Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
649 PM EDT Mon Oct 05 2020
Valid 12Z Fri Oct 09 2020 - 12Z Tue Oct 13 2020
...Overview and Guidance/Predictability Assessment...
Over the course of the Fri-Tue period, the ensemble means and a
reasonable proportion of recent operational models continue to
indicate consolidation of initial features (Bering Sea upper low,
elongated trough/upper low over the Northeast Pacific/Gulf,
energy/closed low reaching the mainland from the northeast) into a
mean trough aloft that settles over the mainland. Consensus shows
fairly persistent surface low pressure over or near the Gulf of
Alaska while low pressure initially over the Bering gradually
weakens as it tracks southeastward and then becomes absorbed into
the larger scale circulation.
There are still embedded uncertainties. In particular the latest
GFS runs hold onto the initial Bering system longer than other
guidance that loses definition after early Sun. Some of the
differences aloft appear fairly subtle and have low predictability
several days out in time, so the GFS scenario cannot be ruled out.
Still there has been gradual consolidation of guidance for low
pressure evolution over the Gulf of Alaska. Meanwhile some of the
details remain in doubt but there is reasonable continuity in the
general signal for the shortwave energy/possible upper low
reaching the mainland from the northeast, encouraged by flow
around a weak early-period Arctic upper ridge. The one notable
continuity change from yesterday is to provide some reflection of
recent GFS/ECMWF/UKMET clustering and continuity for a
strengthening mid-latitude Pacific system that may track over or
just north of Haida Gwaii around day 6 Sun.
Today's forecast blend started with input from the 12Z
ECMWF/GFS/UKMET for days 4-5 Fri-Sat. Then the forecast
transitioned toward a later-period model/mean blend that
incorporated the 12Z/00Z ECMWF runs (more 00Z run) and 12Z GFS
along with the 12Z GEFS/00Z ECMWF means. Both ECMWF runs were
plausible over/near the mainland but late in the period the new
run began to diverge with the larger scale Pacific pattern--albeit
with hints reflected in the new ECMWF mean. Some aspects of the
CMC were reasonable around the mainland early in the period but
elsewhere it strayed significantly from consensus in various ways.
...Weather/Threats Highlights...
The evolving pattern aloft and Gulf of Alaska low pressure should
support precipitation along the southern coast and Panhandle
during most of the period. Best potential for some locally
enhanced amounts will be late this week into the weekend. It will
be a close call regarding possible effects on the southern
Panhandle from the progressive system expected to track near Haida
Gwaii around Sun. Lowering heights aloft may produce scattered
precipitation at times over the mainland which will be in the
gradient between Arctic high pressure and Gulf low pressure. The
Aleutians should see brisk and unsettled conditions.
Continue to expect above normal temperatures over a majority of
the state. The main exceptions remain the Panhandle and some
portions of the southern mainland where highs may be modestly
below normal. A few isolated near to below normal highs may be
possible over the north. Min temperatures will likely be above to
well above normal over most areas, except for near to slightly
below normal over the Panhandle.
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:
-No significant hazards are expected over Alaska during this
forecast period.
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