Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
754 PM EDT Thu Jun 03 2021
Valid 12Z Mon Jun 07 2021 - 12Z Fri Jun 11 2021
...Overview and Guidance Evaluation...
The full array of guidance suggests a regime of rather low
predictability over a significant portion of the forecast area.
Models and some ensemble members continue to be all over the place
for the ultimate evolution/track of a concentrated upper low
expected to be over or near the far western mainland at the start
of the week while separate details of central/northern Pacific
flow are also problematic. Arctic flow is questionable as well.
Thus confidence is below average for some aspects of sensible
weather during the period. In order to navigate the current
guidance spread and provide some elements of continuity until
confidence increases in any alternative scenario, today's forecast
started with a 60/40 blend of the 00Z ECMWF/12Z GFS for days 4-5
Monday-Tuesday followed by a rapid trend toward increasing
ensemble input so that days 7-8 Thursday-Friday leaned 70-80
percent to the means.
For the western mainland upper low, the ECMWF has been the most
consistent over the past couple days with a south/southeast track.
GFS runs have varied among a southward track (12Z), meandering
over the mainland for most of the time (06Z and new 18Z runs), or
eventually lifting northward (00Z). 12Z UKMET/CMC runs take a
northward track at differing speeds. Recent GEFS/ECMWF means have
had a southward tilt but likely weaken the upper low too quickly
due to the expanding member spread. The new 12Z ECMWF mean hints
at a splitting of energy per divergence of members, with the 12Z
CMC mean even more dramatic in that regard. With minimal
confidence, preference was to maintain the southward track
scenario per the ECMWF runs and 12Z GFS to hold onto some degree
of continuity while awaiting better clustering.
Meanwhile details across the North Pacific/Aleutians are ambiguous
as well. The 12Z ECMWF differs from other guidance in quickly
picking up a cutoff low over the central Pacific and producing a
rather strong Aleutians storm by midweek (though the CMC mean uses
weaker northern stream energy to produce a modest surface low of
its own). As a result the 12Z ECMWF could not realistically be
used in today's forecast blend. Details of leading North Pacific
waviness are also in doubt, with the 12Z CMC on the northern side
of the spread among solutions that have a progressive wave.
By the latter part of the period the ensemble means and some
models develop the general theme of a northeastern Pacific upper
low/surface system (from some combination of the mainland upper
low and leading North Pacific energy) while a moderate upper ridge
builds over the mainland. At the same time a western Pacific
system may begin to extend its influence toward the Aleutians.
Recent trends in the GEFS mean for the western system have been
slower toward the ECMWF mean, while the operational 12Z ECMWF has
jumped to a faster timing. The GFS/CMC/00Z ECMWF all compare more
favorably to the means. Meanwhile the character of high-latitude
flow is not lacking in uncertainty either. The GEFS/ECMWF means
both show an evolution from initial Arctic ridging toward greater
progression that could ultimately bring a front near the northern
coast, but operational models offer different ideas for possible
patterns at the surface and aloft.
...Weather/Threats Highlights...
If the initial western upper low tracks southward/southeastward as
currently anticipated, expect it to produce some areas of focused
precipitation over parts of the western and southern mainland
during at least the early part of next week. Precipitation would
become lighter and more scattered mid-late week if it either
continues into the Gulf/Northeast Pacific or instead lifts
northward from the western mainland. Persistence for a longer time
is yet another potential scenario to be monitored. How much
moisture reaches the Aleutians during the period is another
question mark. It is fairly likely that at least some rainfall
reaches the Aleutians during the first part of the week with
leading waviness but there is a lower-end possibility of a drier
scenario with a more southward track. Flow ahead of a western
Pacific system may bring another area of moisture into the
Aleutians during the latter half of the week.
Expect below normal high temperatures early next week underneath
the western mainland upper low, while locations across
Southcentral and the Panhandle should see below normal highs for
most of next week. The western mainland will see a moderating
trend mid-late week assuming the upper low departs in some
fashion, while northern areas will tend to be below normal though
possibly with some pockets of above normal highs. Anomalies for
lows should generally be somewhat warmer than those for highs,
with the Panhandle and Interior most likely seeing moderately
below normal lows versus above normal readings elsewhere.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
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