Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
750 PM EDT Wed May 19 2021
Valid 12Z Sun May 23 2021 - 12Z Thu May 27 2021
...Overview...
Guidance still shows considerable spread and run-to-run
variability over how the more blocky regime aloft will evolve,
with some significant differences for embedded smaller-scale
details as well. The established consensus per recent ensemble
means would still have an Aleutians/Bering Sea upper low with
associated surface system drifting eastward with time while
ridging builds over a decent portion of the mainland. An
alternative scenario would keep the upper low farther westward
with differing shapes/positions of upper ridging to the north and
east. What happens to short-range energy as it reaches the
mainland and vicinity by the start of the period early Sunday is
also up for debate.
...Guidance/Predictability Assessment...
What happens to shortwave/upper low energy passing just north of
Kamchatka around Wednesday-early Thursday does not appear any
closer to being resolved than it was yesterday. The notable
guidance shift today is that the 12Z CMC/UKMET have adjusted
closer to the persistent GFS idea of dropping the larger
proportion of this feature southeastward into the Gulf to merge
with or replace the leading upper low/trough over that area--in
contrast to latest ECMWF runs that have a compact upper low
drifting over the mainland during the first half of next week.
There is an area of potential weakness over the northern/eastern
mainland, between a ridge to the northeast of the mainland and the
ridge building into the western mainland, so cannot discount the
possibility of some energy getting stuck within that area.
However the consensus evolution of rising heights aloft over most
of the mainland still recommends leaning away from the deep side
of the upper low envelope regardless of the model (was the CMC 24
hours ago). Also of note, a more non-ECMWF handling of the energy
leads to a somewhat deeper/closed and slightly slower upper
feature nearing the Panhandle by Sunday.
Farther west, an operational model blend provides a reasonable
depiction of detail for the Aleutians/Bering Sea system during
days 4-5 Sunday-Monday. By Monday this consensus is a bit south
of continuity and the ensemble means. After that time operational
runs begin to diverge with the upper low and character of the
upper ridge to the north/east, as well as with details of Pacific
flow. In varying ways both ECMWF and GFS runs have been erratic
and at times very divergent from the ensemble means. Thus prefer
to maintain the best continuity possible by trending the forecast
more toward the ensemble means later in the period with lingering
operational input from solutions that are closest in principle--in
today's case the 12Z ECMWF and CMC.
From Sunday into Monday the initial blend started with even weight
of the 12Z GFS/ECMWF/CMC/UKMET. This helped to temper the
lower-confidence mainland upper low in the ECMWF. The resulting
blend actually provided fairly good continuity for the Arctic
surface low and trailing front extending into the mainland as of
early Sunday. Day 5 Tuesday served as the transition toward days
7-8 Wednesday-Thursday that started with about two-thirds total
12Z GEFS/00Z ECMWF means relative to the 12Z ECMWF/CMC. Arrival
of the new 12Z ECMWF mean lowers confidence a little more as it
retrogrades the Bering upper low in a manner similar to the 00Z
operational run (and in contrast to multiple previous ensemble
runs). Also it trends a bit weaker with the mainland upper ridge.
...Weather/Threats Highlights...
The evolution and path continue to be uncertain for shortwave
energy that may be over the mainland from Sunday into next week.
Currently the most likely scenario would have a weak form of this
energy supporting the potential for some scattered precipitation
through Sunday and possibly into Monday over eastern areas. The
Panhandle may see light precipitation continue into Sunday as a
nearby system weakens. Expect Bering Sea/Aleutians low pressure
and a potentially wavy leading frontal system to spread one or
more organized areas of rain across the Aleutians, likely into the
Alaska Peninsula, and possibly reaching as far east as the
southern coast/Panhandle late in the period. Confidence is still
low over the precise eastward extent and magnitude of
precipitation, considering for example that guidance for the
Alaska Peninsula ranges from nearly dry (in the minority) to some
degree of front/wave-aided enhancement.
Below normal highs are likely to be most persistent over the
Panhandle and parts of the far southern mainland. Elsewhere there
should be a mix of above/below normal highs on Sunday followed by
a warmer trend that would increase coverage of above normal
readings, with highest anomalies over western/interior areas.
Uncertain details of upper level energy that may be over the
mainland could have some influence on high temperatures. Low
temperatures will also see a trend toward broader above normal
coverage with highest anomalies expected to be over western and
northern locations.
Rausch
Hazards:
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