Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Sun Aug 23 2020
Valid 12Z Thu Aug 27 2020 - 12Z Mon Aug 31 2020
...Overview and Guidance Evaluation/Preferences...
Behind a late-week system tracking from the Alaska
Peninsula/Kodiak Island across the Gulf of Alaska and Panhandle,
guidance continues to advertise a significant large-scale pattern
change toward a mean upper low over the Bering Sea and downstream
ridge aloft building over the Northeast Pacific and eastern
mainland/western Canada. There is good agreement that a strong
North Pacific into Bering Sea storm will usher in this change,
with a broad area of wet/windy conditions. However there are
still question marks with the specifics of system's evolution and
greater uncertainty over the role of a possible upstream wave that
could influence the forecast by next Sun-Mon.
For the dominant North Pacific into Bering Sea system, multi-day
trends continue to be toward slower northward progression. Other
details differ in some guidance, such as the 12Z GFS reflecting
separate waves to a greater extent than earlier runs or the 12Z
ECMWF/UKMET/CMC early-mid period. The 06Z GFS compared better to
the majority 12Z cluster. By day 8 Mon the CMC strays a bit to
the fast side with the main surface low. Upstream, over recent
days the guidance has been showing very inconsistent signals
regarding a potential wave that could either influence the leading
system or merge into its circulation. Very strong solutions such
as the 00Z ECMWF (which was due to questionably fast ejection of
Typhoon Bavi remnants and phasing with northern stream energy)
have tended to adjust back to a weaker/suppressed wave in later
runs. The 12Z ECMWF and other 12Z models do indeed favor a less
influential wave that tracks around the southern side of the main
system--possibly near the Aleutians/Alaska Peninsula by next
Sun-Mon.
Overall clustering remains good for the system tracking across the
Gulf of Alaska Thu-Fri. Especially at the start of the forecast
there is still evidence of a modest slower trend. By day 5 Fri
there are noticeable differences in exact strength/position of the
surface low but the very small scale leads to low predictability
at that time frame. Farther north, the consensus of guidance
(discounting the 12Z CMC's tiny upper low north of the mainland)
has trended toward more upper ridging that extends into the
northern mainland from north of Russia late this week. Then the
evolution trends toward the previously advertised col region that
develops near or just north of the northern coast.
Above considerations led to using the 06Z GFS and 12Z
ECMWF/UKMET/CMC as the starting point for the forecast late this
week before trending toward a model/mean blend, reaching 40-65
percent means (12Z GEFS/00Z ECens) by days 7-8 Sun-Mon. A little
manual editing improved definition of the possible late-period
wave south of the main Bering system. Today's blend produced some
eastward adjustment for the Bering system around next weekend but
with minimal change in progression of the leading fronts.
...Weather Highlights/Threats...
Continue to expect a broad area of precipitation and stronger
winds to accompany the storm forecast to track from the North
Pacific into the Bering Sea during the period. Recent trends have
held back the northward progression of impacts a bit but there is
still reasonable consensus that enhanced precip/winds should reach
the Alaska Peninsula and then extend into the southwestern third
to half of the mainland over the course of the weekend. Some
moisture could reach as far east as the Panhandle by Sun or Mon.
Significant precip totals will be possible along the southern
coast and over some areas of favored terrain in the southwestern
mainland. The system tracking from near Kodiak Island to the
Panhandle Thu-Fri will bring some focused precipitation to areas
along the southern coast and Panhandle. There will also be a
broader area of scattered precip over portions of the mainland
late this week. A drier trend will progress west to east behind
this system and before the approach of moisture ahead of the
Aleutians/Bering Sea storm.
Below normal highs will prevail most of the period from the North
Slope through the eastern and southern-southeastern mainland and
Panhandle. Western areas will see the best potential for above
normal highs. There will be greater coverage of above normal
readings for low temperatures. Expect highest anomalies for lows
late this week to be over the northeastern mainland. Flow ahead
of the Aleutians/Bering Sea storm should spread warmer min
temperatures across much of the state during the weekend and early
next week.
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