Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
805 AM EDT Fri Jul 20 2018

Valid 00Z Sat Jul 21 2018 - 00Z Sat Jul 28 2018

Today's guidance agrees well that moderate trades will prevail
through next week.  Expect any shower activity into at least the
first part of the weekend to be relatively light and
windward-focused given precipitable water values near or slightly
below climatology and presence of modest ridging aloft. 
Model/ensemble consensus shows a band of enhanced moisture
reaching the islands from the east in the Sunday-Tuesday period,
likely accompanied by a weak surface trough.  However solutions
differ regarding to what extent rainfall will increase with this
moisture/surface feature (06Z GFS/00Z GEFS mean heaviest).  There
is also disagreement over trailing moisture that may reach the
islands after Tuesday with GFS/GEFS guidance generally showing
higher precipitable water values than the ECMWF.  Overall would
favor an intermediate approach from the weekend onward.  During
Sunday-Tuesday the modest ridge aloft may restrain activity a bit
from the heaviest end of the spread in a scenario that does
generally favor some rainfall enhancement.  Then most guidance
shows gradual strengthening of the ridge aloft mid-late week,
which should keep rainfall in the lighter half of the spectrum
with less sensitivity to precise precipitable water values.

Rausch