Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
642 PM EDT Fri May 08 2020
Valid 12Z Tue May 12 2020 - 12Z Sat May 16 2020
...Much above normal temperatures over parts of mainland Alaska to
decrease in coverage with time...
...Guidance/Predictability Assessment...
From about day 4 Tue into day 6 Thu there is general consensus
that an extension of the strong upper ridge over the Arctic will
progress southwestward across parts of the mainland while a
separate upper ridge initially over western Canada into the
mainland steadily weakens. The main forecast problem from Thu
onward involves the evolution/progression of upper ridging and
adjacent shortwave energy that most guidance suggests will drop
into northwestern Canada just after midweek. Thus far individual
operational models have been widely divergent and inconsistent
regarding how shortwave energy may retrograde beneath the Arctic
ridge. As a whole the ensemble means have been more conservative
with the weakness aloft that may develop south of the ridge. Over
the past couple days and now including the new 12Z run, the ECMWF
mean has been the most consistent guidance source for depicting
the Arctic upper ridge and showing only a modest weakness to its
south/southeast during the latter half of the period. The 00Z/12Z
CMC means support the ECMWF mean idea as well. GEFS means have
varied a bit in comparison but merit some input for the forecast.
Ensemble mean consistency thus far, plus the historical tendency
for solutions that are deepest/fastest with retrograding energy
beneath an upper ridge to be less probable, favor trending more
toward the means with time. Thus today's blend emphasized the 12Z
operational models Tue into early Wed and then steadily increased
ensemble mean weight to 40 percent by day 6 Thu and 75 percent by
day 8 Sat. Recent continuity favored more ECMWF mean weight
relative to the GEFS mean. At the moment the uncertainties aloft
would appear to have greater impact on temperatures than the
surface pattern.
Meanwhile the primary storm track will remain over the Pacific
through the period. Overall there is good clustering and
continuity for the vigorous system that should track just south of
the Aleutians through midweek and then make gradual progress into
the northeastern Pacific. Enough spread develops toward the end
of the period to indicate uncertainty over whether flow around the
northern periphery of the system could begin to have some
influence on the Panhandle and Alaska Peninsula. The very
northern fringe of an emerging western Pacific system (likely
tracking somewhat south of the first system) could reach the
western Aleutians by day 8 Sat. At that time the last couple
ECMWF means provide the best intermediate solution. The
aforementioned blend with modest manual enhancement represented
preferences well across the North Pacific.
...Weather/Hazard Highlights...
Significant uncertainty develops with the specifics by late week
and next weekend but guidance shows the general trend toward
cooler temperatures with above normal readings being confined
increasingly toward the southwestern part of the mainland as
cooler air pushes into northeastern areas. The warm anomalies
should gradually decrease in magnitude while some areas in the
north and east may eventually see below normal readings.
Northwestward moving shortwave energy may produce a brief period
of precipitation over the southwestern mainland on Tue. The next
day or so should be fairly dry and then the front that settles
into the southern mainland could provide a focus for precipitation
late week into the weekend. Moisture around the northern fringe
of northeastern Pacific low pressure may also reach some locations
between the Panhandle and Alaska Peninsula by next Fri or Sat.
Earlier in the period this system will track just south of the
Aleutians and bring a period of rainfall and brisk winds.
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:
- Much above normal temperatures across portions of mainland
Alaska, Mon, May 11.
- Much above normal temperatures across portions of mainland
Alaska, Mon-Tue, May 11-May 12.
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