Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Thu Apr 30 2020
Valid 12Z Mon May 04 2020 - 12Z Fri May 08 2020
...Guidance/Predictability Assessment...
The past couple days or so of GEFS/ECMWF/CMC ensemble mean runs
have generally been more agreeable and consistent than average
with the pattern evolution over most of the forecast domain. They
continue to show the upper high initially over eastern Siberia and
the mainland drifting eastward and likely merging with another
ridge over western Canada, with a mean ridge persisting from
western Canada through the mainland for the rest of the period.
At the same time a long-term mean low (with multiple embedded
centers) should persist well south of the mainland. The northern
side of this circulation may bring one or more periods of
precipitation to areas from the Panhandle to Alaska Peninsula.
The ensemble mean consensus depicts a col region over portions of
the Bering Sea during the period and a mean trough emerging over
the western Pacific with downstream ridge extending into the
Aleutians.
In contrast to the agreement/continuity of the means, operational
model guidance has become more inconsistent and varied across a
broad area covering the North Pacific and Bering Sea. This is
particularly true of the operational ECMWF, whose 00Z run brought
a system from near Kamchatka into the central Bering Sea (an idea
most other guidance had abandoned previously) and 12Z run was fast
to bring a strongly intensifying western Pacific storm into the
Bering. Neither solution compared very well to the ensemble
means. In addition ECMWF runs have been placing emphasis on
different systems than consensus over the Northeast Pacific. The
most prominent difference in the 12Z run was its southeastward
diversion/delayed development associated with the energy/surface
wave over the western Aleutians around day 4 Mon. Most remaining
models and means suggested that this energy should consolidate
into low pressure near 50N 150W by the latter half of the period.
The 12Z GFS/CMC are somewhat south or west of the means but their
overall evolution is closer to the means in principle. It is
still plausible for other low centers to exist over the Northeast
Pacific though.
Meanwhile operational runs have also varied with specifics of
upper ridging over/near the mainland and possible embedded or
surrounding shortwave impulses, though a blend has tended to
provide a more stable solution with a reasonable comparison to the
means. The 12Z CMC has continued its recent tendency to be more
aggressive than consensus to amplify one or more Arctic troughs
into the mainland and/or northwestern Canada.
Based on today's array of guidance the forecast blend started with
a 12Z operational model blend for day 4 Mon and was able to
maintain mostly operational model input through early Tue.
However the increasing model spread required a fairly rapid
increase of the 12Z GEFS/00Z ECMWF means thereafter, to 50 percent
by day 6 Wed and 80 percent for days 7-8 Thu-Fri. Some aspects of
the GFS were close enough to the means to allow holding onto a
modest weight through late week.
...Weather/Threats Highlights...
A leading wave likely to track south of the Alaska Peninsula by
Tue should bring a period of precipitation to areas from the
southern coast to the Peninsula during the first half of the week,
while the Panhandle should see a drier trend. Some localized
enhancement is possible over eastward-facing terrain. By the
latter half of the week there is a signal toward some increase of
moisture reaching at least a portion of the region spanning the
Panhandle to Alaska Peninsula. However so far the
models/ensembles have provided very mixed messages on the timing,
location, and intensity of precipitation so confidence remains low
for details at this time. Uncertainties over specifics from the
western Pacific into the Bering Sea/Aleutians also lower
confidence in the details of any precipitation over the Aleutians.
Above normal temperatures should be most prevalent over the
southwestern mainland while below normal readings are more likely
over northern and eastern areas. Most of the state should see a
gradual warming trend, leading to expanding coverage of above
normal anomalies with time.
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