Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
814 AM EDT Tue Mar 20 2018

Valid 00Z Wed Mar 21 2018 - 00Z Wed Mar 28 2018

The most significant aspect of the forecast continues to be the
heavy rainfall threat that the majority of guidance is showing
around this weekend.  There are still varying detail differences
with embedded shortwave energy but there is general agreement that
upper troughing to the northwest of the state will approach the
region in that time frame.  Southwesterly flow ahead of this
feature will likely pull up a band of enhanced moisture, with
precipitable water values possibly reaching or exceeding two
inches at some locations.  Meanwhile a surface trough/embedded
wave may provide some added low level focus for activity. 
Upstream mid-latitude troughing should help to push the upper
shortwave across the islands around Monday, leading to a drier
trend over most locations.  Note that the 00z CMC and a minority
of 00z ensemble members suggest that the shortwave could be a
little slower which would prolong the heavy rainfall threat to
some degree.  For a single deterministic forecast an average among
the ECMWF/GFS and their means would provide a reasonable starting
point for this weekend's event.

From now into late this week, strong east-central Pacific high
pressure will produce brisk easterly winds with some variation in
precise direction depending on location.  The majority of guidance
indicates that the surface ridge will keep most of the deep
moisture/rainfall to the south and west of the state but the
ECMWF/ECMWF mean hold more moisture in the vicinity of the western
islands.  At this time prefer leaning somewhat toward the
non-ECMWF majority but with modest confidence.

Rausch