Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
717 AM EST Sun Jan 28 2018

Valid 00Z Mon Jan 29 2018 - 00Z Mon Feb 05 2018

Gradual progression of the upper shortwave currently crossing the
state will allow for a brief period of drier conditions over most
locations for Monday into Tuesday, while low level winds turn from
northeasterly/easterly to southerly as a front approaches from the
northwest.  Note that even so early in the forecast, there are
some timing differences for the shortwave.  The 00z UKMET/CMC
recommend a compromise between the fast 00z-06z GFS/00z GEFS and
slow 00z ECMWF/ECMWF ensembles.

The front approaching on Tuesday will herald the start of a
generally wetter pattern as upper troughing becomes established to
the northwest of the state.  However guidance thus far has varied
within runs and from day to day with respect to details of frontal
timing/position at any particular time and the amount of moisture.
 Thus confidence has been lower than desired in pinpointing
particular time frames and extent of heavier rainfall episodes.

Into Wednesday expect the aforementioned front and accompanying
rainfall, with potential for some locally moderate-heavy activity,
to progress into the central islands.  This front should
stall/weaken as heights aloft stabilize or slowly rise with the
approach of upstream energy aloft and another front.  The trailing
front has shown greater timing differences over recent days.  GFS
and to some degree GEFS means have been on the fast side of the
spread only to trend slower toward other solutions the next day. 
Slower GFS trends into today have led to better agreement that a
southwest-northeast band of enhanced moisture should persist over
the area late week into the weekend.  However it remains to be
seen if precipitable water values reach levels seen in the GFS, as
other guidance shows somewhat higher heights aloft and thus less
concentration of deep moisture/rainfall.  On the other hand the
general pattern may support greater moisture/rainfall than
indicated by the 00z ECMWF.  Recent late-period GFS biases may
come into play again by next Sunday as it brings much lower
heights aloft over/north of the state versus other solutions,
pushing yet another front farther eastward than most solutions--
aside from the 00z CMC which interestingly is slower than the GFS
with the upper trough.

Rausch