Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Thu Apr 29 2021
Valid 12Z Mon May 03 2021 - 12Z Fri May 07 2021
...Overview and Guidance Evaluation...
Latest models and ensembles have maintained a similar pattern for
the extended forecast period but with continued uncertainty for
some details. Behind a weakening wave that should reach a little
south of the Gulf of Alaska early in the week, continuity is
initially good for the upper low likely to drop south or southeast
from the southeastern Bering Sea. Then solutions diverge somewhat
compared to yesterday with respect to the path of the upper low
and associated Northeast Pacific surface development. Farther
north the consensus maintains moderate mean ridging aloft from
western Canada into the mainland. Some degree of mean troughing
should prevail over the Arctic, and after Monday, perhaps down
along parts of the mainland's western coast. Meanwhile guidance
continues to show a general area of low pressure (likely
consisting of multiple waves) that should track near the Aleutians
mid-late week but with persistent spread for track and evolution
details.
A composite of 12Z operational model guidance provides a
reasonable depiction of significant features early in the week.
Then from about late Tuesday onward the 06Z-12Z GFS/GEFS and 12Z
UKMET hold the descending Bering upper low farther west, with the
GFS/GEFS ultimately tracking the feature and associated surface
development well northwestward of continuity as well as recent
runs of the ECMWF/CMC and their ensemble means. This difference
may be due in part to the GFS/GEFS/UKMET being on the faster and
more suppressed side of the spread for the system that may track
near the Aleutians. Prefer to lean away from the GFS/GEFS
scenario over the northeastern Pacific until there is more
pronounced trending in that direction.
An additional day of guidance has not really improved guidance
clustering for the Aleutians system. There are significant
differences for details of the parent low (ECMWF runs deepest and
farthest north with a concentrated low tracking near the far
western Aleutians Tuesday-Wednesday) and beyond that, models have
various ideas for how one or more frontal waves may evolve to
become the dominant low as the overall system continues along. In
addition to the GFS/UKMET attributes, late in the period the 12Z
ECMWF holds back its main low farther west than the remaining
majority. Amid the operational detail spread, the ensemble means
have been more stable with their typically more consolidated
representation of the system--providing some degree of confidence
in the overall system even if details remain uncertain.
For the latter half of the week, preferences to maintain a
model/ensemble compromise for the Aleutians system and stay closer
to continuity (or at most a very slight northwest nudge) for the
northeastern Pacific system lead to transitioning the early-week
blend toward an even model/ensemble mean weight. This blend uses
more 12Z CMC/ECMWF relative to the GFS on the operational side and
more 00Z ECMWF mean relative to the 12Z GEFS mean among the
ensembles. The new 12Z ECMWF mean offers reasonable support for
this approach.
...Sensible Weather...
The upper low and surface reflection dropping southward from the
southeastern Bering Sea early in the week will initially focus
precipitation over areas from the eastern Aleutians into the
Alaska Peninsula. The Peninsula may see some degree of
enhancement for a time. A portion of this moisture may reach the
southern coast. At the same time a leading weakening system may
bring a modest amount of precipitation into the Panhandle. Much
of the mainland will be mostly dry aside from perhaps some
light/scattered activity. It remains to be seen if any moisture
from mid-late week Northeast Pacific development lifts far enough
north to reach the Gulf and vicinity. Currently the more likely
scenario is for most of it to stay to the south. Best consensus
of guidance still has a broad area of precipitation progressing
from west to east across the Aleutians and surrounding areas
during the week as one or more surface waves and associated fronts
progress eastward. This area of moisture may reach as far
eastward as the Alaska Peninsula by the end of next week.
Confidence remains low for the system's track/evolution details
though, with the southern-most minority scenario suppressed enough
to keep the Aleutians mostly dry.
The state will see various areas of above or below normal readings
for daytime highs. The Panhandle and parts of the southern
mainland are most likely to be below normal, as is the case for
portions of the northern third of the mainland. Central latitudes
will see the best opportunity for moderately above normal highs.
Warmer anomalies for morning lows should yield mostly above normal
readings. Localized pockets of below normal min temperatures may
include areas near Kotzebue Sound, parts of the northeastern
mainland, and in the Panhandle/far southeastern 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