Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EDT Tue Apr 28 2020
Valid 12Z Sat May 02 2020 - 12Z Wed May 06 2020
...Guidance/Predictability Assessment...
Most of today's guidance is fairly similar in principle.
Consensus for this weekend shows separate ridges aloft over
eastern Siberia/northwestern mainland and western Canada while a
weakness separates the two. By the first half of next week the
operational models differ on evolution of the Siberia ridge but
the ensemble means indicate that the two ridges should eventually
merge. Models/means have come into better focus in showing Arctic
flow to the north of the overall ridge bringing a front to around
the northern coast of the mainland by Mon-Tue. Meanwhile expect a
mean low to meander over the northeastern Pacific with one or more
embedded centers. The northern periphery of this circulation may
bring some precipitation to the Panhandle as well as the southern
coast and Peninsula. To the west a short-range Aleutians system
should become a part of the Northeast Pacific mean low and then a
system crossing Kamchatka should reach the Bering Sea by around
Tue.
The forecast blend started with a compromise among 12Z operational
models for this weekend to reflect consensus for significant
features while resolving lingering detail differences. Individual
model runs began to diverge more significantly for some specifics
after Sun. This led to a transition toward the 06Z GFS from the
12Z run, using a mix of 12Z and older 00Z runs for ECMWF input,
and increasing total weight of 12Z GEFS/00Z ECMWF means to 35-50
percent by days 7-8 Tue-Wed. The 12Z GFS broke off energy aloft
and an associated surface system over the western Aleutians on day
6 Mon while the remaining upper low back toward Kamchatka did not
really lead to much of a surface reflection until a western
Pacific wave tracked northeastward into the western Aleutians by
next Wed. Remaining guidance did not provide much support for the
12Z GFS scenario while the 06Z run compared better. ECMWF runs
showed various detail differences with decreasing confidence in
which run would be better at later valid times.
In addition to the increasing clarity with the Arctic front,
latest guidance has generally trended toward holding the eastern
Siberia upper high in place for a longer period of time and with
greater strength, before possibly lifting northeastward and
merging with the Canada ridge. General trends have been slower
for the system forecast to reach the Bering Sea early-mid week.
ECMWF runs have varied some with the evolution/timing of this
system but thus far have been much more consistent in principle
than GFS runs which have been very erratic with western Pacific
through Bering evolution in general. GEFS mean runs have trended
northward to consensus over the past day. Over the northeastern
Pacific the latest operational cluster is close to the ensemble
means which have provided the most stable solution for initial low
pressure expected to wobble back to the northwest during the
weekend and early next week. Models/individual ensembles quickly
diverge for this and other possible low centers after early Mon,
leading to decreasing confidence in specifics during the latter
half of the period.
...Weather/Threats Highlights...
The circulation around Northeast Pacific low pressure should
spread some precipitation across parts of the Panhandle and then
the southern coast and possibly into the Alaska Peninsula. There
may be some enhancement along and southwest of the Kenai Peninsula
depending on the surface low track during the first half of next
week. Lingering moisture from a short-range Aleutians system may
keep some light precipitation over the islands during the weekend
and then next week low pressure emerging into the Bering Sea and
its leading frontal system may bring some moisture into the
Aleutians. Temperatures should be fairly steady during the period
with a mix of positive/negative anomalies depending on location,
though with somewhat of a tendency toward below normal readings
over the Panhandle and eastern mainland and above normal over the
western mainland.
Rausch
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
Hazards:
No significant hazards are expected over Alaska during this
forecast period.
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