Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
655 PM EDT Tue Jul 07 2020
Valid 12Z Sat Jul 11 2020 - 12Z Wed Jul 15 2020
...Overview and Guidance/Predictability Assessment...
An Arctic ridge aloft should continue to favor varying degrees of
upper troughing over the mainland from the weekend into the middle
of next week. Initial energy should yield a northeast-southeast
trough, with some uncertainty over exact orientation and position
of a possible embedded low. Then there is a steadily growing
signal for the approach of a deeper upper low from the northeast.
This evolution should keep a front draped over the
central-northern mainland during the period. Farther south, with
typical detail uncertainty there is reasonable agreement in
principle that the system initially south of the Aleutians will
weaken and stretch apart with a lingering wavy front likely
progressing northeastward with time. This will occur as energy
aloft generally becomes more sheared--but with meaningful embedded
detail uncertainties. One or more trailing western Pacific
features may come into the picture behind the wavy front. A
system south of the Panhandle will rapidly weaken after early Sat.
For the initial upper trough over the mainland the primary
forecast problem involves the precise westward/southwestward
elongation and path of a possible embedded low. Currently the
guidance is about evenly split between the GFS/CMC and their means
pulling the energy far enough west to bring an upper low into the
Bering versus the ECMWF/ECMWF mean and UKMET that keep the core of
the upper trough farther east. Typical verification of
retrograding energy favors a conservative approach represented by
a multi-model blend that keeps the center of the trough closer to
the ECMWF cluster.
The GFS/ECMWF/CMC have maintained the signal for a fairly deep
upper low expected to approach from the northeast. The 12Z and
prior 00Z ECMWF runs offer good support for the operational runs
while the 12Z GFS/CMC keep the feature a bit farther
east/northeast--well within guidance error ranges several days out
in time. GEFS/CMC ensemble means so far have been shearing out
this feature into a mere trough. Ensemble means over the past day
indeed favor more troughing while it seems the operational models
have established enough stability to depict an upper low, just
slightly weaker and with a compromise location.
Across the Pacific a consensus blend represents consensus where
agreement exists while downplaying less confident details. The
best cluster for the system initially south of the Panhandle takes
the weakening surface low approximately between the southern
Panhandle and Haida Gwaii. The 12Z GFS is on its own with a
slower/southward wave. Confidence is lower with specifics of the
wavy front expected to lift across the North Pacific into the
Aleutians/Alaska Peninsula. This is due to a combined dependence
on Pacific shortwave energy as well as possibly flow across the
Bering and mainland. One or more trailing waves have low
confidence/predictability as well. There is somewhat more
clustering toward a western Pacific system that could extend its
influence into the Aleutians by late day 8 Wed or soon thereafter.
The 12Z GFS is a bit on the fast side with this system.
Forecast considerations led to starting with an operational model
blend for days 4-5 Sat-Sun, the 12Z ECMWF/GFS/UKMET/CMC in order
from most to least weight. Changes from continuity were fairly
modest in this time frame. By days 7-8 Tue-Wed the blend rapidly
adjusted to 50-60 total ensemble input (12Z GEFS/00Z ECens) with
remaining weight from the 12Z ECMWF/GFS.
...Weather/Hazard Highlights...
The forecast continues to show below normal temperatures over much
of the state due to the mean troughing aloft. The greatest
negative anomalies should be over the North Slope and
south-southeast/Panhandle. Western coastal areas will have the
best opportunity for above normal readings. Expect highest
precipitation totals during the period to be over the southern
mainland and in particular near the Alaska Range where daytime
heating and cool temperatures aloft may enhance activity. Precip
may extend farther northward with time in response to the
approach/possible arrival of the upper low from the northeast
during the first half of next week. The weakening system over the
Northeast Pacific may bring a little moisture to parts of the
Panhandle during the weekend. The weakening/stretching system
initially south of the Aleutians and wavy front lifting
northeastward should bring varying amounts of rainfall to the
Aleutians and Alaska Peninsula. Depending on uncertain details
some of this activity may eventually extend into the extreme
southern mainland/Panhandle. One or more trailing features could
also bring some rain to the Aleutians.
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