Heavy Rain in Hawaii, August 11, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
947 AM HST Wed Aug 11 2021

.SYNOPSIS...

Over Big Island, widespread rain over windward areas this morning
will diminish in favor of locally heavy showers over the leeward
slopes by this afternoon. Otherwise, dry moderate trades will
prevail into next week.

&&

.DISCUSSION...

Widespread light to moderate rain is ongoing over windward and
south-facing sections of Big Island, including the summits, as
the moisture laden low-level trough advertised yesterday advances
through that area. The rain has mainly been stratiform in nature
which has contributed a rather uniform smattering of totals on the
order of a half to one inch in the last 6 hours. The Laupahoehoe
gauge checks in with the highest 6-hr total (ending 8am) of 1.07
inches. As this moisture emerges over the leeward side of Big
Island during the next few hours, diurnal heating will fuel deep
convection capable of producing locally heavy rainfall. In
contrast to yesterday's showers, the increased reservoir of low-
level moisture and diminished trade inversion may allow heavy rain
to develop further downslope today. Thus, although flooding is
not necessarily anticipated, isolated minor flooding cannot be
entirely ruled out. For the remainder of the forecast area, the
weakened inversion courtesy of the upper low parked northwest of
Kauai may contribute to brief isolated moderate or even heavy
showers, but otherwise dry trades will prevail. Rainfall will
remain limited for at least the next several days as moderate
trades struggle to generate showers beneath building mid-level
ridging and strengthening subsidence.

&&

.AVIATION...

Light to moderate easterly trade winds will continue through the
period with clouds and showers favoring windward and mauka
locations. A batch of low clouds associated with a weak trough
moving in from the east will support periods of MVFR CIGS/VSBYS
through the morning hours today over the eastern end of the
island chain. As a result, AIRMET Sierra for mountain obscuration
will likely continue this morning from Maui to the Big Island.

&&

.MARINE...

Not that much change from previous forecast. A surface high
pressure system located far north- northeast of the state will
maintain moderate trades through Thursday. A new surface high
pressure system will build north of the islands as we head into
the weekend. Locally strong trade winds could return starting
Friday. Small Craft Advisory (SCA) conditions are likely
developing across the typically windier waters adjacent to the
islands of Maui County and the Big Island through Saturday.
Weakening trades are forecasted starting Sunday.

Nearshore PacIOOS buoys are reporting a swell height of 3 to 4
feet with a period of around 14 seconds due to the current south-
southwest swell. This swell will maintain slightly elevated surf
along south facing shores and is expected to peak later today
before gradually lower tonight through Thursday. A small,
reinforcing south- southwest swell is expected to arrive Friday,
followed by a larger swell from the same direction starting
Sunday. The latter swell will likely be long- lived, with a peak
in the Tuesday through Wednesday time period next week.

Active tropical cyclone activity in the northwest Pacific has
been sending small, back-to-back west-northwest swell energy
toward the region. The PacIOOS buoy at Hanalei shows a small west-
northwest swell picking up slightly at 4 feet with a period of
about 13 seconds this morning. This will cause surf to continue to
trend up along exposed north and west facing shores today. This
swell may lower by Thursday morning, with another small,
reinforcing west- northwest swell reaching Kauai late Thursday. A
slightly larger west-northwest swell may arrive this weekend.

Surf along east facing shores will continue to lower due to the
lighter trade winds locally and upstream of the islands. A tiny,
short-period northeast swell may arrive this weekend, but surf
will likely remain small along east facing shores into early next
week.

&&

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

&&

DISCUSSION...JVC
AVIATION...TS
MARINE...Almanza