Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
809 AM EDT Fri Apr 21 2017

Valid 00Z Sat Apr 22 2017 - 00Z Sat Apr 29 2017

A weakening surface front whose associated band of moisture is
bringing some rain to the central part of the state will gradually
dissipate heading into the weekend.  Expect somewhat of a drying
trend as the lingering moisture lifts off to the northwest
Saturday-Sunday.  Precipitable water values should be lowest early
next week, perhaps reaching as low as around an inch at times. 
With differing details guidance still shows some pockets of higher
moisture reaching the area from the northeast around
Wednesday-Thursday.  High pressure in the process of tracking
north of the state will settle over the eastern Pacific for
several days.  A ridge extending west from the high will maintain
trades over the islands well into next week, tending to focus what
rainfall occurs over windward terrain. 

The primary guidance difference of interest arises by
Thursday-Friday as a trough aloft to the northwest of the state
amplifies and pushes a wavy front closer to the area.  The 00z GFS
which forms a pronounced upper low to the west-northwest by 00z
Saturday becomes a deep/amplified outlier relative to the ensemble
spread as mid level heights within the trough locally reach at
least six standard deviations below normal.  To a less extreme
extent the 06z GFS and 00z CMC also become fairly deep with the
trough while recent ECMWF runs and 00z ensemble means offer what
may be a more plausible trough evolution at this time.  Relative
to typical uncertainty/forecast error at the day 7 time frame,
surface forecasts from the non-00z GFS solutions cluster fairly
well.  Thus a compromise among the 06z GFS/00z ECMWF and 00z
GEFS/ECMWF means offer a reasonable starting point for frontal
position through next Friday.  As this front approaches expect
trades to turn more from the southeast late in the week. 

Rausch