Heavy Rain in Hawaii, August 11, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Sun Aug 11 2019

.SYNOPSIS...
The islands lie in a rather light trade wind environment allowing
for daytime sea breezes and nighttime land breezes to develop.
Showers will be most prevalent during the afternoon and evening
hours as clouds buildup over the islands due to daytime heating. A
more typical trade wind weather pattern is expected during the
middle and latter half of the week as high pressure builds in to
our north.

&&

.DISCUSSION...
A surface high is centered far northeast of the area while a weak
trough of low pressure lingers a couple of hundred miles north of
Kauai. This places the islands in a rather light trade wind
environment. Daytime sea breezes and nighttime land breezes are
dominating under this wind scenario. The 00Z sounding from Hilo
shows that the airmass over the Big Island has become drier and
more stable than early this morning. However daytime along with
lingering moisture over the leeward Big Island slopes has allowed
for showers to develop. There is also a slight chance of
thunderstorms over the leeward Big Island slopes early this
evening with cloud top estimates currently running up to around
35000 feet. Most of the remainder of the state has generally been
dry as indicated by radar imagery.

This rather light trade wind regime is expected to linger on into
Tuesday as the high remains far to our northeast and the trough
lingers north of Kauai. Models do however indicate a slightly
drier airmass moving in from the east. This will likely limit
shower development with daytime heating being the main ingredient
for any showers mainly during the afternoon and evening hours.

During the middle and latter half of the week, the high to our
northeast will strengthen and drift westward while the trough to
our north weakens and lifts further away from the area. This
should allow for trade winds to gradually strengthen to moderate
and locally breezy levels by the end of the week. Shower activity
will be highly dependent on incoming moisture availability with
clouds and passing showers favoring windward and mauka areas with
most leeward locations being generally dry. The exception will be
for some afternoon clouds and showers over the leeward Big Island
slopes due to daytime heating.

&&

.AVIATION...
A light trade wind will persist through at least tomorrow, with
some clouds and showers favoring the windward sides of the
islands. Afternoon seas breezes bringing some clouds and showers
to the leeward and interior areas are expected to diminish
overnight. Some afternoon afternoon towering cumulus over the Big
Island could continue into the early evening hours, before
clearing overnight.

No AIRMETs are in effect this afternoon, and no AIRMETs are
anticipated into the evening hours.

&&

.MARINE...
High pressure northeast of the state will keep light to moderate
trade winds through Tuesday. Winds speeds will increase Wednesday
through the end of next week, likely requiring a Small Craft
Advisory (SCA) for at least the typically windy waters around Maui
County and the Big Island.

The lighter trades will keep east shore surf small through early
next week, with a bump expected during the middle to latter part
of next week as the trades strengthen. A series of southeast and
south swells will keep south shore surf near or slightly above
the summer average this week. Westerly swells from typhoons in
the west Pacific will give surf a noticeable boost along exposed
north, west, and south facing shores Monday through late this
week.

&&

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

&&

DISCUSSION...Burke
AVIATION...M Ballard
MARINE...Foster