Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Sat Sep 08 2018

Valid 12Z Wed Sep 12 2018 - 12Z Sun Sep 16 2018

...Guidance/Uncertainty Assessment...

A longstanding upper ridge with height anomalies in the 3 to 4
sigma above climatology range will begin the forecast over far
western Alaska. The ultimate pattern changer is a strong
north-central Pacific system taking aim at the Aleutians late
Wednesday. While it will not break the ridge axis down, its
position does adjust eastward while heights re-lower across the
Bering Sea during the Day 6-8, September 14-16 window. It appears
to be the catalyst to finally erode this mentioned upper ridge
toward the end of the period. While this occurs, several upstream
intrusions will take place as embedded shortwaves push out of the
Bering Sea into mainland Alaska. Operational models continue to
waver with the timing of the progression of this pattern change.
The 12Z GFS/00Z ECMWF each have been quicker to do this although
ensemble spaghetti plots depict a variety of scenarios in play.
Given the strength of the multi-day upper ridge, preferred to keep
a solution that kept the ridge in place longer. Thus, stuck closer
to the 06Z GFS and ensemble means throughout the period with only
minor contributions from the 00Z ECMWF. Eventually a more ensemble
mean-based approach was utilized toward the end of next week.


...Weather Highlights/Hazards...

During the period, temperatures will gradually cool as the
persistent upper ridge advances eastward and weakens in time.
Initial highs should stay in the mid 60s across southern Alaska
before falling back down into the mid/upper 50s by next weekend.
Additionally, precipitation chances will be on the increase from
west to east given the onslaught of shortwave energy moving across
the Bering Sea. On Wednesday, initial chances should stay across
the Aleutians with the upper low approaching and northeastern
Alaska along a northwest-southeast oriented convergence zone. The
driest spot will likely be southeastern Alaska with neither the
GFS or ECMWF showing any precipitation chances given the
prevalence of ridging overhead.


Rubin-Oster


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