Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
741 PM EDT Tue Aug 03 2021
Valid 12Z Sat Aug 07 2021 - 12Z Wed Aug 11 2021
...Overview and Guidance/Predictability Evaluation...
The majority of latest guidance agrees fairly well for the large
scale evolution through the period. Consensus shows an initial
Bering Sea upper low with a mean trough extending into the North
Pacific. The Bering Sea low may open up by next Tuesday-Wednesday
but troughing should persist from the Bering Sea southward.
Meanwhile a downstream ridge should build over the eastern Pacific
but with uncertainty over how far northward the ridge could
amplify north of 50N latitude on any particular day. This overall
pattern will likely promote a period of wet weather over portions
of Alaska with highest precipitation totals tending to be over
southern areas. Farther north, a col region over or near the
northern coast during the weekend may transition to broad cyclonic
flow aloft as an Arctic upper low drifts southward toward 80N
latitude.
Operational model and ensemble mean comparisons show one prominent
discrepancy in terms of the large scale forecast, namely the
12Z/18Z GFS runs straying faster than other guidance for the
entire North Pacific into Mainland Alaska pattern late in the
period. The 18Z run is even more extreme in that regard than the
12Z version. Thus the manual forecast phased out GFS input after
early next week. Otherwise model/ensemble spread and run-to-run
variability suggest low confidence in the specifics of one or more
Pacific waves that may either reinforce the area of mean low
pressure over the Bering Sea from the weekend into early next week
or else track farther to the southeast. There is a decent signal
for such waves to exist though. The fairly small scale of these
waves and supporting dynamics lead to low predictability so it may
take a while to resolve the details that will be important for the
precipitation forecast. A 12Z operational model blend (though
splitting ECMWF input between the 12Z/00Z runs) followed by a
trend toward about half weight of models/means (ECMWF runs/CMC and
12Z GEFS/00Z ECens) provided a reasonable balance of representing
the agreeable mean pattern and some indication of Pacific waves.
The similarity of the 12Z ECMWF and CMC for a wave tracking into
the southeastern Bering Sea next Wednesday is probably mere
coincidence but including a toned-down version of such a wave in
the blend represents the plausibility of a wave in principle,
given the position of the mean trough aloft.
...Weather/Threats Highlights...
During the weekend a northwest-southeast front over the mainland
could provide a focus for locally moderate to heavy rainfall at
times. Then expect Bering Sea low pressure, leading front(s), and
possibly one or more waves, to bring widespread precipitation to
half or more of the mainland and vicinity. Wave details that
currently have low confidence will determine the location and
magnitude of precipitation, some of which could be fairly heavy
depending on strength/track. Ensemble guidance shows the best
potential for highest five-day totals from the eastern side of the
Kenai Peninsula into the Panhandle as well as over favored terrain
in the Alaska Range. Other areas could see significant totals as
well. The Aleutians will be unsettled for most of the period but
again with low confidence in how much rain falls due to dependence
on uncertain wave details.
The unsettled pattern over a majority of the state will likely
lead to below normal high temperatures through the period.
Meanwhile somewhat warmer anomalies should prevail for min
temperatures, with only the North Slope and northeastern mainland
consistently below normal while a mix of above/below normal
readings should exist elsewhere.
Rausch
Hazards:
- Heavy rain across portions of mainland Alaska, Fri, Aug 6.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook 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