Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
651 PM EDT Fri Oct 30 2020
Valid 12Z Tue Nov 03 2020 - 12Z Sat Nov 07 2020
...Guidance/Predictability Assessment...
Guidance shows little change or disagreement over most aspects of
the large scale pattern compared to recent days but there are
persistent question marks over embedded details. A strong bundle
of closed low/shortwave energy to the west of the northern
mainland as of early day 4 Tue will drop rapidly southward across
the far western mainland and into the northeastern Pacific. This
feature will pull the initial mean trough--whose core will be
centered over the southern coast of the mainland Tue into
Wed--farther to the south/southeast after midweek. A trailing
upper ridge crossing the Bering Sea through midweek will settle
over the mainland late this week into the weekend while a strong
Kamchatka/Siberia system will spread a broad area of
moisture/strong winds across the Bering Sea as well as parts of
the Aleutians and mainland mid-late week. The majority of
guidance suggests broad troughing aloft should persist over the
western Aleutians and Bering Sea into next Sat.
The early to middle part of the forecast period (Tue-Thu) shows
detail differences and variability with low pressure across the
Arctic, the shortwave energy dropping southward over the far
western mainland, and Northeast Pacific low pressure that could
have peripheral influence on the southern Panhandle. A 12Z
operational model blend represents the most common ideas for the
first two features with details being sufficiently small in scale
to have low predictability in the extended time frame. Looking in
greater detail at the Northeast Pacific, there is still a majority
cluster showing a wave tracking near Haida Gwaii around Tue.
However most 12Z models are generally signaling greater potential
for a trailing wave along a similar track, but the 12Z GFS trended
away from such a wave relative to its 06Z run. Ensemble means
show little indication of the trailing wave but there is troughing
within which a wave could fit. The aforementioned model blend
would provide a moderate trend toward a defined second wave while
awaiting confirmation from future runs. By around Thu there is
also some question over northeastern Pacific low pressure that
evolves in response to the western mainland energy dropping into
the mean trough aloft, with the blended approach helping to
reflect a better-defined version of some ensemble mean ideas.
Farther upstream there is reasonable agreement and continuity for
the strong Kamchatka/Siberia system and associated wind/moisture
field as well as for the upper ridge that settles into the
mainland (albeit with typical differences for strength). Late in
the period the 12Z ECMWF trends faster than most other guidance
with a shortwave reaching the Bering Sea and extends an upper
low/trough closer to the northeast corner of the mainland versus
other solutions. Thus in addition to incorporating some 12Z
GEFS/00Z ECMWF ensembles after early Thu (reaching 40 percent
total weight by day 8 Sat), the ECMWF component of the blend
transitions from the 12Z run to the 00Z run that generally
compared better to other models/means.
...Weather/Threats Highlights...
Uncertainty over one or more Northeast Pacific waves will continue
to keep confidence lower than desired for precipitation specifics
over the southern Panhandle, with meaningful totals possible for a
low track in the northern half of the spread. Exact timing is
still a question mark but there will be a drier trend later in the
week when low pressure progresses farther southeast. The far
western mainland will see mostly light precipitation around Tue
with the wave dropping rapidly southward over the area. Arctic
low pressure and accompanying front(s) could promote some light
precip over far northern areas as well. During mid-late week the
forecast is consistent for the broad circulation ahead of strong
Kamchatka/Siberia low pressure to bring a period of precipitation
and strong winds to the western half of the Aleutians, much of the
Bering Sea, and then western parts of the mainland. Favored
terrain in the west/northwest could see significant totals by
Thu-Fri. Some precipitation may extend to other portions of the
mainland as well.
Much below normal temperatures over the south and east through
midweek should moderate late week into the weekend as the upper
ridge approaches/settles over the mainland. The Panhandle will
remain well below normal into the weekend though. On the other
hand expect the North Slope to see well above normal readings
through the period, possibly with anomalies increasing further
late in the period while somewhat less extreme warm anomalies
spread into the western and eventually central parts of the
mainland south of the North Slope.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Mon-Tue, Nov 2-Nov 3.
- Much above normal temperatures across portions of mainland
Alaska, Mon, Nov 2 and Fri, Nov 6.
- Much below normal temperatures across portions of mainland
Alaska, Mon-Thu, Nov 2-Nov 5.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Tue-Fri, Nov 3-Nov 6.
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