Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
344 PM EDT Tue Sep 03 2019
Valid 12Z Sat Sep 07 2019 - 12Z Wed Sep 11 2019
For the large scale pattern the majority of guidance shows a mean
trough centered over the Bering Sea into the North Pacific and a
mean ridge over mainland Alaska/northwestern Canada. There is
significant spread/uncertainty for embedded features--in
particular trough energy heading into the mean ridge, and toward
the latter half of the period for specifics of low pressure that
should have some input from one or more tropical systems.
A leading system should be approaching the southwest mainland at
the start of the period very early Sat. Even with the relatively
shorter lead time there are meaningful differences for how upper
support splits and relative strength/position of low centers over
the eastern Bering and northern Pacific/Gulf of Alaska. The scale
is sufficiently small (for lower than average predictability) that
a consensus blend provides the best starting point for the
forecast.
From Sun onward there is a widening array of possible solutions
for the system that starts the period over/near the western
Aleutians. Based on the current array of guidance and continuity,
the 00Z ECMWF's idea of bringing a fairly strong shortwave
progressively across the mainland by Tue appears to be a less
probable option. At least the new 12Z run has trended more
sheared with the shortwave. Otherwise there is still the
significant difference between the ensemble means that have a
slow-moving open trough and GFS/CMC that undercut the mainland
Alaska/northwest Canada ridge closed low (with important latitude
differences determining how much precipitation reaches the Alaska
Peninsula/Kenai Peninsula). Especially late in the period prefer
leaning toward the ensemble means to represent the favored non-00Z
ECMWF scenario for now given the uncertainty in the upstream
forecast.
During the first half of the period most guidance shows an
interaction of mid-latitude flow over eastern Asia and
northward-tracking Typhoon Lingling. The best clustering of
guidance suggests that low pressure related to this interaction
may reach the vicinity of the western Bering Sea by day 8 Wed.
Also worth noting to the east is Tropical Storm 14W whose track
should approach Japan in a few days per the 12Z JTWC advisory.
Most solutions suggest this feature will be absorbed into the
larger scale upper trough/surface low. Models/ensembles typically
have difficulty resolving mid-latitude/tropical interactions and
in this case there are two tropical systems involved, so there is
definite the potential for significant changes in the forecast
over coming days.
Based on the above considerations today's forecast started with a
primary emphasis on operational model solutions from early day 4
Sat into early day 6 Mon. Then the forecast rapidly transitioned
toward greater ensemble mean weight (more 00Z ECMWF mean versus
06Z GEFS/00Z NAEFS to provide better continuity) and downplaying
the 00Z ECMWF over the mainland. While details remain uncertain,
the combination of systems originating from the Bering Sea upper
trough should promote highest five-day precipitation totals over
the southwest corner of the mainland and Kenai Peninsula. It will
take additional time to determine wind/rain effects over the
Aleutians and Bering Sea by next Tue-Wed.
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
- Heavy precipitation across portions of mainland Alaska and the
Aleutians, Fri-Sat, Sep 6-Sep 7.
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