Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
830 AM EDT Sat Oct 27 2018

Valid 00Z Sun Oct 28 2018 - 00Z Sun Nov 04 2018

...Heavy Rain Threat for Hawaii Next Week...

Models and ensembles reasonably agree that modest trade/moisture
flow under a weakness aloft should support windward favored
showers over the next few days. Mid-upper level trough
amplification is expected to the northwest of the state early next
week as impulse energies dig to the lee of an amplified west
Pacific subtropical ridge aloft. This will disrupt trades and
force a convection focusing cold front across the islands
Tuesday/Wednesday. 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 act to closed off a low in a sufficient subset of
models/ensemble members to suggest a reasonable threat to linger
moisture and heavier precipitation near the Big Island Thursday
into next Saturday in a pattern with return trade flow.

Schichtel