Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EST Tue Jan 07 2020

Valid 00Z Wed Jan 08 2020 - 00Z Wed Jan 15 2020

A persistent ridge aloft will help to maintain a strong
east-central Pacific surface high, with surrounding flow promoting
brisk trades over the islands during most of the forecast period. 
This flow will tend to favor windward/mountain focus for rainfall
while other locations could see less frequent periods of rain. 
Over the course of this week an upper low initially east of the
state will elongate westward to the south of the upper ridge.  The
upper low energy and associated moisture reaching/passing through
the region should lead to a period of increased rainfall from
about Thursday into the weekend.  A positive trend in the guidance
over the past day is better clustering for timing the arrival of
enhanced moisture, with the GFS finally adjusting faster toward
the ECMWF/ECMWF mean which have consistently been on the faster
side of the spread.  The GEFS mean has been supporting faster
timing than the GFS though generally not quite to the extent of
the ECMWF.  Now that timing is more agreeable, primary differences
arise Friday onward as GFS redevelopment of the upper low just
west of the main islands is stronger than other guidance.  The
stronger upper low sends the axis of highest precipitable water
values farther north than other solutions.  Among the 00Z
ECMWF/12Z ECMWF mean/00Z GEFS mean the main difference is that the
GEFS mean is slower to push the moisture westward as heights aloft
gradually rise (to a greater degree than in the GFS).  The
difference among non-GFS solutions is generally within typical
guidance error at that time frame, favoring a compromise among the
ECMWF and ECMWF/GEFS means for a single specific forecast.  The
gradual reduction in precipitable water values by next
Monday-Tuesday should lead to somewhat lighter rainfall versus the
Thursday-Sunday period.

Rausch