Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Tue Jul 30 2019
Valid 12Z Sat Aug 03 2019 - 12Z Wed Aug 07 2019
There is a loose consensus that ridging will prevail at the
surface and aloft to the south of the mainland while some degree
of upper troughing persists over parts of the Bering Sea and
eastern Asia. This pattern would focus highest precipitation
totals for the five-day period from the eastern Aleutians through
the northern two-thirds or more of the mainland. Beyond this
expected mean pattern there are multiple embedded uncertainties
that limit confidence in specifics.
The strong system expected to cross the northern mainland is one
such forecast problem. There is good agreement for its existence
and potential to bring an episode of strong winds/areas of heavy
precipitation, but agreement and continuity for timing have been
poor thus far. As of the 00-06Z cycles, the 00-06Z GFS/00Z ECMWF
and their ensemble means (plus the CMC mean) provided the majority
cluster--significantly faster than yesterday but not as fast as
the 00Z UKMET or even faster CMC. New 12Z guidance including the
ECMWF has come in with stronger clustering in favor of faster
timing than the aforementioned model/ensemble grouping but slower
than the 00Z CMC. Today's forecast started with the 00-06Z
model/ensemble cluster given the already faster adjustment from
continuity and the GFS being the only 12Z run available at the
start of forecast preparation. Given the adjustments in the full
array of 12Z guidance, an average of the new runs would provide a
more likely--though still not truly confident--solution.
Behind this system there are hints of waviness over the Bering in
response to shortwave energy within the upstream mean trough
aloft. The 06Z GFS/00Z ECMWF offer a reasonable starting point to
reflect this waviness relative to the weaker ensemble means. The
00Z ECMWF mean has the best detail among the means. Specifics
become even more uncertain with upstream energy possibly closing
off an upper low that reaches the Bering, with considerable spread
and run-to-run variability. In addition there is also the
possibility for higher latitude energy to reach the mainland from
Siberia and the Arctic. These issues favor a steady increase of
ensemble weight with time.
Based on the guidance available through arrival of the 12Z GFS,
the forecast started with a 06Z GFS/00Z ECMWF blend for day 3 Sat.
After that time incorporation of ensembles (06Z GEFS/00Z ECMWF
means) led to an even model/mean blend by day 6 Mon and 100
percent ensemble means by day 8 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 rain across portions of mainland Alaska, Fri, Aug 2.
- Heavy rain across portions of mainland Alaska, Sat, Aug 3.
- High winds across portions of mainland Alaska, Sat, Aug 3.
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