Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
714 AM EST Fri Jan 26 2018

Valid 00Z Sat Jan 27 2018 - 00Z Sat Feb 03 2018

Into the weekend the models have developed some disagreement over
the westward extent of enhanced moisture forecast to be most
concentrated east of the state, influencing the duration of
rainfall over/near the Big Island.  The ECMWF with higher
precipitable water values and rainfall did trend somewhat lower
versus its prior run while the 00z GEFS mean is similar to GFS
runs so a compromise seems appropriate at this time.  Regardless
of moisture levels, southeasterly flow ahead of a front
approaching/crossing the state should provide some enhancement to
Big Island rainfall.

Behind the weekend front expect winds to turn
northeasterly/easterly while a pocket of drier air will likely
keep any scattered showers on the light side into Tuesday.  Albeit
with some uncertain details, guidance is consistent at least with
the idea of the pattern becoming more active by midweek as fairly
deep mean troughing aloft becomes established to the northwest of
the islands.  Into Wednesday there is reasonable consensus that a
front approaching the northwestern islands will be accompanied by
a band of rainfall that could be locally moderate/heavy for a time
before weakening as best dynamics aloft lift northeastward. 
Guidance is still suggesting that this front may stall over the
islands as an upstream front approaches, allowing for a continued
southwest to northeast flow of moisture aloft.

Guidance differences become more pronounced for the upstream front
by Thursday-Friday, with GFS runs becoming quite progressive
relative to most other guidance due to having greater southeastern
amplitude for the upper trough.  The 00z GEFS mean hedges a bit
faster than the ECMWF/CMC and their means but in principle is much
closer to those solutions than the GFS.  Therefore would recommend
a compromise among the GEFS mean and ECMWF/ECMWF mean as the best
starting point at this time.  This scenario would keep more
moisture over the state than GFS runs by the end of the period
next Friday.

Rausch