Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
714 AM EST Thu Jan 31 2019

Valid 00Z Fri Feb 01 2019 - 00Z Fri Feb 08 2019

Expect light to moderate showers to continue into and through the
weekend.  Initially there will be a strong gradient between a
surface trough to the east of the islands and high pressure to the
north, supporting fairly strong winds that will focus rainfall
over northward/northeastward facing terrain.  Some rain may stray
into other locations as well.  Upstream shortwave energy and a
narrow band of moisture settling southward over the state will
maintain rainfall potential through at least a portion of the
weekend.  Meanwhile the surface trough east of the state will
weaken late this week as a supporting upper low and energy
dropping into the western side of the circulation shear out. 
Greater dominance of high pressure will lead to a more easterly
component for low level winds and corresponding focus for rainfall.

Early next week there is general model/ensemble agreement for a
brief drying trend but most solutions suggest that precipitable
water values will not decrease as much as forecast by 00Z/06Z GFS
runs.  From late Tuesday to Thursday upstream shortwave energy
will gradually amplify as it moves toward/into the area, likely
increasing rainfall coverage and amounts once again.  Overall the
guidance is reasonably well clustered for a forecast 6-7 days out
in time--though with the GFS/GEFS mean slightly faster than the
ECMWF/CMC and their means for the upper trough, an intermediate
timing likely provides the most reasonable starting point for
specifics.  The same preference applies to the surface where the
slower cluster aloft would lead to greater interruption of the
trades.

Rausch