Heavy Rain in Hawaii, September 23, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sat Sep 23 2017

.SYNOPSIS...
Trade winds will gradually ease during the next couple of days as a
surface ridge to the north weakens. Mainly windward clouds with
light passing showers will prevail through Tuesday. A possible boost
in shower activity is expected around Wednesday as an area of more
organized clouds is forecast to reach the state from the east.
Gentle trade winds will likely persist for the later part of next
week.

&&

.DISCUSSION...
Trade wind weather continues across the islands this evening as a
surface high stays far northeast. Not much low clouds are in the
island vicinity this evening as revealed by the satellite imagery,
though some heavier downpours have developed over the lee area of
Kohala and these downpours may persist through the next couple of
hours. Radar data indicated there are some showers affecting
windward areas of Oahu at this time, with more showers along the
Hamakua coast of the Big Island as well as windward coastal waters
of the rest of the islands. Thus expect mainly fair weather to
prevail for the lee areas of the smaller islands while lee areas of
the Big Island will see some clouds and showers through rest of the
evening, with clearing overnight. Windward areas of all the islands
will see more passing low clouds and showers as well.

Slow changes to the ongoing weather pattern are in store for the
islands though the next few days. High far northeast as well as its
associated ridge will slowly weaken as a cold front far northwest
advances east. This will lead to a gradual decrease in the trades
over the islands. With the lighter trade winds, sea breezes over the
lee areas will become more prominent, with possible more afternoon
clouds over the lee areas. Passing low clouds embedded in the trade
wind flow will continue to reach the area, bringing some showers to
the windward areas at times. There will be enough pressure gradient
across the state for gentle trade winds to persist for much of the
work week.

Moisture associated with a weakening surface trough about 500 miles
east-northeast of the Big island is expected to reach the area by
Wednesday. This will probably lead to an increase in shower
activities for the islands, though forecast solutions indicated a
weak upper level ridge may move to the north of the islands,
with airmass in the island vicinity may remain slightly stable.

&&

.AVIATION...
High pressure northeast of the State will keep a light to moderate
trade wind flow in place through Sunday, with clouds and showers
favoring windward and mauka areas. Brief MVFR cigs/vsbys will be
possible in passing showers across windward areas, but predominantly
VFR conditions are expected through 06Z Monday.

No AIRMETs are in effect and none are anticipated through Sunday.

&&

.MARINE...
Trade wind speeds will remain below the Small Craft Advisory
threshold through the forecast period.

Surf along south facing shores will remain small through early next
week, with mainly background southeast and southwest swell energy
expected. A small, long-period pulse out of the southwest from the
Tasman Sea is expected Sunday, so a slight increase in south shore
surf is anticipated. For the extended forecast, strong to gale-force
southwest winds associated with another low in the Tasman Sea may
lead to a reinforcing, small southwest swell by Tuesday and
extending through midweek.

A small northwest swell will ease tonight, before another small
reinforcement from that direction fills in around Sunday night,
lasting through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

Jelsema/Hui/Kinel