Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
839 AM EDT Mon Oct 29 2018

Valid 00Z Tue Oct 30 2018 - 00Z Tue Nov 06 2018

...Heavy Rain Threat for Hawaii...

It remains the case that modest trade and moisture flow under a
weakness aloft will favor lingering windward showers today.
Mid-upper level trough amplification is meanwhile expected to the
northwest of the state as impulse energies dig to the lee of an
amplified west Pacific subtropical ridge aloft. This will disrupt
trades by Tuesday coincident with potential approach of a surface
low/tropical moisture surge. This will be followed into midweek
with passage across the islands of a convection focusing cold
front. Deep layered moisture influx/pooling in advance of the
front will fuel a marked increase in rainfall/thunderstorm
potential including a local risk of runoff issues. The heavy
rainfall threat potential is high with this pattern given the
surge of tropical moisture from the ITCZ with both the GFS and
ECMWF showing a 2.25 inch precipitable water maxima in advance of
the wave crossing the islands, plus the synoptic lift provided by
the approaching low pressure center. Mesoscale
processes/convection will also play a role in focusing rainfall.
Reinforcing energy feeding into the base of the main trough
position aloft will then act to closed off a low to offer a
continuing threat of lingering moisture and heavy downpours for
the Big Island over the weekend in a pattern with return trade
flow as high pressure becomes re-established to the north and
northeast of the state.

Schichtel