Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
711 AM EST Wed Dec 26 2018

Valid 00Z Thu Dec 27 2018 - 00Z Thu Jan 03 2019

Compared to 24 hours ago, guidance agreement has improved
significantly regarding the duration of the heavy
rainfall/thunderstorm threat that has been forecast from late this
week into the weekend.  From about late Thursday onward a cold
front heading into the region will decelerate and eventually stall
as energy within the southern periphery of a sharpening shortwave
aloft drops over and south of the state, possibly closing off an
upper low.  The front will disrupt trade flow and serve as a focus
for areas of potentially heavy rainfall/thunderstorms with the
northwestern half of the main islands seeing the greatest
probability of occurrence.  By mid-late weekend the stalled front
should begin to dissipate.  Trades will become established once
again as trailing high pressure reaches well northeast of the
islands and merges with another high already over the eastern
Pacific.  During the first half of next week models/ensembles
agree in principle upon a pronounced decrease in precipitable
water values and terrain-focused rainfall as elongated upper
ridging to the west/north sags southeastward.  The primary
guidance difference is how quickly this drying trend occurs. 
Currently the 00Z ECMWF is slowest to bring in drier air, with
even the ECMWF mean showing a greater decrease in precipitable
water values though not to the extent of latest GFS/GEFS mean runs.

Rausch