Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Wed Apr 28 2021
Valid 12Z Sun May 02 2021 - 12Z Thu May 06 2021
...Overview and Guidance Evaluation...
The best consensus of latest guidance shows a leading shortwave
and surface system tracking south of the Alaska Peninsula and
eventually weakening after they reach near the southern Gulf of
Alaska. Behind this feature, expect a fairly deep upper low
initially over the Bering Sea (with its own surface system) to
drop southeastward and energize a mid-latitude Pacific wave that
should reach a ways south of Haida Gwaii by early next Thursday.
Models/ensembles are more ambiguous with details from the mainland
into the Arctic but with a tendency for upper ridging from western
Canada into the mainland and possibly a mean low aloft over high
latitudes of the Arctic. Meanwhile low pressure emerging over the
western Pacific and its leading frontal system will likely affect
areas from the North Pacific into the Bering Sea after Monday.
This pattern will focus most precipitation from the Aleutians
through the Alaska Peninsula, and perhaps the southern coast and
Panhandle for a period of time.
A composite of latest guidance accounts for remaining detail
differences for the leading wave that tracks south of the Alaska
Peninsula and eventually weakens. Guidance is coming into better
agreement for the ultimate evolution/track of the Bering Sea upper
low, with a strong consensus now dropping most if not all of the
upper low's energy southeastward into the Pacific. Prior
model/ensemble runs had varied between keeping it over the Bering
(especially some earlier ECMWF/ECMWF mean runs that had been
fairly far north) and somewhat farther south/southeast motion or
interaction with additional northern stream flow. At the moment
only the 12Z CMC keeps a greater proportion of upper low energy
over the Bering after early Tuesday. Otherwise comparisons show
that the 12Z ECMWF pulls the upper low farther south than other
solutions by the latter half of the period. For the system
tracking into the Aleutians or vicinity, models and ensembles show
a fair amount of north-south track spread (12Z ECMWF farthest
north) and differences in evolution but overall are providing a
consistent signal for the system in general.
Based on guidance comparisons, the part of the starting blend
focused on the 12Z operational runs and with more ECMWF/GFS weight
relative to the UKMET/CMC. Lower confidence in the aforementioned
more extreme attributes of the 12Z ECMWF led to shifting ECMWF
input a little more toward the prior 00Z run by mid-late period.
Also the 12Z GEFS/00Z ECMWF means provided a reasonable component
of a multi-guidance compromise for primary features later in the
period (half models/half means by day 8 Thursday). The newly
arrived 12Z ECMWF mean provides reasonable support for the
preferred blend.
...Sensible Weather...
From late weekend into early next week expect Bering Sea low
pressure and its associated frontal system to focus precipitation
over and just north of the Alaska Peninsula. There may be some
localized enhancement within this area of precipitation. Precise
evolution of this system and the leading wave weakening over or
near the southern Gulf will determine how much moisture may reach
Kodiak Island and the southern coast. Also the flow ahead of the
leading wave may bring a period of mostly light to moderate
precipitation into portions of the Panhandle. The eastern
Aleutians may see some light precipitation on the back side of the
initial Bering Sea system. Then from Monday night or Tuesday
onward the western Pacific system coming into the picture will
likely spread precipitation eastward across a broad area from the
North Pacific into the Bering Sea. There may be some areas of
enhanced winds as well. However confidence in system details is
fairly low at this time so it will take additional time to resolve
specifics of precipitation coverage and totals as well as winds.
Southern parts of the mainland along with the Panhandle will
likely see below normal highs for most of the period, with other
areas seeing a mix of above/below normal readings. Anomalies for
morning lows should be somewhat higher, leading to greater
coverage of above normal min temperatures and more localized
pockets of below normal values--mostly in parts of the
north-central mainland, Southcentral, and Panhandle.
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