Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Fri Oct 02 2020
Valid 12Z Tue Oct 06 2020 - 12Z Sat Oct 10 2020
...Overview and Guidance/Predictability Assessment...
Today's guidance shows remarkably good agreement and continuity
for the general pattern during the period while some typical
detail uncertainties arise by the latter half of the forecast.
Strong and massive low pressure should track across the far
western Bering Sea through midweek and then gradually weaken as it
drifts into the north-central/east-central Bering. As progressive
North Pacific flow begins to promote elongated upper troughing to
the southeast of the parent system, consensus develops a frontal
wave near the Alaska Peninsula or Kodiak Island with low pressure
persisting over the northeastern Pacific/Gulf area into next Sat.
Ahead of this evolution expect initial low pressure near the Kenai
Peninsula to weaken Tue-Wed before dissipating, while an upper
ridge likely progresses from the Bering Sea across the mainland
and ends up extending from western Canada northwestward into the
Arctic by next Sat.
From day 4 Tue into early day 6 Thu a 12Z operational model blend
represented consensus well and with good detail. During the rest
of the period the primary uncertainty involved the specifics of
low pressure to the southeast/east of the Alaska Peninsula.
Operational models have displayed various ideas for evolution of
the wave that develops from the parent system's front, as well as
for interaction among the frontal wave and another wave that may
lift northward ahead of it. The importance of subtle details will
likely give this evolution fairly low predictability for a while,
favoring a transition from the earlier operational model blend
toward an even mix of models (12Z GFS/ECMWF) and means (12Z
GEFS/00Z ECens) by day 8 Sat. The 12Z CMC strayed to the northern
side of the spread by the latter half of the period so it was
phased out of the forecast by Fri-Sat.
...Weather/Threats Highlights...
Weakening low pressure initially near the Kenai Peninsula may
support lingering mostly light precipitation along the southern
coast and Panhandle through Tue. Other parts of the eastern half
of the mainland may see light/scattered activity as well.
Meanwhile the strong Bering storm will spread a broad area of
precipitation and strong winds across the Aleutians and Bering Sea
Tue-Wed. Some of the leading moisture and brisk winds may reach
the far western mainland by Wed-Wed night. During the late
week/weekend time frame the evolving pattern aloft and low
pressure to the southeast/east of the Alaska Peninsula will tend
to favor precipitation over the Peninsula/Kodiak Island and
southern coast/Panhandle. Coverage, duration, and intensity of
precip will depend on low-predictability specifics of the one or
more low pressure centers. Winds across the Aleutians will
moderate somewhat with time as the Bering Sea low trends gradually
weaker.
Expect a majority of the state to see above normal temperatures
during the period. The Panhandle and some portions of the
southern mainland may see modestly below normal highs. Min
temperatures should be above climatology nearly state-wide with
anomalies tending to be greatest over central/northern locations.
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
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Mon, Oct 5.
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