Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
640 PM EST Sat Feb 01 2020
Valid 12Z Wed Feb 05 2020 - 12Z Sun Feb 09 2020
...Overview...
If anything, the forecast at higher latitudes has become more
divergent over the past day regarding the ultimate evolution/path
of the Arctic upper low that should descend to a position just
north or northwest of the mainland by the start of the period
early Wed. Differences with this feature lead to a wide spread
for southward extent of colder/drier air over the mainland.
During mid-late week guidance still displays a progressive pattern
across the North Pacific and vicinity with one system reaching
near the southeastern coast/Panhandle early Wed and an upstream
Wed-Thu Aleutians system most likely weakening thereafter. From
Thu onward the issues with higher latitude flow affect the
latitude of the second system's track. Models/ensembles remain
agreeable with the general idea of fairly strong low pressure
tracking into the Aleutians/Bering Sea Fri-Sat with steady
weakening during the weekend as upper dynamics open up. Ahead of
this system expect a downstream ridge aloft to build into the
northeastern Pacific and mainland Alaska/northwestern Canada.
...Guidance Evaluation...
Regarding the evolution of the initial Arctic upper low and
surrounding trough, GFS/GEFS mean runs have been consistently on
the eastern side of the full guidance envelope (in current runs,
over the northwestern mainland) while the past couple ECMWF/ECMWF
mean runs have jumped well westward of continuity and consensus
with retrogression over Siberia. A number of CMC runs had been on
the western side of the spread but the latest 12Z run along with
the UKMET now define the middle of the envelope by early day 5
Thu. The wide solution spread, along with support from the 12Z
CMC mean, argue for maintaining a middle-ground solution among the
eastern GFS troughing aloft and greatest southward extent of
cold/dry air versus western ECMWF that promotes a greater expanse
of warmer air and moisture.
Farther south there is still a suggestion of multiple surface lows
possible from the Bering Sea into the mainland early in the period
with the small scale of important details leading to low
predictability/confidence. For this general area of low pressure
and the system initially near the southeastern coast/Panhandle, a
model blend looks reasonable. Continuity is good for at least the
existence/definition of the Wed-Thu Aleutians system but the
differences with higher latitude flow play an increasing role in
the system's track after Wed. Latest GFS runs are farthest south,
south of the Alaska Peninsula, while most other models/means are
at least as far north as the Peninsula. Combination of an
intermediate solution with flow to the north along with upper
support that may be a little stronger than depicted in the means
could support a low track a bit farther north than the GEFS/CMC
means and closer to the CMC.
The fairly strong system reaching the Aleutians/Bering Sea by
Fri-Sat has good agreement and continuity in concept while models
and ensemble means still form two general clusters--models
developing a frontal wave ahead of the parent low and the ensemble
means keeping the system more consolidated though at times with
some elongation in a faint nod to the operational model scenario.
Continue to favor a model-mean blend with somewhat more model
weight into early day 7 Sat. After that time specifics become
more uncertain, leading to a little more ensemble mean weight as
the system weakens in response to the upper support likely opening
up.
Early in the period a blend among the 12Z ECMWF/GFS/UKMET/CMC
followed by manual adjustments where appropriate provided the
desired centered solution for flow over/near the northern mainland
and yielded the preferred attributes for features over the Bering
and Aleutians/North Pacific. The forecast started to incorporate
a little 12Z GEFS/00Z ECMWF mean on day 6 Fri with the means
comprising 40-60 percent of the blend by days 7-8 Sat-Sun.
...Weather Highlights...
Highest precipitation totals for the full five-day period should
be along the southern coast and Panhandle. Most activity will be
light-moderate over the southern coast/Panhandle early-mid period
as well as with the system tracking near the Aleutians Wed-Thu and
weakening thereafter. During the late-week/weekend time frame the
flow ahead of Aleutians/Bering Sea low pressure may bring a brief
period of enhanced precipitation to the eastern Aleutians/Alaska
Peninsula with some moisture eventually extending eastward.
During mid-late week expect below normal temperatures over
northern and western parts of the mainland with above normal
readings likely to prevail over the south and east, followed by a
trend toward greater coverage of above normal readings next
weekend. A front draped over the south-central mainland,
beginning to lift northward as a warm front during the weekend,
may provide some focus for precipitation. Confidence is low for
precise coverage of precipitation and southward extent of cold air
over the mainland due to the wide spread of solutions for the
upper low initially over the Arctic.
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