Rainfall Summary for January 2021
The first half of January was relatively quiet with respect to rainfall across the main Hawaiian Islands. Around half of the days during this period involved trade wind conditions. On Jan. 13, a weak cold front moved eastward across the west half of the state, then stalled and dissipated over Maui County on Jan. 14. This weather system was not much of a rainfall producer over Kaua‘i and O‘ahu, with event totals of less than one inch on both islands. The east half of Moloka‘i received about 2 to 4 inches as the front stalled, which resulted in some minor flooding.
During the second half of January, the weather pattern over the North Pacific changed significantly. This allowed rain-producing low pressure systems to move closer to the island chain, resulting in a couple of heavy rain events. The first event took place during the Martin Luther King Day weekend in the form of a cold front that reached Kaua‘i on Jan. 17. The front moved eastward and stalled near the end of the Big Island the following day. Strong north to north-northeast winds to the west of the frontal boundary, along with ample low level moisture and instability aloft, helped produce heavy rainfall over portions of Maui County on Jan. 18. Remnant moisture from the dissipated front moved westward and caused heavy rainfall over Kaua‘i on Jan. 19. The highest totals occurred along the north-facing slopes of Maui County and Kaua‘i, with several peak totals exceeding 10 inches in 48-hours. Flash flood impacts included the closure of Kamehameha V Highway on south Moloka‘i near Kamalō and Pūko‘o. On Maui, a road closure due to flooding occurred in Makawao, and “washing machine size” boulders closed Honoapi‘ilani Highway near Honokōhau. Not surprisingly, a rain event of this magnitude also resulted in several rockslides along the Hana Highway on the north flank of Haleakalā. The heavy rainfall on Kaua‘i caused the Hanalei River to overflow its banks and forced the closure of Kūhiō Highway for several hours on the afternoon of Jan. 19.
The following week, a low pressure system aloft combined with enhanced moisture in the low level east-southeasterly winds to produce another heavy rain event. This time, the Big Island took the brunt of the event, with a large swath of 4 to 10 inches of rainfall occurring on Jan. 24 and 25 along the southeast-facing slopes of the island. While there were several secondary roads in the South Hilo, Puna, and Ka‘ū Districts that were briefly closed due to flooding, the main closure was Highway 11 at a flood prone stretch called Kāwā Flats in the lower Ka‘ū District. This closure lasted several hours and prevented vehicular travel along the main public road in that part of the island. By comparison, O‘ahu rainfall totals were only about half as much as the amounts on the Big Island. However, it occurred in only 3 to 4 hours along a highly urbanized portion of the island. As a result, the impacts were much more significant. The intense rainfall was the result of a highly localized effect caused by the convergence of low level winds in the lee of the islands, in this case Lanai and Moloka‘i. During normal trade wind conditions, these convergence cloud bands pass over the leeward coastal waters. On January 25, the veering of winds to east-southeasterlies put Lanai’s and Moloka‘i’s lee-side cloud bands directly over the south O‘ahu urban corridor. The enhanced atmospheric instability that day made the rainfall much more intense as well. Peak 15-minute rainfall at the Niu Valley rain gage was 0.81 inches from 3:30 to 3:45 p.m. (3.24 inches per hour rain rate). This was within the peak 1-hour accumulation of 2.47 inches during the period from 3:15 to 4:15 p.m. There were media reports of flood damage in the ‘Āina Haina and Pālolo areas, and a large sewage spill in Mānoa. In Waipahu, two young boys were rescued from a flooded drainage canal, with one sustaining minor injuries. The heavy rain also impacted the afternoon commute due to significant ponding along portions of the H-1 Freeway in Honolulu. The atmosphere over the island chain stabilized on Jan. 27 and closed out the rest of the month with fresh to strong trade wind conditions.
Rain gages along the northern half of Kaua‘i had mostly above average monthly totals for the month of January. Gages on the southern half had below average totals. The US Geological Survey’s (USGS) rain gage on Mount Wai‘ale‘ale had the highest monthly total of 38.45 inches (155 percent of average). This was followed closely by the USGS’ Kilohana total of 38.33 inches (255 percent of average). The Kilohana rain gage had the highest daily total of 17.62 inches on Jan. 19. The Wailua UH Experiment Station gage had its highest January total since 2005.