Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
807 AM EDT Mon Apr 20 2015

Valid 00Z Tue Apr 21 2015 - 00Z Tue Apr 28 2015

High pressure initially to the north of the state will settle over
the eastern Pacific, weakening gradually as it drifts southward. 
This high will support fairly brisk trades for at least the first
half of the forecast period.  Meanwhile into late week there
remains good model/ensemble clustering with respect to an
unusually deep/southward upper low to the northwest of the
islands.  Operational models continue to indicate that heights
with this feature may reach as low as 4-6 standard deviations
below normal for a time.  This feature will reflect at the surface
as an inverted trough or frontal zone around 165W longitude,
helping to turn low level winds over the islands more from the
southeast.  Currently the associated band of heavy rainfall should
remain just west of the northwestern islands.  However after early
Sat the 00z ECMWF/ECMWF mean suggest the system may drift farther
eastward than latest GFS/GEFS mean runs thus increasing potential
for the enhanced rainfall to extend into the northwestern islands.
 Consensus has nudged east compared to 24 hours ago, but with such
features it is still reasonable to give at least partial weighting
to slower solutions.  Therefore would ultimately recommend a
compromise between the 00z ECMWF and the 00z/06z GFS.  Even if the
band of heaviest rainfall does not reach the state, the models
generally agree that precipitable water values should gradually
increase.  As a result shower activity generally but not
exclusively confined to windward locales may become a little
heavier with time.  By next Sun-Mon another surface high reaching
the central Pacific may begin to strengthen trades once again.

Rausch