Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EDT Tue Aug 11 2020
Valid 12Z Sat Aug 15 2020 - 12Z Wed Aug 19 2020
...Guidance Evaluation and Preferences...
Most guidance shows a slow moving system to the southwest of the
Alaska Panhandle through the weekend and likely into early next
week, with the flow of moisture ahead of it bringing significant
precipitation to portions of the Panhandle. Somewhat more
progressive flow upstream will bring a western Pacific system into
the Bering Sea during the weekend while an upper ridge between
these systems builds from the Bering into the mainland.
After Sat the guidance diverges into two very different clusters
with respect to how quickly eastern Bering/mainland upper ridging
erodes and the degree of phasing that occurs between
Arctic/Siberia flow and the Bering system--leading to
significantly different effects on the mainland by the first half
of next week. In varying ways the latest runs of the ECMWF/ECMWF
mean along with CMC/CMC mean/UKMET all show a more persistent
ridge aloft and minimal phasing. The CMC does show more digging
of northern stream flow than others in this cluster but it occurs
behind the Bering system. The result in the majority cluster is a
weak and suppressed solution for the surface low. On the other
hand recent GFS runs have displayed quicker erosion of the upper
ridge and much more phasing, leading to a deep northern Bering
storm by next Mon-Tue with significant precip/wind impacts over
parts of the western mainland. The GEFS mean is more subdued but
still hints a lot more at the GFS idea than the ECMWF cluster.
Since yesterday's continuity more closely resembled the ECMWF
scenario and there has been significant model spread/variability
for details of Arctic/Siberia flow thus far, preference favors
holding onto the majority ECMWF idea. The GFS/GEFS solution and
potential effects still require monitoring even though their
probability is low enough not to include in the manual
deterministic forecast at this time. Also of note, the 12Z GFS is
essentially on its own in how far southward it pulls its
northeastern Pacific system by early next week.
In light of the above considerations today's forecast emphasized
the 12Z and 00Z ECMWF runs along with the 12Z CMC/UKMET during the
first half of the period, aside from the 12Z GFS still being close
enough to consensus to include on day 4 Sat. Then the blend
trended toward a more even model/mean weight to reflect the
majority cluster for large scale evolution and downplay less
confident details. Total ensemble mean input was about 2-1 in
favor of the 00Z ECMWF mean relative to the 12Z NAEFS mean which
at least had some CMC ensemble influence to tone down the GEFS
portion.
...Weather Highlights/Threats...
The most confident part of the forecast is with meaningful
precipitation over portions of the Panhandle, with current
consensus suggesting the southern half will likely see the highest
totals. If the more likely pattern verifies then flow between the
northeastern Pacific system and upper ridging over parts of the
mainland could bring some of this moisture into the southern
mainland. Meanwhile the system tracking into the Bering Sea and
its associated fronts will bring precip of varying intensity to
the Aleutians followed by a drier trend next week. Assuming the
more likely evolution of a suppressed track next week, this system
should bring minimal moisture to the mainland. However the less
likely GFS solution with strong northern Bering low pressure and
heavy precip/strong winds over parts of the western mainland still
requires watching.
The temperature forecast continues to show cooler than normal
daytime highs over far southern/southeastern areas and the
Panhandle as well as the northern mainland. Upper ridging will
promote warmer than average highs over the Interior and in
particular the west/southwest part of the mainland. Expect lows
to be above normal over most 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
Hazards:
- Heavy rain across portions of the Alaska Panhandle, Sat-Mon, Aug
15-Aug 17.
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