Flooding, Rainfall, and Storm Events in South Maui

What the Numbers Actually Tell Us

South Maui flooding is often described as rare, unpredictable, or “once-in-a-generation.”
The data tells a different story.

The extreme storms of March 2026 did set a new benchmark in some official records. But when placed into a longer timeline — including instrument data, rainfall records, and lived experience — a clearer pattern emerges:

Flooding in Kīhei is not rare. It is recurring. And it is measurable.

What is missing is not history — it is consistent measurement where it matters most.

The 2026 Storm: What Was Actually “Record-Breaking”

The March 2026 Kona storm produced the wettest single day ever recorded at Kahului Airport, Maui’s official long-term climate station.

• 7.40 inches in 24 hours (March 13, 2026)
• Previous record: 6.40 inches (Dec 20, 2017)
• Official climate record span: 1954–present

This is where the phrase “highest rainfall in 72 years” comes from.

But this statistic has an important limitation:

Kahului is not South Kīhei.

Rainfall distribution during Kona storms is uneven, and South Maui — especially the leeward slopes of Haleakalā — often receives very different rainfall patterns than the central valley.

During the same March 2026 event:

• 49.57 inches — Haleakalā Summit
• 46.57 inches — Kula
• 17.55 inches — Lipoa (South Maui)
• 16.62 inches — Keālia Pond
• 7.16 inches — Kīhei gauge (#2)
• 15.96 inches (7-day total) — Kahului Airport

This matters because:

Water falling mauka does not stay mauka.

Runoff from Haleakalā drains through gulches like Kūlanihākoʻi, Waipuilani, and others — directly into the Kīhei floodplain.

Even when some water infiltrates into the aquifer, extreme events overwhelm that buffering capacity.

How Long Have We Been Measuring Rainfall on Maui?

Rainfall observation in Hawaiʻi dates back to 1837, with increasingly organized mapping by the early 1900s.

For Maui specifically:

• Kahului Airport data:
  
  • Long-term records: ~1905 onward
  • Official NWS daily record baseline: 1954–present
• Kīhei station records:
  
  • Early observations: early 1900s
  • More consistent summaries: ~1949–2012
• USGS stream flow data (Kūlanihākoʻi Gulch):
  
  • Peak flow measurements: 1963–2008

So when something is called “record-breaking,” it usually means:

Record-breaking within a specific station and timeframe — not across all of South Maui history.

The Strongest Quantified Evidence: Kūlanihākoʻi Gulch Flow Data

The most direct measurement of flooding impact in South Kīhei comes from the USGS stream gauge at Kūlanihākoʻi Gulch.

This is critical because:

Flooding is not caused by rainfall alone — it is caused by runoff volume and flow rate.

Peak Recorded Flow Events (1963–2008)

The 2007 storm also provides a useful rainfall reference:

• 3–13 inches over 4 days across watershed
• Average ~7 inches
• Peak daily: 5.3 inches

That event alone produced:

• 2–4 feet of water over South Kīhei Road
• Documented roadway flooding and closures

This is important context:

Flooding comparable to modern events was already happening with far lower recorded rainfall totals.

A Timeline of Major South Kīhei Flood Events

This combines stream data, rainfall records, road closures, and documented impacts.

What the Pattern Shows

Across five decades of data:

• Major flood events appear roughly every 3–5 years
• Severe disruptions (road closures, property impacts) appear even more frequently
• Multiple events cluster during Kona storm seasons

This is not random.

It is a recurring hydrologic pattern tied to storm type, watershed behavior, and infrastructure limits.

The Measurement Problem in South Kīhei

Despite the repeated flooding:

• Rain gauges in South Kīhei are sparse and inconsistent
• Long-term gauges have been discontinued or poorly maintained
• Stream gauges are no longer active in key gulches
• Much of the most relevant data is mauka, not at the floodplain

This creates a critical gap:

The area experiencing the flooding has the least reliable measurement.

As a result:

• Kahului is often used as the “official” reference
• South Maui conditions are underrepresented
• Extreme local rainfall may go unrecorded

This is why lived experience matters:

Residents have repeatedly documented:

• Rapid-onset flooding
• Debris-choked culverts
• Road overtopping within minutes
• Flood depths reaching multiple feet

These observations align with known flow data — even when rainfall numbers are incomplete.

Where the Water Actually Comes From

Flooding in South Kīhei is driven by combined sources:

1. Mauka rainfall (Haleakalā slopes and summit)
   
   • Large volume, delayed runoff
   • Feeds gulches and drainage channels
2. Local rainfall in Kīhei
   
   • Faster response, surface flooding
   • Adds directly to runoff volume
3. Watershed saturation
   
   • Reduces infiltration
   • Increases surface flow
4. Infrastructure constraints
   
   • Undersized culverts
   • Debris accumulation
   • Roadway acting as a barrier

Not all rain becomes runoff. Some infiltrates into the aquifer.

But during extreme events:

The system exceeds its capacity — and flooding becomes inevitable.

What Can Be Said — And What Cannot

What can be said with confidence:

• March 2026 produced a record daily rainfall at Kahului
• South Maui experienced significant storm totals and flooding
• Historical data shows repeated major flood events
• Instrumented flow data confirms large flood volumes decades ago

What cannot be said with precision:

• Exact rainfall totals for every South Kīhei flood
• True peak runoff volumes for recent storms
• Whether 2026 was the “largest ever” in Kīhei specifically

Because the measurements are incomplete.

The Real Conclusion

The question is not whether flooding in Kīhei is rare.

It is not.

The question is whether it is being:

• Measured accurately
• Understood correctly
• Planned for responsibly

The historical record — even with gaps — shows a consistent truth:

Flooding in South Maui is not an anomaly.
It is a known, recurring condition.

And without better measurement and infrastructure alignment,
future storms — whether larger or simply more frequent —
will continue to produce the same outcome.

Sources and References

National Weather Service (NWS) — March 2026 Storm Summary
https://www.weather.gov/hfo/marchSevere2026
(Official event summary with rainfall totals, impacts, and statewide data)

NWS Kahului Climate Records (1954–present)
https://forecast.weather.gov/product.php?issuedby=OGG&product=CLI&site=NWS
(Official daily climate record showing Kahului record period 1954–present)

NOAA / NWS Rainfall & Hydrologic Summaries (Hawaiʻi)
https://www.weather.gov/hfo/HRS_archive
(Archived rainfall summaries and storm data products)

University of Hawaiʻi Rainfall Atlas & Climate Portal
https://rainfall.geography.hawaii.edu/
(Comprehensive rainfall mapping using over 1,000 stations statewide)

History of Rainfall Mapping in Hawaiʻi (UH Mānoa)
https://rainfall.geography.hawaii.edu/history
(Documents rainfall studies on Maui going back to the 1940s and earlier)

USGS Stream Gauge — Kūlanihākoʻi Gulch (Station 16660000)
https://waterdata.usgs.gov/monitoring-location/16660000/
(Official USGS streamflow monitoring location for South Kīhei watershed)

Kīhei Environmental Assessment Area Drainage Study (2021 draft)
https://gokihei.org/wp-content/uploads/Kihei-EAA-Report_DRAFT_11.22.21.pdf
(Includes peak flow data 1963–2008 and documented flood history)

Kīhei Drainage Master Plan (Maui County / Public Works)
https://gokihei.org/wp-content/uploads/Kihei-DMP-Pre-Final-Report-Complete-11-04-16.pdf
(Regional drainage analysis and flood modeling context)

NWS Monthly & Hydrologic Summaries (Hawaiʻi Climate Data Portal access)
https://www.weather.gov/hfo/hydro_summary
(Monthly precipitation summaries and hydrologic conditions)

NWS Kona Low Event Summary — December 2021
https://www.weather.gov/hfo/konaLowEventSummaryDec2021
(Official analysis of a major recent flooding event)