Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
752 PM EDT Sun Jul 11 2021
Valid 12Z Thu Jul 15 2021 - 12Z Mon Jul 19 2021
...Overview...
Today's guidance continues to suggest that the upper pattern will
open up somewhat after late this week. An upper high initially
near the western coast of the mainland should gradually lose
definition as it drifts southward with the overall ridge dropping
down into the southern mainland by the weekend. Another upper
ridge, ahead of mean troughing most likely aligning between
Kamchatka and the western Aleutians (supporting one or more
surface systems), may build into the mainland behind the first
ridge. The evolving pattern will eventually push away initial
upper low/trough energy over the Bering Sea but with continued
uncertainty over what path this energy will take. Mean troughing
is likely to persist over the northeastern Pacific with various
possibilities for specifics over the course of the period.
Northern/eastern parts of the mainland will see an oscillation of
surface fronts in response to Arctic shortwaves that will be
partially influenced by the precise amplitude of ridging over the
mainland.
...Guidance/Predictability Evaluation...
The one detail that has relatively lower confidence is the 12Z/00Z
ECMWF idea of taking the initial Bering Sea upper low/trough and
bringing it through the southwestern mainland next weekend.
Consensus that includes latest ECMWF mean would have more
persistent upper ridging. Thus it seems more likely that the
Bering Sea energy will eject either weaker/northward or to the
south (or split between these two options).
Meanwhile guidance shows various ideas for how features may evolve
to the east of Kamchatka after clustering fairly well at the start
of the period on Thursday. Initial low pressure could linger for
a while to the south/southwest of the western Aleutians, whether
through persistence or redevelopment, or eject to be replaced by
another system. Operational models have not been very agreeable
or consistent over recent days. So far the ensemble means have
not been too keen on the eastern side of the model spread for
height falls/low pressure. Even with the uncertainty for
individual low centers, there is better agreement for existence of
the gradient between the low(s) and mid-latitude Pacific high
pressure.
Across higher latitudes, like yesterday there is a general theme
in the guidance that late this week Arctic energy will provide a
northward lift to a northwest-southeast front initially over the
mainland as of early Thursday and then a trailing front will take
its place over northern areas. At the initial time the front is a
bit southwest of continuity due to slightly more amplitude with
the upper trough extending from northern Canada. By the weekend
and early next week solutions diverge for the shape of Arctic flow
with combined uncertainties for the amplitude of mainland ridging
at any particular time and details of flow over/north of Siberia.
The most common theme is for mean troughing to develop over
Siberia, with low pressure lifting the northern mainland front
back to the north once again.
An operational model blend for about the first half of the period
downplayed the ECMWF handling of the initial Bering Sea energy
while emphasizing the most common details of guidance elsewhere.
The forecast trended more toward the ensemble means by day 8
Monday due to the widening guidance spread east of Kamchatka as
well as for Arctic flow. This approach provided fairly good
continuity with modest adjustments where agreed upon by enough
guidance.
...Weather/Threats Highlights...
Organized precipitation over the southeastern quarter of the
mainland may extend into Thursday with the initial front over the
region. Most of the Panhandle should see a drier trend with time
as the upper trough axis moves just far enough east. Northern
portions of the mainland will likely see one or more episodes of
precipitation with the oscillating fronts over the region. The
gradient ahead of one or more low centers emerging from Kamchatka
should bring an increase of precipitation and winds into at least
parts of the Aleutians and Bering Sea. There is still a lot of
uncertainty over how much moisture may extend into the mainland
and when/where--not only associated with these systems but also
the initial closed low/trough energy over the Bering Sea.
The majority of the Panhandle and southeastern coast should see
below normal highs during most of the period. The North Slope
should tend to see below normal highs too but may see some
variability depending on day-to-day frontal position. Elsewhere
expect a mix of above/below normal highs, with a tendency for a
cooling trend from west to east by the weekend. Min temperature
anomalies will tend to be higher, yielding greater coverage of
above normal readings.
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
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