Heavy Rain in Hawaii, August 23, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
947 AM HST Mon Aug 23 2021

.SYNOPSIS...
A surface low will move from east to west across the state today
and tonight. Strong, gusty winds are possible across the western
half of the state, while heavy rainfall and thunderstorms are
possible across all of the islands. Trade wind weather will
return Tuesday and continue through the week.

&&

.DISCUSSION...
A surface low located just north of Maui County will move slowly west
today bringing the potential of strong, gusty winds for Lanai,
Molokai, Oahu, and Kauai; and heavy rain, thunderstorms, and the
possibility of flash flooding to the entire state. Satellite wind
estimates overnight showed an area of 30-35 mph sustained winds
along the northern side of the low, and these stronger winds may
reach northern Oahu this afternoon and Kauai this evening. A wind
advisory is in place for those affected areas.

Morning radar imagery shows quick hitting showers moving into the
windward areas of the smaller islands with some rainfall reaching
leeward locations as well. The low is carrying a big slug of
moisture as the CIMSS MIMIC precipitable water (PW) estimate
shows PW above 2 inches for much of the area With that amount of
moisture, any showers that do form will have the potential to
produce heavy rainfall. As the low moves toward Oahu this morning
expect increasing showers for both Oahu and Kauai with scattered
rainfall continuing over the rest of the state. With the onset of
the afternoon, the potential for convective showers and
thunderstorms increases over the entire state, and this will
increase the chances of heavy rainfall and flash flooding.

High PW values remain into tonight with low level wind
convergence on the low's east side being a focal point for showers
and thunderstorms. East southeast flow behind the low will also
create potential for anchored convection over the higher terrain
of Oahu and Kauai late this afternoon and evening. Expect
diminishing showers tonight and early Tuesday as the low moves
west of Kauai.

High pressure will fill in behind the low Tuesday with breezy trade
winds returning and windward and mountain showers. Trade wind
weather will continue through the rest of the week as the high
pressure center remains far north to northwest of the area.

&&

.AVIATION...
A surface low is centered just north of Maui as of this writing,
and heading slowly west. Showers and accompanying MVFR conditions
extend across the state, with Maui and Molokai currently seeing
the most precipitation. The chance of gusty winds and
thunderstorms will last through the rest of day as this feature
progresses toward Oahu and Kauai.

AIRMETs for mountain obscuration are in effect from Maui westward,
and will likely be extended later today.

An AIRMET for low level turbulence also continues in areas
downwind from the mountains statewide, and will be extended as
conditions warrant.

&&

.MARINE...
A gale low just north of Maui County will continue to move west
today through Tuesday. The strongest winds will be just north and
west of the center. A Gale Warning is currently posted for
portions of the coastal waters around Oahu and Maui County. A
Small Craft Advisory (SCA) is posted for waters south of Oahu and
Maui County and northeast of Kauai. The SCA will likely be expanded
to the remaining waters around Kauai later today as winds and
seas increase. The SCA will also be expanded to typically windy
waters around Maui County and the Big Island as high pressure
builds in to our north. Winds will drop off during the second half
of the week as the high moves a bit further away from the state.

A High Surf Advisory (HSA) remains in effect for east facing shores
of Oahu, Molokai and Maui through this afternoon and for the east
facing shores of Kauai through tonight. Select north and south
facing shores could experience some swell wrap along exposed reefs.
This swell will quickly decline below HSA thresholds by Tuesday
as the low moves west and away from the state. A series of small
southerly swells can be expected through Friday with a slightly
larger south swell expected over the weekend. No other significant
swells are expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for all Hawaii islands-

Wind Advisory until 6 AM HST Tuesday for Niihau-Kauai-

Wind Advisory until 6 PM HST this evening for Oahu-

Wind Advisory until noon HST today for Lanai Mauka-Molokai-Lanai
Windward-Lanai Leeward-Lanai South.

High Surf Advisory until 6 PM HST this evening for Olomana-Maui
Windward West-East Honolulu-Koolau Windward-Molokai Windward-
Molokai Southeast-Windward Haleakala-Kipahulu.

High Surf Advisory until 6 AM HST Tuesday for Kauai East-Kauai
South.

Small Craft Advisory from 6 PM this evening to 6 PM HST Tuesday
for Kauai Northwest Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Windward
Waters-Oahu Leeward Waters-Maui County Leeward Waters-Maalaea
Bay-Pailolo Channel.

Gale Warning until 6 PM HST this evening for Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters.

&&

DISCUSSION...Morrison
AVIATION...Kinel
MARINE...Burke