Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Tue Sep 18 2018

Valid 12Z Sat Sep 22 2018 - 12Z Wed Sep 26 2018

Rather complicated Pacific upper pattern will unfold over the next
week or so with several distinct anomaly centers each attempting
to drive the flow. Teleconnections from a few of the stronger
centers show an east-west spread in the favored ridge axis
along/east of 150W by next week which leads to a fair amount of
disagreement among the guidance.

For the weekend, an upper high is forecast to drift eastward
between 40-45N south of Alaska with SW flow through the Aleutians
into Southcentral. The GFS runs were quicker with the associated
surface system but was on the far east side of the ensemble spread
and was not preferred. The 00Z ECMWF/UKMET were close to the 00Z
ECMWF ensemble mean and formed a good starting point. Next week,
that system should dissipate as heights rise into the Gulf but the
boundary should remain in place back to the southwest. Another
wave is forecast to lift northeastward Mon/Tue but there was even
more spread in the guidance as it was unclear if the source would
be from the south or the west off Japan (or both, per the GFS).
Trended mostly toward the 00Z ECMWF ensemble mean with some GEFS
influence and detail from the ECMWF, but with low confidence. At
least the ensembles agree on building the ridge into the Gulf
and/or the Panhandle as a deeper system takes shape near the
central Aleutians.

Rainfall will be widespread across the Aleutians and Southcentral
into at least the northern Panhandle with a fairly consistent
storm track. Both the parallel NBM and the in-house bias-corrected
ensemble mean show a swath of 2-5" over the 5-day period and
locally 5-10" in favored areas.

Fracasso


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