Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Sat Aug 10 2024
Valid 12Z Wed Aug 14 2024 - 12Z Sun Aug 18 2024
...Overview...
Most guidance develops an amplified mean trough aloft from the
Arctic through the Bering Sea and North Pacific by mid-late next
week, with the Bering Sea/North Pacific part of the trough likely
pushing gradually eastward with time and possibly reaching the
mainland by next weekend. Embedded dynamics should support a
leading well-defined system that tracks from the Aleutians
northeastward through the Bering Sea Wednesday onward, spreading a
broad area of precipitation and brisk/strong winds from the
Aleutians/Bering Sea into portions of the mainland. However,
confidence is not very high yet for important specifics of upper
feature interactions and the resulting surface system's exact
track and depth. Farther east, upper ridging initially building
into the mainland will progress into the Panhandle and
northwestern Canada, displacing a modest trough over the Panhandle
and vicinity as of Wednesday. Height falls aloft may the reach
the Panhandle next weekend.
...Guidance Evaluation/Preferences...
Regarding the Aleutians into Bering Sea storm, detail
predictability looks below average even early in the period. This
is due to uncertainty with the specifics of an initial shortwave
over the Aleutians and separate northern stream energy digging
southeastward from Siberia and vicinity, plus their possible
interaction. Dynamical and machine learning (ML) guidance shows
various ways that the upper dynamics could evolve, leading to the
observed spread in guidance for surface low track and depth. The
00Z/12Z ECMWF runs have started a fairly new trend of keeping the
Aleutians shortwave more progressive with less influence from the
upstream digging energy, leading to a faster/eastward surface
system. Most other guidance (including a majority of the 00Z
initialized ML models) suggests that the Aleutians energy will in
some fashion combine with the upstream energy to yield a closed
upper low within an area covering the central Bering Sea into far
eastern Siberia around Thursday-Friday. This keeps the the
surface low somewhat farther west late next week, with a depth
between the upper 970s and upper 980s mb. Within this cluster,
the 12Z GFS/GEFS mean are initially on the northward side of the
spread, with the 06Z GFS better for a while before possibly
straying fast by Friday, while the 12Z CMC is a little on the
south side. Among the means, the 12Z CMCens/00Z ECens fit the
favored scenario well for track but are likely weak due to the
spread of individual members. The system should quickly weaken
and trend more progressive after early Friday given indications
that the supporting dynamics should open up. Once that happens,
the GEFS mean is another viable contributor to the forecast blend.
Behind this system there has been some signal for trailing frontal
waviness, with varied effects on locations from the Aleutians into
the mainland. The 12Z UKMET is the quickest to introduce such a
wave and becomes rather strong with it (reaching the southeastern
Bering Sea by early Friday) while some recent GFS/ECMWF runs have
had a well-defined wave reaching the mainland by next weekend.
The 12Z GFS/ECMWF backed away from this signal and the ensembles
do not agree well enough among their members to depict a
discernible wave, while there is not much of a signal in the ML
models either. Thus current preference is to downplay such
waviness, only showing the weak waviness that typically develops
along the southern coast as the upper trough axis pushes eastward.
Speaking of the upper trough energy later in the period, guidance
diverges considerably among each other and from run to run by next
weekend. An emphasis on the ensemble means, which ultimately
bring flat to slightly cyclonic flow aloft into the Gulf of
Alaska/Panhandle region by next Sunday, looks like the best
intermediate approach between the negatively tilted trough seen in
the 12Z GFS and slower/deeper 12Z CMC versus the ridge axis still
in place near the southern Panhandle in the 00Z ECMWF/06Z GFS.
The 12Z ECMWF and 18Z GFS are closest to the means for this part
of the forecast.
Above considerations led to starting with a blend of 70 percent
total 06Z GFS/12Z CMC/12Z UKMET and 30 percent total 12Z
CMCens/00Z ECens means Wednesday-Thursday. Thereafter, the blend
split GFS input between the 06Z/12Z runs, eliminated the UKMET
(after Thursday) and eventually the CMC (after early Saturday)
while raising total ensemble mean weight to 70 percent by day
8/Sunday. The ensemble mean component included the 12Z GEFS
during next weekend. This provides reasonable continuity for the
Aleutians/Bering Sea storm aside from a modest eastward nudge late
in the week.
...Weather/Hazards Highlights...
The storm forecast to track northeast from the Aleutians mid-late
week should spread a broad area of windy conditions and rain of
varying intensity from the Aleutians/Bering Sea into parts of the
mainland. Western and southern portions of the mainland should
see relatively greater totals of rain, with favored
southward-facing terrain likely to see the heaviest rainfall.
Some snow could be possible at highest elevations. Strongest
winds should be over the Bering Sea into the far western mainland.
Focused precipitation may reach Southcentral and possibly the
northern Panhandle by next weekend. At this time there is still
considerable spread for track/timing/intensity of this system,
limiting confidence in refining specifics of impacts at particular
locations. Trailing frontal wave(s) could produce additional
rainfall from the Aleutians into parts of the mainland but
confidence is well below average regarding if/when/where this may
occur.
Maximum temperatures will generally be near to below normal except
for a brief warm-up over the eastern interior mid-late week in
association with upper ridging moving across that part of the
state. In general expect a cooler trend with time from west to
east as Bering Sea upper troughing approaches/reaches the mainland
by the weekend. Areas of the North Slope may remain above normal
with offshore flow, downsloping from the Brooks Range. Minimum
temperatures will generally be near to above normal. Exceptions
may include the southeastern half of the state on Wednesday
(possibly continuing into Thursday over parts of Southcentral and
the Panhandle) and western locations by next Sunday.
Rausch
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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