Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
648 PM EDT Tue Oct 20 2020
Valid 12Z Sat Oct 24 2020 - 12Z Wed Oct 28 2020
...Heavy precipitation possible for the Panhandle and coastal
Southcentral areas late weekend/early next week...
...Guidance/Predictability Assessment...
Latest models and ensemble means are similar for the large scale
pattern evolution during the days 4-8 Sat-Wed period. The upper
low initially associated with the system over the Bering Sea
should drift toward the western mainland and then open up early
next week as shortwave energy over the Arctic and northern/eastern
Russia drops southeastward, ultimately leading to a trough
extending from the mainland into the southeastern Bering/eastern
Aleutians by next Wed. Meanwhile energy streaming across the
North Pacific/Aleutians may feed into a central/east-central
Pacific trough.
Guidance agrees well that the Bering Sea low should reach close to
the western coast of the mainland by 12Z Mon. There has been more
spread and variability for a Pacific frontal wave expected to
reach the Gulf of Alaska or vicinity by late Sun/Mon. ECMWF runs
before the new 12Z version had been consistent in tracking the
wave to the Panhandle but the ensemble means and 12Z CMC/UKMET
offer support for the new run's adjustment closer to the Kenai
Peninsula. This is also close to WPC continuity. The 12Z GFS is
on the deep/western side of the spread. Operational runs show a
small scale wave tracking along or south of the Aleutians after
the weekend. Later in the period the ensemble means support the
idea of a farther north track than seen in the 12Z ECMWF. Both of
the North Pacific waves are fairly small in scale and thus have
lower predictability than the Bering system. Finally, progressive
Arctic flow will likely bring a front into the extreme northern
mainland during the weekend and then more amplified upstream
energy should push another cold front southeastward into the
mainland and Bering Sea/Aleutians by early-mid week.
Based on guidance comparisons today's forecast started with a 12Z
operational model blend for most of the weekend. Then the
forecast trended to a mix of models and 12Z GEFS/00Z ECMWF means,
with lower GFS weight to downplay its Mon system over/near the
Gulf and less 12Z ECMWF weight late in the period to emphasize
consensus for the North Pacific/Aleutians wave. For the latter
the old 00Z ECMWF was closer to the majority.
...Weather/Threats Highlights..
Expect the heaviest precipitation during the period to be over the
far southern mainland and Panhandle with the developing frontal
wave currently expected to reach the Gulf of Alaska or vicinity
around late Sun/Mon. Meanwhile the southwestern mainland/Alaska
Peninsula may see continued precip through the weekend ahead of
the Bering Sea system drifting toward the western coast of the
mainland. There may be localized areas of focused activity.
Weakening/progression of the Bering system should promote a drier
trend over the west after the weekend. Lingering low pressure
over the Gulf should keep some precip chances along the southern
coast/Panhandle after Mon but with lesser totals than with the
Sun-Mon system.
Above to much above normal temperatures should prevail over a
majority of the mainland during the period. The North Slope and
western portions of the mainland should see the greatest positive
anomalies with min temperatures tending to be more above normal
than the daytime highs. The upper trough/cold front dropping into
the state from the northwest should lead to a cooling trend by
midweek. The Panhandle and parts of the far southeastern mainland
are likely to see below normal readings, moderating gradually with
time.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Peninsula and
southwestern Alaska, Fri, Oct 23.
- Heavy precipitation across portions of the Alaska Panhandle and
coastal Southcentral Alaska, Sun-Mon, Oct 25-Oct 26.
- High winds across portions of the Alaska Peninsula, Fri, Oct 23.
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