Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 PM EDT Fri Aug 31 2018

Valid 12Z Tue Sep 04 2018 - 12Z Sat Sep 08 2018

An already amplified upper flow regime across much of the northern
hemisphere appears likely to become even more amplified over the
next week, influenced at least in part by the extratropical
transition of Typhoon Jebi across eastern Asia. The deterministic
models diverge in details but at least clustered in principle
toward a highly amplified or cutoff pattern as a lead upper low
drifts along 45-50N south of the Gulf and another meanders across
the Aleutians. The ensembles seem to have come into somewhat
better agreement over the past day on the track of Jebi as it
undergoes extratropical transition across eastern Asia by the
middle of next week. The resultant trend has been toward further
amplification of an upper ridge axis across the Aleutians and the
Bering Sea by late next week, and a slower eastward progression of
the upper low amplifying west of the ridge. Additionally, greater
ridge amplification would potentially open the door to northerly
mid/upper-level flow across Alaska with increasing influence of
Arctic shortwave energy. The details surrounding this expected
flow amplification still remain somewhat murky, however, with the
position and strength of the expected ridge still uncertain.

The ECMWF and GFS showed relatively good consensus with the
ensemble means initially on day 4 (Tue), and a majority
deterministic blend based on these solutions was used as a
starting point. From day 6 onward, as uncertainty begins to
gradually increase, ensemble mean (ECENS and GEFS) weighting was
boosted to comprise a majority of the forecast. The ECMWF
continued to show the best clustering with these ensemble means
through next week, and thus a substantial minority component of
the ECMWF was continued through the period.

Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html