Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
659 PM EDT Thu Jul 09 2020
Valid 12Z Mon Jul 13 2020 - 12Z Fri Jul 17 2020
...Overview and Guidance/Predictability Assessment...
The consensus of latest guidance advertises a general increase in
heights aloft over the mainland through next week. This trend
corresponds to the departure of a fast-moving retrograding Arctic
upper low and the arrival of upper ridging from the Bering Sea
ahead of a western Pacific system forecast to reach near the
Aleutians by late in the week. Meanwhile features within a
somewhat complex surface pattern from south of the Aleutians into
the northeastern Pacific during the first half of the week will
progress along/weaken as supporting energy aloft shears out.
Specifics within the initial northern/northeastern Pacific pattern
still have moderately low predictability due to the scale of
important features and uncertainty over their interaction. The
wave expected to track just south of the Panhandle around Tue-Wed
now clusters closer but not completely to the more northern ECMWF
runs from the past day, with the 12Z ECMWF a bit south of its
previous run. The 12Z GFS is on the slow/deep side while the 06Z
run compared better to the model/mean average. A complicating
factor for this feature is a suggestion from some guidance that
the initial wave could give way to or merge with another wave
immediately upstream.
There is still a good signal for the system tracking toward the
Aleutians but guidance is showing some spread and run-to-run
variability. Currently the ECMWF mean shows the best consistency
for track and timing through next Fri and fits between the last
two ECMWF runs (12Z run to the west/northwest and prior run to the
southeast). The GEFS mean had been close but the 12Z version has
adjusted southward and more elongated. As was the case farther
east, the 06Z GFS compared better to the majority cluster for this
system versus the 12Z run that brings its best-defined low farther
east.
After a couple days of considerable trending, the models/means
appear to have stabilized for the retrograding Arctic upper low.
In the wake of the low there is lingering uncertainty for exact
details of flow aloft over portions of the Arctic and northern
mainland. The most common theme is for a modest east-west
weakness to persist along/north of the northern coast, most likely
drifting slowly northward with time.
Guidance comparisons led to starting the days 4-5 Mon-Tue forecast
with a blend of the 12Z ECMWF, 06Z GFS, 12Z CMC, and 12Z UKMET in
order from most to least weight. The remainder of the forecast
introduced some ensemble input, more 00Z ECMWF mean than the 12Z
GEFS due to preferences for the Aleutians system. Maintaining
components of the ECMWF (both 12Z and 00Z runs)/06Z GFS/12Z CMC
helped to hold onto a little more detail than seen in the means.
...Weather/Hazard Highlights...
The mainland, especially the southeastern half to three-fourths,
will see scattered precipitation during the period with best focus
along favored terrain. The wave expected to track south of the
Panhandle may bring some moisture to that region for a period of
time but totals remain uncertain due to sensitivity in the precise
evolution/track of the system. The Aleutians should see
increasing precipitation and stronger winds with the western
Pacific system approaching the area during the latter half of the
week. Expect a majority of the state to see below normal
temperatures at least through midweek, with the most extreme
anomalies tending to be for highs and over the North Slope and
southeast/Panhandle regions. Western coastal locations and parts
of the southwest may have some opportunity for above average
readings. Temperatures may begin to moderate some late in the
week though northern and southeastern areas are most likely to
remain below normal.
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