Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
710 PM EDT Sun Oct 04 2020
Valid 12Z Thu Oct 08 2020 - 12Z Mon Oct 12 2020
...Overview and Guidance/Predictability Assessment...
A strong and massive Bering Sea storm as of early Thu should
slowly trend weaker as it drifts closer to the southwestern coast
of the mainland. Ahead of this system the consensus still shows
late week upper trough elongation into the northeastern Pacific,
supporting development of an associated wave. The combination of
the elongated trough/possible embedded low lifting northward and
eventual approach of the Bering Sea upper low should bring the
surface low northward into the Gulf of Alaska. Meanwhile most
guidance has held onto yesterday's trends of taking an initial
eastern mainland upper ridge and closing off the northern part
into an upper high that quickly tracks into the Arctic. Around
this upper high, shortwave/upper low energy may progress
southwestward into the mainland by next weekend. There is a fair
amount of agreement that some combination of this energy as well
as what remains of the Bering upper low and possibly
surrounding/incoming flow should comprise a mean trough aloft
whose axis reaches about 160W longitude by day 8 Mon.
Today the operational guidance is more mixed regarding progression
of the initial Bering system at the surface and aloft. There are
also considerable differences regarding the details of the
trough/upper low that may reach parts of the mainland from the
northeast. On the positive side latest guidance appears to have
simplified the northeast Pacific surface low forecast somewhat.
Meaningful differences remain with specifics but at least models
have really dampened the prior signal for involvement from any
northward-tracking waves ahead of the low that breaks off from the
front reaching the Alaska Peninsula by early Thu. Finally, the
12Z GFS/CMC offer potential for a strengthening mid-latitude
Pacific system to affect the Panhandle by day 8 Mon. Thus far the
signal is too low in other guidance to incorporate that idea into
the forecast.
A 12Z operational model blend represented consensus well as of
early day 4 Thu. Solutions diverged after that time, as has been
the case recently. The forecast quickly adjusted to primary
weight on the 12Z/00Z ECMWF runs and 06Z GFS with a small ensemble
component (a bit more 00Z ECens than 12Z GEFS) through mid-period
followed by a more even model/mean weight late. This approach
helped to maintain continuity for moderately slow progression of
the Bering system (versus the faster 12Z GFS/CMC; GEFS mean hedged
between the ideas, showing some early progression but also holding
an implied upper low back) and to a reasonable extent over the
northeastern Pacific/Gulf. Meanwhile the 12Z ECMWF/06Z GFS
inclusion meshed with WFO AFG coordination supporting some degree
of upper trough/low arriving from the northeast--also suggested by
the past couple ECMWF means. This feature supported introduction
of a leading cold front that pushes southward over parts of the
mainland.
...Weather/Threats Highlights...
Guidance is still in the process of trying to resolve important
details for enhanced precipitation potential from the eastern
Alaska Peninsula across the southern coast and into the Panhandle.
The most common theme is for some locally higher totals to focus
along the eastern Peninsula to the Kenai Peninsula in association
with the overall pattern evolution at the surface and aloft late
this week, with signals regarding coverage/amounts being more
mixed across locations to the east. Confidence further decreases
for specifics during the weekend and early next week. The upper
trough/low forecast to reach into the mainland from the northeast
could eventually promote some scattered precipitation during the
latter half of the period while the Aleutians will likely see
continued brisk and unsettled conditions.
The forecast continues to show above normal temperatures over most
of the state. The Panhandle will be the main exception with below
normal readings. Pockets of the southern mainland may see
modestly below normal highs. The upper trough/closed low reaching
into the mainland from the northeast by the weekend may begin a
cooling trend, with highs at some locations over the north
possibly declining toward normal depending on the depth of the
upper feature.
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