Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
743 AM EDT Sun Mar 18 2018

Valid 00Z Mon Mar 19 2018 - 00Z Mon Mar 26 2018


...A heavy rain/flash flood threat in the western islands this
coming Thursday and Friday...

The models high pressure building to the north of the islands
today.  The anticyclonic flow keeps the deep er moisture to the
south and west at bay temporarily with shower coverage
predominantly in windward terrain.

A transition occurs early in the week as the high pressure ridge
drifts east of the islands.  Low level flow becomes southeast to
south and low level moisture returns to the region. There is a
lack of large scale low level convergence so showers will continue
to be driven by windward low level convergence areas, including in
terrain.

A new system moves slowly in from the west/Papahanaumokuakea
Marine National Monument which has an associated cold front that
drifts slowly towards the western islands late next week and
weekend. Precipitable water values are forecast to rise into the
1.75-2" range.  Low level convergence increases as the front
approaches, so showers should increase Thu and Fri in the western
islands, with the gfs and ecmwf showing higher coverage and
amounts Thu and Fri as a result.  The high available moisture
which would lead to the possibility of high rain rates.
Considering wet conditions from recent rainfall, flash flooding is
a possibility.

The models and ensemble means are in better than average agreement
on the movement of the boundary through Friday.  The guidance
splits with the 06z GFS showing a slower moving boundary drifting
east across Hawaii next weekend, continuing a heavy rain threat
for the central and eastern islands.  The 00z ECMWF shows the
boundary dissipating quickly and drying aloft, with much less
rainfall.  The ECMWF and GEFS ensemble members are split, with
some showing drying and other continuing the rainfall threat.

Petersen