Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
826 AM EDT Tue Apr 21 2015

Valid 00Z Wed Apr 22 2015 - 00Z Wed Apr 29 2015

High pressure initially NNE of the state will settle over the
eastern Pacific, weakening gradually as it drifts southward. This
high should support fairly brisk trades into midweek before
weakening. 

Meanwhile, there remains good model and ensemble clustering with
respect to an unusually deep/southward closed upper low developing
to the northwest of the islands. 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/frontal zone around 165W
longitude, helping to turn low level winds over the islands more
from the southeast allowing for deeper moisture return. However,
the heaviest associated band of heavy rainfall settles just west
of the northwestern islands late week into the weekend. 

The 00z ECMWF/ECMWF ensemble mean suggest the system may drift
farther eastward than recent GFS/GEFS mean runs over the weekend
into early next week, thus increasing potential for the enhanced
rainfall to extend more through the islands. Consensus has nudged
eastward over the past 48 hours, but with such closed features it
is still reasonable to give at least partial weighting to slower
solutions. Recommend a compromise between the 00z ECMWF and the
00z/06z GFS that would bring heavier rainfall potential gradually
into the northwestern islands and allow precipitable water values
and showers to gradually increase over the state. In the wake of
this scenario, another surface high reaching the central Pacific
may begin to strengthen trades once again in about a week.

Schichtel