NOAA: One of the Wettest Decembers in Half Century

It was one of the wettest Decembers in the last 50 years, with some areas on the Big Island receiving 300% to 500% of their monthly average.

After a slow start to the 2021-2022 Hawaiian Islands wet season, with dry overall conditions in October and November, the month of December made up for lost ground, the NOAA/NWS Honolulu Forecast Office stated in its monthly rainfall summary for December.

The first low pressure system of  the month sent a cold front across the main Hawaiian Islands on Dec. 3. Brief heavy showers within the frontal rain band resulted in minor flooding over portions of Molokai, Maui, and the Big Island. The low pressure system subsequently developed into a strong subtropical cyclone, known locally as a kona low, to the northwest of the state on Dec. 5. While additional analysis is needed to refine the details, this kona low appears to have been one of the strongest to affect the State of Hawaii in the last 50 years.

Here’s the Big Island map of rainfall broken down by areas: Rainfall map.

In terms of impacts, the Dec. 5-8 kona low was comparable to previous significant kona lows such the events in January 1980, December 1987, and December 2008. In this case, “kona low” is not capitalized since “kona” in this case is an adjective meaning “leeward” in the Hawaiian language. This usage is consistent with several research papers covering subtropical cyclones in the central North Pacific.

Several locations on the Big Island, Maui, and O‘ahu received 10 to 15 inches of rainfall during the event. While there are no active real-time rain gages along the south-facing slopes of Haleakalā, credible rainfall estimates from the Multi-Radar, Multi-Sensor (MRMS) system showed more than 20 inches of rainfall in the area over the Kahikinui Forest Reserve.

After the kona low weakened on Dec. 8, trade winds at mostly fresh to strong intensities filled in to replace the southerly low level flow. A low pressure system developed to the northwest of the island chain, resulting in a slight veering of the low level winds to an east-southeasterly direction on Dec. 17 and 18. Unstable conditions aloft helped produce enhanced rainfall and minor flooding over portions of Upcountry Maui, windward O‘ahu, and the Big Island during this period. East-northeasterly trade winds resumed on Dec. 19 and continued at mostly moderate to fresh intensities through De. 30. However, the persistence of unstable conditions aloft maintained enhanced rainfall through Dec. 22, especially along the windward slopes of the Big Island and Maui.

One of the wettest Decembers in the last 50 years ended with the return of southeasterly to southerly winds over the state in response to a low pressure system to the west of Kaua‘i.