late period helps to tone down detail uncertainties and
maintain elements of continuity.
Latest ensemble means show better than average agreement for low
pressure developing over the western Pacific and possibly reaching
near the western Aleutians by early day 8 Mon. Operational runs
show various ideas with the GFS closest to the means in principle.
The forecast incorporated a 12Z multi-model blend for day 4 Thu
followed by exclusion of the CMC due to its less likely solution
for the Arctic upper low. The late Fri through Sat period used
both 12Z and 00Z ECMWF runs on the way to only the 00Z run being
used Sun-Mon. Some 12Z GFS input continued through day 8 Mon,
while total weight of the 12Z GEFS/00Z ECMWF means increased from
20 percent on day 5 Fri to 60 percent by days 7-8 Sun-Mon.
...Weather Highlights...
Continue to expect areas along the southern coast and Panhandle to
see the highest five-day precipitation totals, while meaningful
amounts are also likely over parts of the southern and western
mainland as well as across the Aleutians. Most late-week activity
should be in the light to moderate range. Flow around low
pressure reaching the Bering Sea by Sat may produce some
enhancement from the eastern Aleutians to southern coast and
Panhandle during the weekend. This system should also spread
moisture northward over portions of the mainland. With
considerable uncertainty in specifics, developing western Pacific
low pressure may bring an area of moisture and brisk winds into
parts of the Aleutians/Bering Sea by next Mon. Late this week a
front will separate below normal temperatures over northern and
western areas from above normal readings over the south/east, with
a brief southward push of colder air over western locations. The
system tracking into the Bering and possibly the western mainland
next weekend should push the front northward and promote an
expanding area of above normal temperatures across the mainland.
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