Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
645 PM EDT Sat Jul 25 2020
Valid 12Z Wed Jul 29 2020 - 12Z Sun Aug 02 2020
...Overview and Guidance Evaluation...
The majority of latest guidance shows upper troughing with one or
more embedded lows over the Bering Sea while an elongating
east-west trough eventually reaches into the northeastern
Pacific/Gulf of Alaska. A downstream ridge should build into the
mainland and northwestern Canada, with this ridge gradually
helping to dislodge an initially deep upper low over the Arctic.
There are various uncertainties that persist for some specifics
but overall the models/means appear to be somewhat better
clustered than yesterday.
A blend of 12Z operational models (ECMWF/GFS/CMC/UKMET) represents
consensus well for significant features during the first half of
the period, from Wed into early Fri. Before this cycle there had
been considerable spread and variability for the deep Arctic upper
low, ranging between rapid east/northeast departure and slight
retrogression, so there are some reservations about whether the
current model/ensemble mean cluster (northeastward departure after
Wed-Thu) will be able to maintain continuity in successive runs.
Meanwhile the initial upper low moving northeast across the Bering
will support a surface low and associated frontal system. A wavy
front will likely persist over the northern/north-central
mainland, possibly becoming a little more diffuse after departure
of the Arctic upper low.
By the latter half of the period the primary uncertainty involves
details within and around the Bering Sea mean trough/low.
Specifically the models diverge regarding the evolution and timing
of energy that drops from Siberia across or near Kamchatka and
into the Bering, possibly interacting with the low already in
place there. Increasing model differences and fairly good
agreement among the means for the general pattern recommend a
transition toward 50-60 percent total weight of the 12Z GEFS/00Z
ECMWF means by days 7-8 Sat-Sun. By mid-late period the
operational ECMWF component of the blend splits its weight between
the 12Z run and prior 00Z run, as the new run strays a bit faster
than consensus to bring troughing toward the mainland while the
old run was a tad slow due to being stronger than consensus with
the mainland upper ridge. The new 12Z ECMWF mean provides added
support for a blend/intermediate approach.
Farther west the guidance signals potential for some degree of
surface development over and/or south of the Bering next weekend,
from a combination of initial Siberia energy and western Pacific
dynamics/surface wave that may become embedded within the larger
scale flow. At this time confidence is fairly low for the
specifics of this evolution.
...Weather Highlights/Threats...
The moisture shield ahead of midweek Bering Sea low pressure and
associated wavy front should bring some focused precipitation into
the Alaska Peninsula/southwestern mainland during the latter half
of this week. Specifics will depend in part on frontal wave
details. Some activity may persist over the southern mainland for
a time as the upper pattern evolves toward more elongated
east-west troughing over the Northeast Pacific/Gulf of Alaska.
The wavy front forecast to be draped across the northern mainland
may provide a focus for meaningful precip as well. Possible
surface development over/south of the Bering may bring another
area of moisture into the Aleutians next weekend. The North Slope
and Panhandle/extreme southeast, as well as more of the southern
mainland after midweek, have the best potential for below normal
high temperatures during the period. Locations in-between should
generally experience above normal highs. A broader area will
likely see above average low temperatures, with negative anomalies
more narrowly confined to the extreme north and over/near the
Panhandle.
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