Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
806 AM EDT Fri Apr 17 2015

Valid 00Z Sat Apr 18 2015 - 00Z Sat Apr 25 2015

The islands should see lighter and more scattered shower activity
during the weekend and early next week as a ridge aloft builds
over or just north of the state and precipitable water values
decline to near or a little under an inch.  Expect trades to
become stronger in this time frame, first due to a southwestward
building of eastern Pacific high pressure and then a central
Pacific high passing to the north of the region behind a
mid-latitude cold front.  Low level winds will likely turn more
southeasterly by next Tue or Wed as the high continues eastward.

Since yesterday the majority of guidance has made some significant
changes in the handling of closed low energy reaching just east of
180 longitude by early Tue, with an increase in spread as well. 
The latest trend is to keep this feature more separated than
before, which ultimately leads to meaningful differences in when
it gets picked up by the westerlies.  The 00z GFS/ECMWF/CMC and
ECMWF mean form the most similar and intermediate cluster with the
06z GFS much faster to capture the upper low and the 00z GEFS mean
the slowest.  Consensus trends aloft increase the probability that
a leading front will approach/reach the northwestern islands in
the latter half of the week and increase rainfall to some degree. 
This front would separate easterly/southeasterly winds to its east
from northerly/northeasterly winds to its west.

Rausch