Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
825 AM EDT Sun Sep 23 2018

Valid 00Z Mon Sep 24 2018 - 00Z Mon Oct 01 2018

An upper ridge drifts east of the islands early in the week.  The
latest models and ensembles agree that the upper low to the
northwest of Hawaii early in the week moves slowly northeast with
time. The trailing low level front remains west of Hawaii.  The
confluent flow east of the front sets up deep return moisture
advection, as the models indicate potential for high 2-2.25 inch
precipitable water plume to cross the western islands in the
middle of the week. This supports enhanced showers and potential
for locally heavy rainfall.

Both the 00z ECMWF and 06z GFS forecast low level ridging to build
back in next weekend, keeping the moisture plume with highest
precipitable water values from reaching the eastern islands, and
even producing drying aloft in the big island and Maui. The
resumption of easterly trades next weekend brings scattered
showers with a focus on areas of coastal convergence and upslope
flow in terrain.

The 06z GFS lags a few hours behind the 00z ECMWF and Canadian in
moving the upper low northeast of HI early in the week, but
catches up afterward. There is good agreement among the GFS/GEFS
Mean/ECMWF/ECMWF Mean in building a low-mid level ridge north of
HI next weekend. 

Petersen