March was the wettest across the state since 2006. The March 2006 totals set a very high bar so monthly rainfall records were broken at only a couple of locations.
Most of the heavy rainfall and 8 of the flash flood events took place from March 7 through March 13. The weather pattern over the area included a strong low pressure system aloft tapping into an area of enhanced low level moisture embedded within the trade wind flow.
Heavy rainfall shifted to Oahu and Kauai, and continued over the Big Island on Tuesday, March 9.
By March 10, the low pressure system aloft induced a low pressure system at the surface southwest of Kauai, which resulted in a veering of the low level winds to southeasterlies across the island chain. The shift in the winds caused a change in the movement of rainfall areas, which in turn changed the areas of flood vulnerability. On Kauai, bouts of heavy rain on March 10 did not produce significant flooding. However, days of rain soaked the soils, which generated a large landslide on the morning of March 11. The landslide covered the portion of Kuhio Highway leading into Hanalei, and cut off all communities west of the Hanalei Bridge from the rest of the island, including Hanalei town, Wainiha, Waipa, and Haena.
From its position southwest of Kauai, the surface low moved northward on March 11, then northeastward, with a cold front extending from the low toward the southwest. The front reached Kauai on the early morning hours of March 12, moved eastward across the state, and was over the north end of the Big Island on March 13. The front produced another round of significant flooding across portions of the island chain during its passage. Kauai was the hardest hit, with numerous reports of flooding on the south side of the island. Waikomo Stream in Koloa overflowed its banks and inundated several nearby properties. Several vehicles stalled in deeply flooded roads, especially from Koloa to Kalaheo.
Although some enhanced rainfall occurred over portions of the state after March 13, atmospheric conditions slowly stabilized to help keep intensities in check and prevent significant flooding. Most of the rest of the month included trade winds over the main Hawaiian Islands. The main break in the trades occurred from March 23 through 25 as a cold front approached the state from the northwest. Low level winds veered in response, coming from a southeasterly through southerly direction. Pre-frontal convergence rain bands set up over Oahu, producing 1 to 2 inches of rainfall on March 24 but no significant flooding problems. The front itself stalled northwest of the main Hawaiian Islands.
Lastly, on March 29, a low pressure disturbance aloft made the atmosphere unstable enough to support heavy rainfall over portions of Kauai. The rainfall over the interior of the island caused Hanalei River to once again overflow its banks and close Kuhio Highway near the Hanalei Bridge briefly in the morning and again for several hours in the late afternoon and early evening.
All of the rain gages on Kauai recorded monthly rainfall totals that were 2 to 4 times greater than the average for March. The USGS’ rain gage on Mount Waialeale had the highest monthly total of 111.72 inches (295 percent of average) and the highest daily total of 14.44 inches on March 13. While this monthly total is incredibly high, it did not come close to breaking the March record of 148.83 inches set in 1982. There were no March rainfall records broken across the island. However, the Kalaheo, Omao, and Port Allen gages recorded their highest March totals since 2006. Lihue Airport’s 11.89 inches (258 percent of average) marked its sixth wettest March, well below the record of 36.13 inches from 2006.
Rainfall totals for 2021 through the end of March were above average at all of the gages on Kauai. Mount Waialeale had the highest year-to-date total of 191.03 inches (219 percent of average). On average, this amount of rainfall is not reached until July at this location.