Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Fri Apr 30 2021
Valid 12Z Tue May 04 2021 - 12Z Sat May 08 2021
...Overview and Guidance Evaluation...
The best consensus of guidance maintains a similar theme from
recent days for the large scale pattern evolution. By the start
of the period a short-range Bering Sea upper low should reach just
south of the Alaska Peninsula, with this feature continuing
southeastward and helping to develop low pressure southwest of
Haida Gwaii by Wednesday-Thursday before weakening and departing.
Farther west expect a mean upper low to settle over the Aleutians
and/or southern Bering Sea by the latter half of the week. A
leading system (possibly multiple waves/redevelopment) will
establish this feature and then one or more trailing pieces of
energy and corresponding surface reflection(s) may merge into it
toward the end of the period. Still expect varying degrees of
mean ridging aloft to extend from western Canada into parts of the
mainland. One or more shortwaves could pass through the mainland
part of the ridge and separate impulses could exist for a time
near the western/northern mainland. Meaningful detail questions
persist within the more agreeable big-picture evolution though.
Models continue to vary with respect to the precise handling of
upper low energy that initially reaches just south of the Alaska
Peninsula. The established consensus/continuity maintain a far
enough southeastward pull of the strongest dynamics (albeit with
some energy possibly lingering farther north for a time) to keep
surface low pressure near 50N latitude. Meanwhile a minority
still offers potential for a farther north position for at least
some of the upper energy/surface low pressure. Today it is the
ECMWF that shows the latter possibility after some earlier
GFS/GEFS runs had been well north before subsequently trending
back to the more southern cluster. Latest GEFS/ECMWF/CMC ensemble
means all support the southern track.
For the leading system forecast to track near the Aleutians, the
most notable adjustment in the guidance is for some narrowing of
previous latitude differences--by way of trimming the southern
part of yesterday's spread that had been well south of the
Aleutians. There is still considerable uncertainty over evolution
details such as possible development of a frontal wave versus an
eastward wobble of the initial parent low. By mid-period the 12Z
GFS/CMC stray to the northern side of the spread so prefer to lean
away from those solutions mid-late period. A recent added
complication is a compact upper low that latest guidance is
suggesting may track over or near the far western Bering Sea.
This feature could have some influence on the system of interest.
Or it may interact with progressive shortwave energy emerging into
the western Pacific, with guidance signaling that this overall
bundle of energy may promote another system reaching near the
central Aleutians by day 8 next Saturday. Meanwhile the leading
system is likely to weaken by that time.
As is typically the case, the starting blend used operational
models early in the period but split ECMWF/GFS input between the
12Z runs and the prior 00Z run (ECMWF) and 06Z run (GFS) to
downplay less confident 12Z ECMWF specifics over the northeastern
Pacific and 12Z GFS details for the Aleutians system. A
transition toward an even model (06Z GFS and last two ECMWF runs)
and ensemble mean (12Z GEFS/00Z ECens) blend later in the period
represented consensus well with fairly modest adjustments in
continuity. This approach also toned down low-predictability
small scale features that may exist for a time near the
western/northern mainland. The new 12Z ECMWF mean provided no
meaningful deviations from the preferred solution.
...Sensible Weather...
Light to maybe locally moderate precipitation along the southern
coast and Panhandle on Tuesday should taper off by Wednesday.
Some of this moisture could combine with one or more
low-confidence shortwave impulses to produce scattered areas of
precipitation farther north over the mainland around mid-late
week. By the start of the period early Tuesday expect a front
with accompanying band of brisk to strong winds to push an area of
precipitation into the western Aleutians. Progression of one or
more surface waves and the overall frontal system should steadily
push this moisture eastward across the Aleutians, reaching the
Alaska Peninsula after midweek. Since yesterday confidence has
increased that the Aleutians will see the moisture from this
system, though precip amounts remain in question. There could be
some localized enhancement of winds and rainfall depending on
exactly how the system evolves. A trailing system that may reach
the central Aleutians by next Saturday would bring another episode
of precipitation but with lighter totals.
Temperature anomalies across the state are likely to maintain a
fairly stable pattern during the period. Central latitudes and
perhaps eventually areas along the northern coast should see above
normal highs while the southern mainland and Panhandle will tend
to be below normal. Expect a much larger percentage of the state
to see above normal lows, with some pockets of below normal min
temperatures possible over the Panhandle/far southeastern mainland
as well as near Kotzebue Sound and northeastern mainland.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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