Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
717 AM EST Sun Mar 04 2018

Valid 00Z Mon Mar 05 2018 - 00Z Mon Mar 12 2018

An upper trough/embedded low currently settling over the
east-central Pacific will begin the longer-term drier trend that
has been advertised by guidance.  Precipitable water values will
decrease fairly slowly over the next couple days ahead of a
weakening front which may be accompanied by some pockets of
moderately enhanced rainfall.  Then expect a push of drier air
(PWATs possibly decreasing below 0.75 inch at some locations) that
should reach at least as far south as the northwestern/central
islands.  Models/ensembles still show southwestward elongation of
upper troughing behind the initial east-central Pacific trough but
today's solutions indicate a farther northwest axis.  As a result
the moisture gradient ends up being closer to the Big Island than
seen in continuity.  The 00z GFS could be a little tight with its
gradient however as the 06z GFS has weakened the core of upper
energy a bit in favor of the 00z ECMWF while the ensemble means
are a little weaker yet.

Continuing from yesterday, GFS to a modest degree GEFS solutions
maintain a somewhat broader/eastward depiction of the amplified
Pacific pattern aloft by next weekend versus the ECMWF/CMC and
their means.  The main effect of this difference on Hawai'i is
that western Pacific high pressure that merges with an initial
central Pacific high extends farther southeastward especially in
the 00z/06z GFS, leading to stronger trades.  The 00z GEFS mean
offers somewhat of a compromise.  The amplified nature of the
overall pattern seems to favor leaning somewhat toward the ECMWF
cluster and to some extent GEFS mean but teleconnections relative
to the strong core of positive height anomalies south of the
Aleutians by late in the period leave the door open to some
broadening of the pattern.  Either way expect a farther
southeastward push of lower PWATs next weekend.

Flow around central Pacific high pressure and eventual merging
western Pacific high will support northeasterly trades through the
period--though subject to uncertainty in strength by next weekend
as noted above.  What rainfall occurs should be most favored over
windward terrain and the Big Island which will tend to see higher
PWATs relative to other parts of the state from midweek onward.

Rausch