Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
641 PM EDT Mon Jul 06 2020
Valid 12Z Fri Jul 10 2020 - 12Z Tue Jul 14 2020
...Overview and Guidance/Predictability Assessment...
The large scale pattern appears rather blocky and slow to evolve,
aside from parts of the Northeast Pacific during the first half of
the period when an initial wave well southwest of the Panhandle
will weaken as it tracks near/south of Haida Gwaii. The best
consensus of guidance shows an elongated trough aloft with a
possible embedded upper low becoming established over the northern
mainland in response to flow around an Arctic upper high. Farther
south there will be a system over the North Pacific late this week
into at least the weekend. An associated front may eventually
lift into the Aleutians as the eastern side of the system is
pulled eastward. This system could linger into next Mon-Tue in
weakened form or be replaced by another wave approaching from the
western Pacific.
As is typically the case in such patterns there is considerable
spread for the specifics of energy that reaches the mainland from
the northeast. The 12Z CMC (bringing some mainland energy into a
deep Bering closed low) and 12Z GFS (showing a second deep upper
low arriving during the latter half of the period, as does the
CMC) are the two least confident solutions based on the full array
of models/ensembles. The 12Z ECMWF shows this second low but at
least is slower than the 12Z GFS/CMC. Historically in situations
of retrograding energy the deepest and/or fastest solutions tend
not to verify that well.
Details are murky with respect to how North Pacific low pressure
may elongate/split apart, as well as possibly interact with
upstream energy. Toward the end of the period the 12Z GFS is a
fast extreme with a stronger system whose influence reaches the
Aleutians by next Tue in that model. Overall a model-ensemble
approach best resolves the various question marks that exist for
the North Pacific evolution.
Based the latest array of guidance through the 12Z cycle (and 00Z
for the ECMWF mean) today's forecast started with a mostly 12Z
operational model blend early in the period. Then the blend
eliminated the 12Z CMC and transitioned GFS input from the 12Z run
to the 06Z version, while increasing GEFS/ECMWF mean weight to a
total 40-50 percent by days 7-8 Mon-Tue. The ECMWF component used
some of both the 12Z run and prior 00Z run due to different
mainland and North Pacific details which may or may not be better
in the newer run.
...Weather/Hazard Highlights...
The elongated upper trough settling over the mainland will lead to
fairly broad coverage of below normal temperatures (especially
daytime highs) during the period. Expect the greatest negative
anomalies to be over the North Slope and the southeast/Panhandle.
The best potential for above normal temperatures will be over the
extreme western mainland near the Bering coast. Highest rainfall
totals from late this week into the first half of next week should
be over the southern mainland, near the Alaska Range where daytime
heating along with cooling temperatures aloft should promote
multiple episodes of rain. The Panhandle may see some
precipitation from moisture on the northeastern periphery of a
Northeast Pacific system likely to track toward Haida Gwaii late
this week/weekend. Meanwhile one or more surface systems/fronts
over the North Pacific may extend far enough north to bring some
rainfall into 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