Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
647 PM EDT Sat Jun 20 2020
Valid 12Z Wed Jun 24 2020 - 12Z Sun Jun 28 2020
...Overview and Guidance/Predictability Assessment...
The guidance remains fairly similar in principle for the large
scale pattern. A persistent Arctic upper low will have a trough
extending southward into the Bering Sea, with a leading surface
front over the northern mainland likely to lift northward and
weaken toward the end of the week as the upper trough drifts
westward a bit. Latest consensus expects a system initially well
south of the Aleutians to track northeastward into the Gulf of
Alaska by early day 6 Fri. Farther west there is good agreement
on an upper trough reaching/evolving over the central
Pacific/Aleutians by the latter half of the period. However there
is still considerable uncertainty over the specifics of this
trough and the associated surface reflection.
Starting with some 00Z guidance and more emphatically in the 12Z
cycle, there has been a notable shift toward a more progressive
and moderately southern track for the system over the North
Pacific as of midweek and now expected to track into the Gulf of
Alaska by Fri. Yesterday's 12Z CMC was one solution that was
close to this idea 24 hours ago. The key to this change appears
to involve a deeper upper trough/possible embedded low that
settles for a time over the Bering Sea. The 12Z GEFS/CMC ensemble
means are slower than the operational model consensus for the
surface system while they are weaker than other guidance with the
Bering upper trough. Latest ECMWF means are also weaker than
operational guidance with the Bering upper trough but at least
shape the flow in a way that is more similar to the operational
models--thus leading to a surface progression also more like the
models. Additional changes may be in store but at this time it is
hard to argue against 12Z operational model consensus. Thus the
12Z models (more GFS/ECMWF relative to the CMC/UKMET) served as
the basis for the forecast during the mid-late week time frame.
Not surprisingly the operational models continue to vary among
each other and from run to run for specifics of the Arctic upper
low and surrounding trough while the ensemble means have been a
lot more stable. The operational model blend favored during the
first half of the period kept the forecast fairly close to the
means and then increasing 12Z GEFS/00Z ECMWF ensemble input to
50-60 percent provided stability in the forecast through next
weekend.
The ensemble means are remarkably similar through day 8 for the
general upper trough forecast to reach the central
Pacific/Aleutians next weekend, with a modest surface wave
tracking into the Aleutians. Operational models still reflect a
variety of ideas over how this trough will evolve, depending on
specifics of western Pacific energy and northern stream flow.
Thus far they suggest decent potential for an embedded closed low
that would result in a stronger and possibly slower system. The
aforementioned model/ensemble mean blend reasonably accounted for
this range of possibilities.
...Weather/Threats Highlights...
The mainland will continue to see periods of convection/rainfall
from midweek into the weekend, with the front over the northern
mainland mid-late week and shortwaves within the upper trough
aligned near/just off the western coast of the mainland through
the period providing some of the focus. The North Pacific system
currently expected to track into the Gulf of Alaska by late week
should bring a period of enhanced precipitation at least to the
Panhandle and possibly portions of the southern coast if the storm
track is sufficiently far north. Some moisture may reach the
Aleutians by next weekend depending on specifics of the system
forecast to track into the region. Below normal highs will
prevail over much of the state from midweek through next weekend.
Greatest negative anomalies should be mid-late week over the
northwest as well as south/southeast and Panhandle. Expect
anomalies for lows to be more moderate with some pockets of above
normal lows possible away from the cool northwestern/southeastern
areas.
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