Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
717 AM EDT Mon Sep 12 2016

Valid 00Z Tue Sep 13 2016 - 00Z Tue Sep 20 2016

Early this week, a tropical wave and embedded weak area of low
pressure will cross the islands. The wave departs to the west
mid-week, but the mid-upper level trough remains overhead for a
few more days. Anomalous precipitable water (pw) will persist over
the islands, especially the Big Island.  The operational gfs and
ecmwf increase pw values to 2 inches. While this moisture in
place, locally heavy showers are possible in areas where ascent
occurs.

Over this coming weekend the 06z GFS/00z GEFS/00z ECMWF/00z ECMWF
ensemble mean nudge the upper trough east and a deep layer ridge
crests and builds to the north of the islands.  Both the gfs and
ecmwf produce strong advection of drier air with pw values
dropping next Sun 19 Sept. This would results in a sharply reduced
threat of moderate to heavy rainfall in the islands.  Trade winds
become brisk out of the east northeast.

Early to middle portions of next week looks as though high
pressure moves east and there is potential for a tropical wave or
disturbance to approach from the east.  This would result in the
trades slackening and presents the possibility of a return of high
layer relative humidity values.

Petersen