Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
647 PM EDT Sun Jun 21 2020
Valid 12Z Thu Jun 25 2020 - 12Z Mon Jun 29 2020
...Overview and Guidance/Predictability Assessment...
In contrast to operational runs as described below, the latest
ensemble means provide a fairly agreeable and consistent depiction
of the expected large scale evolution during the period. They
show a deep Arctic upper low to the northwest of the mainland
anchoring a broad area of cyclonic flow covering portions of the
mainland, the Bering Sea, and Siberia. With some separation from
the Arctic flow, an upper trough aligned over the eastern Bering
Sea late this week should eject eastward after Fri. Meanwhile a
progressive North Pacific system will track just south of the
Alaska Peninsula and into the Gulf Thu into Fri and then an
evolving central Pacific upper trough with decent potential for an
embedded low will likely bring a system into the Aleutians by
around Sun-Mon.
From Thu into Fri a composite of the 12Z operational model
guidance offered a reasonable starting point with more definition
than the means. Note that the 12Z ECMWF/UKMET were closest to
yesterday's continuity for the system tracking into the Gulf late
this week and the initial blend was adjusted slightly to reflect
their depth for the system. By Sat-Mon nearly all of the 12Z
model runs significantly diverged from each other, their previous
runs, and the means for one or more significant features. This
required a rapid transition to 50-60 percent ensemble mean weight
(12Z GEFS/00Z ECens) while reverting operational model input to
the 06Z GFS and 00Z ECMWF.
For the Arctic upper low and surrounding flow the operational
guidance spread has become more extreme compared to previous days.
The 12Z ECMWF tracked the low well northward of previous run and
the means while the 12Z GFS pulled the low northward as well but
in contrast to other guidance also dropped a significant bundle of
energy into the Bering Sea--ultimately interacting with the
late-period central Pacific/Aleutians system. The 12Z CMC offered
yet another questionable idea, bringing the upper low center to
the northwestern coast of the mainland by the end of the period.
The ensemble means have drifted a bit westward versus 24 hours ago
and the 12Z ECMWF mean adjusted somewhat northward versus the 12Z
run but overall the means have thus far provided the most stable
forecast for the upper low and its broader circulation.
GFS/ECMWF runs have continued to show decent potential for the
evolving central Pacific upper trough to contain an embedded low
and support a well-defined surface system that tracks into the
Aleutians by days 7-8 Sun-Mon. CMC runs have tended to differ for
some details while seemingly underplaying the surface reflection
relative to depth of dynamics aloft at some valid times.
Multi-day trends for this system have generally been slower but
with some oscillation. Given the signal for a fair amount of
separation from flow to the north for a meaningful period of time,
it is a question mark whether the faster adjustment in the 12Z
ECMWF/ECMWF mean will hold. The 12Z GFS evolution over the Bering
differs enough from other guidance to question its specifics as
well. Still there is considerable uncertainty over the degree of
stream interaction toward the end of the period.
...Weather/Threats Highlights...
The system tracking just south of the Alaska Peninsula and into
the Gulf late this week should provide a brief period of
precipitation to the southern coast and likely with greater focus
to the Panhandle. Convection/rainfall over portions of the
mainland should be most common late this week when deepest upper
troughing aligns a little west of the mainland and before a front
draped across the North Slope retreats northward/dissipates.
Ejection of the initial Bering trough energy and rising heights
aloft in general should reduce precipitation coverage somewhat
during the weekend and early next week. Expect the system
forecast to track into the Aleutians late in the period to bring a
shield of moisture into/east of the region, though with ongoing
uncertainty over coverage/intensity/timing. The temperature
forecast is consistent in showing below normal highs over a
majority of the state with greatest negative anomalies over the
south/southeast/Panhandle as well as the northwest. Expect more
moderate anomalies for lows, including some areas of above normal
lows between the northeast and southwest.
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
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