Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Wed May 03 2023
Valid 12Z Sun May 07 2023 - 12Z Thu May 11 2023
...Overview...
Recent guidance still shows some notable differences and trending,
but some prevailing themes for the general pattern have carried
through into today's runs. In particular by the middle of next
week the majority clustering continues to have flow settling into
a regime consisting of a persistent Canadian ridge extending
somewhat into the mainland while troughing anchored by an upper
low becomes centered just south of the Alaska Peninsula. This
evolution will likely increase the amount of moisture reaching
some areas from the Alaska Peninsula through the Panhandle, with
the distribution and amounts very sensitive to the exact direction
of flow between the upper low and Canadian ridge. Specifics are
not yet well resolved in the guidance though. Recent trends have
been faster and better defined for the Sunday-Tuesday southern
Bering Sea/Aleutians storm that should lead into the mid-late week
pattern, while a lot of smaller scale detail uncertainty persists
for upper details over the mainland between when one
trough/weakness is likely over the western mainland on Sunday and
another one develops near the western coast by midweek.
...Guidance Evaluation/Predictability Assessment...
Based on guidance through arrival of the operational 12Z ECMWF
run, an average among the 12Z ECMWF/UKMET (the latter ending 12Z
Tuesday) and 12Z GEFS/00Z ECMWF means provided the best
intermediate track and timing for the system expected to progress
across the southern Bering Sea and settle into a position south of
the Alaska Peninsula by midweek, while reflecting the faster trend
seen in most guidance. Latest GFS runs have been on the slow side
but the new 18Z GFS has nudged a little faster and actually
catches up to the majority by day 8 Thursday. Meanwhile latest
CMC runs have been a bit on the southern side of the spread, along
the Aleutians, though the 12Z CMC mean is close and such a track
is still within the realm of possibility given typical guidance
errors several days out in time. The new 12Z ECMWF mean has
adjusted a little northward across the southern Bering and is
consistent though slightly deeper thereafter, yielding good
agreement with the preferred forecast blend in principle for most
of the period.
By the latter half of the period an important discrepancy arises
regarding the direction of flow between the upper low expected to
position itself south of the Alaska Peninsula and the ridge over
Canada. In addition to differences with shape/location of the
core of the upper trough/low, latest CMC runs have been directing
south-southwesterly flow into northwestern North America in
contrast to most other guidance that extends Canadian ridging more
into the mainland to varying degrees. The 12Z CMC mean is
somewhat of a compromise. Thus the CMC and CMC ensembles indicate
higher precipitation potential along the southeastern coast and
Panhandle than most other solutions, though also showing an
eastern Kenai Peninsula/Alaska Peninsula focus that would result
from the majority scenario that has some easterly component in the
deep-layer flow. ECMWF runs have occasionally been producing
significant totals over this area (including the current 12Z
version) while GFS runs have generally been out of sync with
various surface details or else not sufficiently matched up in
terms of gradient/moisture, leading to lighter amounts.
Predictability is low for some flow details over the mainland from
the weekend into the first part of next week, given the medium to
smaller scale of features involved. There is still a lot of
spread and run-to-run variability with the combination of the
initial ridge over the northern/eastern Bering Sea and upper
trough/weakness over the western mainland. The non-CMC majority
suggests the Bering Sea ridge should continue into the mainland to
break the initial trough in some fashion before another weakness
forms along the western coast of the mainland.
Based on the latest array of guidance, today's forecast started
with a blend of 70 percent models (12Z ECMWF/UKMET/GFS in order
from more to less weight) and 30 percent total means (12Z GEFS/00Z
ECens) for the first half of the period and then trended to 50-60
percent means with the rest 12Z ECMWF/GFS (still more ECMWF
weight). While the GFS had one of the less appealing Bering Sea
solutions, more acceptable comparisons over the mainland merited a
small weight in the blend overall.
...Weather/Hazard Highlights...
The most organized weather of note will be with the strong system
forecast to track across the southern Bering Sea from late weekend
into early next week and then in somewhat weaker form settle to a
position south of the Alaska Peninsula. Earlier in the period
expect this storm to produce a broad area of rainfall across the
Bering Sea and Aleutians along with brisk to strong winds.
Moisture may reach into the Alaska Peninsula by Monday as well.
Based on the more likely scenario currently offered by guidance,
by midweek there should be some easterly component of flow that
would provide precipitation enhancement along favored terrain of
the Kenai Peninsula and Alaska Peninsula. However the magnitude
of this focus is very uncertain at this time. There is also a
lower probability solution that requires monitoring, which
involves winds becoming more southerly and spreading more
precipitation into the southeastern coast and Panhandle. These
latter areas could still see some lighter activity within the
broad flow ahead of the main low pressure, depending on
low-predictability shortwave details. Confidence is also not
great for specifics of areas of precipitation across southern and
eastern areas of the mainland early in the period, ahead of an
initial western mainland upper trough/weakness whose energy may
evolve in various ways after early Sunday. The state should see a
mix of above or below normal temperature anomalies for daytime
highs, likely leaning toward more coverage of below normal
readings. On the other hand above normal morning lows may be more
common, with best potential for some below normal lows to be over
parts of the west and interior.
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 outlook 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