Heavy Rain in Hawaii, September 2 to 5, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Thu Sep 2 2021

.SYNOPSIS...
A breezy trade wind weather pattern is expected for the next
couple of days as high pressure lies north of the area. Clouds
and passing showers will tend to favor windward and mauka areas.
Winds will become lighter later in the weekend and on into the
middle of next week as the high to the north weakens. Clouds and
showers will tend to favor interior and leeward areas especially
during the afternoon and evening hours.

&&

.DISCUSSION...
Trade winds will gradually strengthen tonight and Friday as high
pressure builds in north of the state. Moisture embedded in the
trades will provide for passing showers over windward and mauka
areas with brief passing showers possible over select leeward
areas at times. Winds may reach Wind Advisory levels on Friday
over favored areas of Maui County and the Big Island.

Over the holiday weekend, winds will gradually diminish as the
high to the north weakens and an upper level trough drops down
southward to a position just north of the state. Winds will likely
be light enough Sunday and Monday for widespread sea breezes to
develop during the daytime hours. This would likely lead to
interior and leeward cloud and shower development especially
during the afternoon hours. The proximity of the upper trough may
allow for some showers to become briefly heavy. This lighter wind
pattern is expected to continue into the middle of next week as
the high to the north remains on the weak side. The upper trough
will also linger just north of the area so showers will continue
to favor interior and mauka areas especially during the afternoon
and evening hours.

&&

.AVIATION...
High pressure far to the NE will sink SW over the next 24 ours or
so. Trades have definitely responded and are already locally
windy, and should increase another couple of knots tomorrow. Have
issued an AIRMET for low level turbulence below 080 to the S
through W of mountains and this will almost certainly need to
continue through Friday. The incoming airmass is dry and stable,
so predominantly VFR conditions expected, although a minor trade
wind cloud band about 30 miles upwind of Maui and the Big Island
may bring ISOL to possibly TEMPO MVFR to some windward areas for a
few hours this evening and overnight.

&&

.MARINE...
A high northeast of the state will drive stronger trade winds
across local waters through Saturday. A Small Craft Advisory
(SCA) remains in effect through Saturday afternoon for the winder
waters surrounding Maui County and the Big Island, but has been
expanded to all other waters through Friday afternoon as well.
Trades are expected to further strengthen Friday and impact more
western waters.

A small, long period south swell filling in through Saturday will
be reinforced by small, medium period south and low period
southeast swells over the next several days. These swells will
maintain near seasonal south facing shore surf through the holiday
weekend. A series of small, medium to slightly longer period
south swells will arrive Labor Day and hold through most of next
week. Strengthened trades over and upstream of the islands over
the next few days will boost east facing shore surf by another
foot or two into the weekend.

&&

.FIRE WEATHER...
Conditions are favorable for weather conditions to reach critical
fire weather thresholds during the day Friday. Winds are expected
to increase a bit from todays levels. Relative Humidity levels
are expected to be near or below 45 percent especially from the
late morning through mid afternoon hours. And the KDBI remains
over the critical value of 600 due to the persistent dry
conditions over leeward areas this summer. Thus a Fire Weather
Watch has been issued for portions of the state that are prone to
wildfires.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Friday morning through Friday afternoon
for Kauai Leeward-Oahu South Shore-Waianae Coast-Oahu North
Shore-Olomana-Central Oahu-Waianae Mountains-Molokai Leeward-
Maui Leeward West-Maui Central Valley-Leeward Haleakala-Kona-
Kohala-Big Island Interior.

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

DISCUSSION...Burke
AVIATION...R Ballard
MARINE...Powell