Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
812 AM EDT Fri Oct 13 2017

Valid 00Z Sat Oct 14 2017 - 00Z Sat Oct 21 2017

Latest guidance is consistent in showing a pronounced
strengthening of trades as a surface trough initially over the
western islands departs from the area.  Surface high pressure
expected to settle to the north of the state late Sunday into next
week should begin to weaken somewhat toward next Friday.  Trades
will likely still be brisk at that time but with somewhat less
windy conditions than early-mid week.  Into the early part of this
weekend, rain and potential thunderstorms may be locally heavy in
association with a weakness aloft and a band of 1.50-2.00 inch
precipitable water values passing over the state.  Drier air will
move in late weekend into next week, leading to a lighter trend
for generally windward-focused rainfall.  Toward Thursday-Friday
there is still some disagreement over the potential evolution/path
of some weak upper level energy with corresponding differences in
the northward extent of lower latitude moisture.  Among current
guidance the 00z GFS/GEFS mean show the best defined upper level
feature with more moisture reaching at least as far north as the
Big Island.  The 06z GFS made a significant adjustment in the 00z
ECMWF direction, with a stronger upper ridge just to the north
leading to a more suppressed/sheared weakness aloft and
maintenance of precipitable water values at values below
climatology.  The full range of model/ensemble guidance suggests
there is still room for some energy aloft/moisture return but that
the most likely scenario is somewhat more than halfway toward the
drier 06z GFS/00z ECMWF versus the 00z GFS/GEFS mean.

Rausch