Flash Flooding and Debris Flows in Hawaii, March 6 to 11, 2021

Name: Flash Flooding and Debris Flows in Hawaii, March 6 to 11, 2021
Start: 2021-03-06T06:35
End: 2021-03-11T11:53
Location: Hawaii

March 6 to 11, 2021 · NWS HFO · 27 events in 4 counties and zones

Where it happened

Every event with a place to put it. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: $51 million
Crop damage: $1.8 million
Events: 27

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS HFO wrote it up.

A slow-moving surface trough, heading east to west, and an upper disturbance triggered heavy showers and isolated thunderstorms that caused flash flooding in some instances. Damages occurred to public and private property, including roads and bridges washed out. Nine homes were destroyed, 44 suffered major damage, and 55 suffered minor damage. No significant injuries were reported.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Honolulu, HI wrote its last area forecast discussion before the first event at Mar 6, 3:07 AM HST, 3 hours 28 minutes earlier.

Area forecast discussion, Mar 6, 3:07 AM HST, 3 hours 28 minutes before

Area Forecast Discussion
National Weather Service Honolulu HI
307 AM HST Sat Mar 6 2021

.SYNOPSIS...
A high to the north will maintain breezy trade winds over the area
through the next several days. In the near-term, the most active
showers will be over windward areas of Kauai, Oahu and Maui County.
Starting late Sunday, heavier showers will move over the Big Island,
then spread west through mid-week.

&&

.DISCUSSION...
As of 2 AM, the surface analysis showed a strong 1039 mb high far
north of the main Hawaiian Islands. This high will maintain
breezy trade winds over the area through the middle of next week.
The trade winds are carrying remnants of a front over mainly
windward areas from Kauai to Maui. A trough aloft is making the
atmosphere over the islands a bit unstable, so the frontal remnants
are taller than typical trade-wind clouds. The clouds are tall
enough, and the trade winds are strong enough, for the showers to
spread leeward on the smaller islands.

The trough aloft is forecast to deepen over the next few days.
That will make the atmosphere more unstable and help pull deeper
moisture over the islands from the southeast starting on Sunday.
It is too soon to know the details of this developing pattern, but
it looks as if rainfall will become more active, starting on the
Big Island by late Sunday, then spreading northwest. Heavy rain
and thunderstorms will be possible on the Big Island with ice and
snow over the upper elevations. As this pattern develops, a Flash
Flood Watch and a Winter Weather Watch may need to be issued.

There have been large differences from model run to model run and
between the GFS and ECMWF models next week. That reduces the
confidence we have in the long-term forecasts.

&&

.AVIATION...
High pressure to the north-northwest and frontal remnants to the
east will continue to bring clouds and showers to the islands
today. Over the last couple of hours, clouds and showers have
thinned some, and so AIRMET SIERRA has been canceled for mountain
obscuration. Its possible that it may be needed again, if clouds
and showers build up again over the islands. However VFR
conditions prevail over most of the islands, with passing MVFR
conditions.

AIRMET TANGO continues for low level mechanical turbulence to the
lee of the mountains on Kauai, due to winds remaining elevated in
both observations and the VAD wind profile. Winds are expected to
trend down a bit today which could allow the AIRMET to be
canceled.

&&

.MARINE...
A strong high pressure system far north of the Hawaiian Islands
will drift slowly southeastward over the next few days. Trade
wind speeds will remain in the fresh to strong range today, and
then increase into the strong to near gale range from Sunday
through Wednesday. Gale conditions may develop over the windier
eastern waters and channels next week.

The current Small Craft Advisory (SCA) includes most island
waters due to a combination of strong trade winds and elevated
seas. The SCA will expand on Sunday to include all coastal waters
through Monday morning and will likely continue into much of next
week due to increasing trade winds.

A north swell continues to elevate surf heights along exposed
shores. This north swell may produces surges in north facing
harbors, including Hilo and Kahului harbors. Swell heights will
slowly decrease through Sunday and the High Surf Advisory for
north facing shores of the eastern islands was cancelled this
morning. The forerunners of the next northwest swell are expected
to arrive on Sunday, with overlapping northwest and north swells
producing medium size surf along north and west facing shores
through Monday, and then decreasing to smaller size surf from
Tuesday through Thursday.

Strengthening trade winds will increase surf along east facing
shores through the first half of next week with a building trade
wind swell. Surf may reach HSA levels along exposed east facing
shores on Tuesday and Wednesday.

Background south swells will provide small surf to south facing
shores through Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

Small Craft Advisory until 6 AM Sunday for all Hawaiian waters
except Maui County Leeward waters and Oahu Leeward waters.

Small Craft Advisory from 6 AM Sunday through 6 AM Monday for all
Hawaiian waters.

&&

DISCUSSION...Donaldson
AVIATION...M Ballard
MARINE...Jelsema/Bohlin

The radar

The South Kauai radar (PHKI), 31 km away, was scanning. The volume nearest the first event began at 16:36 UTC.

Open the radar archive at this moment

Event by event

17 heavy rain, 5 debris flow, 4 flash flood, 1 lightning. Local standard time, as filed.

Heavy Rain, Kauai

When: March 6, 6:35 AM HST to 5:23 PM HST
Where: Kauai County, 3.66 mi E of Na Pali Coast Sp to 3.83 mi ENE of Koloa
Reported by: Official NWS Observations
Coordinates: 22.174, -159.593

Debris Flow, Kauai

When: March 6, 10:55 AM HST
Where: Kauai County, 1.45 mi WNW of Hanalei to 1.42 mi NW of Hanalei
Cause: Heavy Rain
Property damage: $3.9 million
Reported by: Department of Highways
Coordinates: 22.210, -159.520

Rocks and soil caused the closure of one lane of Kuhio Highway at mile marker 4.5, near Waikoko on Kauai's north shore.

Heavy Rain, Hawaii

When: March 7, 5:22 PM HST to 11:15 PM HST
Where: Hawaii County, 2.03 mi SW of Papaaloa to 6.23 mi ESE of Glenwood
Reported by: Official NWS Observations
Coordinates: 19.953, -155.245

Heavy Rain, Maui

When: March 8, 10:04 AM HST to 2:20 PM HST
Where: Maui County, 0.87 mi N of Puukolii to 4.13 mi SSE of Nahiku
Reported by: Official NWS Observations
Coordinates: 20.943, -156.680

Heavy Rain, Hawaii

When: March 8, 1:09 PM HST to March 9, 1:10 AM HST
Where: Hawaii County, 6 mi SW of Ookala to 5.59 mi SW of Volcano
Reported by: Official NWS Observations
Coordinates: 19.962, -155.349

Flash Flood, Maui

When: March 8, 4:35 PM HST to 6:15 PM HST
Where: Maui County, 2.06 mi ESE of Haiku to 1.38 mi E of Pauwela
Cause: Heavy Rain
Property damage: $14 million
Reported by: Emergency Manager
Coordinates: 20.910, -156.290

Hana Highway was closed from Peahi Road near Haiku to Keanae in windward East Maui due to flooding. A dam failure in the same area was a possibility as water topped the Kaupakulua Dam upstream from the Haiku and Pauwela communities. The dam survived the downpours, however.

Heavy Rain, Maui

When: March 8, 9:32 PM HST to March 9, 3:25 AM HST
Where: Maui County, 1.16 mi WSW of Haiku to 1.76 mi NNE of Kipahulu
Reported by: Official NWS Observations
Coordinates: 20.915, -156.337

Heavy Rain, Kauai

When: March 8, 10:55 PM HST to March 9, 4:55 AM HST
Where: Kauai County, 0.35 mi NE of Kaunalewa to 3.59 mi SE of Kilauea
Reported by: Official NWS Observations
Coordinates: 22.004, -159.726

Heavy Rain, Honolulu

When: March 9, 5:02 AM HST to 8:33 AM HST
Where: Honolulu County, 0.54 mi S of Kawailoa to 0.52 mi NE of Niu Valley
Reported by: Official NWS Observations
Coordinates: 21.612, -158.080

Flash Flood, Honolulu

When: March 9, 10:35 AM HST to March 10, 12:00 AM HST
Where: Honolulu County, 0.43 mi ESE of Kaneohe to 0.45 mi ESE of Kaneohe
Cause: Heavy Rain
Property damage: $2.7 million
Crop damage: $1.8 million
Reported by: County Official
Coordinates: 21.418, -157.794

Four feet of water from heavy rain flooded William Henry Road in Kaneohe in windward Oahu. A blocked drain caused the backup of water, inundating some homes in the area. Major flooding problems continued over windward portions of Oahu and the North Shore, especially around Haleiwa and Waialua. The Opaeula Stream overflowed at Haleiwa Road and caused flooding of homes in the area. Also in windward Oahu, flooding was reported along Okana Road and a partial road closure occurred on Kamehameha Highway between Wailehua Road and Waikane Store.

Heavy Rain, Hawaii

When: March 9, 10:55 AM HST to 9:26 PM HST
Where: Hawaii County, 1.73 mi NNW of Akaka Falls St Park to 6.68 mi W of Naalehu
Reported by: Official NWS Observations
Coordinates: 19.871, -155.164

Debris Flow, Honolulu

When: March 9, 11:05 AM HST
Where: Honolulu County, 1.19 mi W of Maunawili to 1.15 mi W of Maunawili
Cause: Heavy Rain
Reported by: County Official
Coordinates: 21.368, -157.788

A small section of the Pali Highway, in the Kaneohe-bound lane, was blocked by debris from the nearby steep terrain.

Debris Flow, Maui

When: March 9, 11:20 AM HST
Where: Maui County, 2.58 mi W of Keanae to 2.14 mi W of Keanae
Cause: Heavy Rain
Reported by: Department of Highways
Coordinates: 20.870, -156.190

Hana Highway was closed between mile post 10 and mile post 13, about 2 miles west-northwest of Keanae in windward East Maui. The closure was caused by large boulders blocking the roadway.

Heavy Rain, Kauai

When: March 9, 3:47 PM HST to 4:30 PM HST
Where: Kauai County, 1.82 mi W of Kokee State Park to 0.45 mi NW of Hanamaulu
Reported by: Official NWS Observations
Coordinates: 22.132, -159.698

Flash Flood, Kauai

When: March 9, 5:26 PM HST to March 10, 9:11 AM HST
Where: Kauai County, 0.73 mi SSW of Princeville to 0.84 mi SSE of Princeville
Cause: Heavy Rain
Property damage: $30 million
Reported by: Emergency Manager
Coordinates: 22.210, -159.482

Heavy rain caused the Hanalei River to overflow near the Hanalei Bridge, which then closed Kuhio Highway in that area. Other areas in northern Kauai were affected by downpours that caused debris flows unto roadways, near Princeville, Kilauea, and Wailoko. Other areas that had problems were near Lihue at Niumalu and Hulemalu roads, and in Kapaa where the Keapana Bridge became impassable.

Debris Flow, Hawaii

When: March 9, 6:17 PM HST
Where: Hawaii County, 0.03 mi SW of Hakalau to 0.04 mi SSW of Hakalau
Cause: Heavy Rain
Reported by: Law Enforcement
Coordinates: 19.900, -155.130

One lane of Highway 19 in windward Big Island near Hakalau, and the Hakalau Bridge, was closed due to debris on the roadway.

Heavy Rain, Maui

When: March 9, 10:56 PM HST to March 10, 1:54 AM HST
Where: Maui County, 1.31 mi SSW of Wailua to 1.12 mi WNW of Kaupo
Reported by: Official NWS Observations
Coordinates: 20.833, -156.139

Heavy Rain, Hawaii

When: March 10, 1:02 AM HST to 11:01 AM HST
Where: Hawaii County, 4.51 mi SSW of Hilo to 6.29 mi WSW of Naalehu
Reported by: Official NWS Observations
Coordinates: 19.669, -155.095

Heavy Rain, Kauai

When: March 10, 9:11 AM HST to 9:21 PM HST
Where: Kauai County, 1.68 mi W of Kokee State Park to 1.1 mi E of Hanamaulu
Reported by: Official NWS Observations
Coordinates: 22.128, -159.696

Heavy Rain, Honolulu

When: March 10, 1:09 PM HST to 1:45 PM HST
Where: Honolulu County, 2.51 mi E of Kawailoa to 1.08 mi N of Waialae Iki
Reported by: Official NWS Observations
Coordinates: 21.621, -158.041

Flash Flood, Honolulu

When: March 10, 3:45 PM HST to 10:52 PM HST
Where: Honolulu County, 2.02 mi ESE of Waikane to 2.09 mi ESE of Waikane
Cause: Heavy Rain
Reported by: Broadcast Media
Coordinates: 21.489, -157.851

Kamehameha Highway in windward Oahu was closed at the Waikane Bridge due to flooding. More flooding conditions occurred around Oahu - along Kamehameha Highway near Kualoa Point (and Kualoa Ranch), and in Waikiki at Hobron Lane where cars stalled due to high water there.

Heavy Rain, Maui

When: March 10, 3:45 PM HST to 6:36 PM HST
Where: Maui County, 1.44 mi E of Keawakapu to 1.3 mi SSE of Nahiku
Reported by: Official NWS Observations
Coordinates: 20.698, -156.428

Heavy Rain, Hawaii

When: March 10, 4:30 PM HST to 6:37 PM HST
Where: Hawaii County, 3.21 mi WSW of Glenwood to 7 mi WSW of Naalehu
Reported by: Official NWS Observations
Coordinates: 19.457, -155.193

Debris Flow, Honolulu

When: March 10, 6:20 PM HST
Where: Honolulu County, 0.97 mi SE of Waimea to 0.95 mi SE of Waimea
Cause: Heavy Rain
Reported by: Emergency Manager
Coordinates: 21.639, -158.061

There was a partial closure of Kamehameha Highway near Waimea Bay on Oahu's North Shore as a large boulder blocked traffic for a time.

Lightning, Honolulu

When: March 10, 6:34 PM HST to 10:30 PM HST
Where: Honolulu County, 0.66 mi ESE of Moilili to 0.77 mi SW of Punaluu
Property damage: $20,000
Reported by: Emergency Manager
Coordinates: 21.297, -157.820

Lightning strikes on Oahu led to power outages from Honolulu to Hauula. Power was restored after several hours for most locales as lines were repaired.

Heavy Rain, Honolulu

When: March 11, 2:23 AM HST to 9:26 AM HST
Where: Honolulu County, 0.16 mi E of Barbers Pt Housing to 3.24 mi WSW of Punaluu
Reported by: Official NWS Observations
Coordinates: 21.320, -158.078

Heavy Rain, Kauai

When: March 11, 3:21 AM HST to 11:53 AM HST
Where: Kauai County, 0.39 mi ESE of Kaunalewa to 2.3 mi WSW of Anahola
Reported by: Official NWS Observations
Coordinates: 21.998, -159.725

What these are

Heavy Rain: Rainfall totals unusual for the place and season that did not produce a flood.
Debris Flow: A fast-moving slurry of mud, rock and debris, usually on a slope soaked by heavy rain or stripped by a fire.
Flash Flood: Flooding that begins within six hours of the rain that causes it, or of a dam or levee failure or ice jam release, often with deadly speed.
Lightning: A lightning strike that killed, injured or caused damage.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Hawaii in 2021 Every state in 2021 This day in weather history Warnings in Hawaii, March 2021

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 938296, 938297, 938298, 938299, 938301, 938300, 938302, 938307, 938604, 938605, 938606, 938609, 938610, 938607, 938611, 938614, 938612, 938613, 938617, 938619, 938627, 938640, 938642, 938644, 938653, 938647, 938650.