November’s Kona low brought flash flooding, some relief to drought conditions on Maui

The wet second half of November, driven by a Kona low, is expected to have brought some needed relief from drought conditions across the State of Hawai’i, according to the National Weather Service.

The US Drought Monitor map — which will be updated on Dec. 7 and made available to view on Dec. 8 — will show just how much the initial effects of the Kona low have affected drought conditions.

Due to the severity of the drought in some areas of the state, additional significant rain events will be needed for full drought recovery, which is not likely within the winter months due to the ongoing strong El Niño event. 

On Maui, the National Weather Service’s latest precipitation summaries found that the US Geological Survey’s rain gage at West Wailuaiki Stream had the highest monthly total of 23.89 inches (120% of average), and the highest daily total of 9.83 inches on Nov. 30.

While this is a rather high daily total, bias-corrected radar rainfall estimates indicated nearly 21 inches of rain fell over the slopes of east Maui above the town of Hāna during the Nov. 30 Kona low flash flood event.

Elsewhere in Maui County, the Lahainaluna gage had its highest November total since 2006. Kīhei, Lānaʻi Airport, and Lānaʻi 1 had their highest November totals since 2007.

Maui County rainfall totals for 2023 through the end of November were near average at most of the gages. The rain gage at West Wailuaiki Stream had the highest year-to-date total of 184.82 inches (88% of average).

October’s generally dry weather pattern over the main Hawaiian Islands continued into early November with a surface ridge of high pressure near the island chain that kept winds light through Nov. 3.

The trade winds returned on Nov. 4 at moderate to fresh intensities. Embedded low level moisture from the remnants of a cold front boosted rainfall along the windward slopes of Kauaʻi and Oʻahu on Nov. 6 and 7, but did not produce any significant flooding impacts.

Fresh to strong trade winds from Nov. 10 through 17 produced daily rainfall along the windward slopes of the state, but resulted in generally dry conditions over the leeward areas.

The overall weather pattern changed on Nov. 18 and signaled the start of a much wetter period across the main Hawaiian Islands for the rest of the month. On Nov. 19, the strongest cold front of the 2023-2024 wet season thus far reached Kauaʻi and deposited 2 to 4 inches of rainfall over the upper slopes of the island.

The front weakened as it passed over Oʻahu on Nov. 20, and dissipated over Maui County later that day. Rain gage data and radar rainfall estimates indicated 2 to 5 inches along the windward slopes of Oʻahu’s Koʻolau Range, and 2 to 4 inches over the windward slopes of Molokaʻi.

Although the cold front dissipated, remnant moisture from the front combined with island-scale circulations on Nov. 22 to produce 2 to 4 inches of rainfall over most of the Koʻolau Range on Oʻahu.

Peak amounts of 6 to 10 inches occurred over the interior slopes from Maunawili to Hālawa Valley. Stream gages showed elevated flow levels, but there were no indications of significant flooding from this event.

November closed out with the most significant weather event of the month in the form of a Kona low, which developed late on Nov. 28 several hundred miles northwest of Kauaʻi. Bands of mainly light to moderate rainfall produced 1 to 3 inches of rain over Kauaʻi and Oʻahu on Nov. 28 and 29.

Only minor flooding issues were reported from this rainfall. At this time, the main axis of deep tropical moisture, and the most intense rainfall, remained east of the Big Island. The axis shifted westward on Nov. 30, resulting in a significant increase in rainfall over the southeastern half of the Big Island and the eastern and southern flanks of Haleakalā on Maui.

Flash flooding closed the Hāna Highway at several locations between Paʻuwela and Hāmoa, and along Piʻilani Highway between Kahikinui and Kaupō. Heavy rainfall during the morning of Nov. 30 on the Big Island resulted in the closure of Wood Valley Road near Pāhala, and Pōhaku Drive in Orchidland Estates near Keaʻau.

Combined rain gage and bias-adjusted radar rainfall estimates showed 11 to 22 inches over the eastern portions of the Big Island and Maui from the Kona low. The Pāhoa Beacon automated rain gage recorded peak rain rates in excess of 4 inches per hour during the morning of Nov. 30.