Persistent Trade Winds Kept Leeward Side of Big Island Mostly Dry

The windward side of the Big Island saw near to above average rainfall totals at the close of 2020 while persistent trade winds over the past couple months have kept the leeward areas mostly dry.

December’s trend matches rainfall averages that were reported islandwide for the past year by the National Weather Service — near to above average.

The most significant rainfall event took place during the middle of December during a period of fresh to strong trade winds. Most of the unstable conditions occurred from Dec. 16-19 over Hawai‘i Island and Maui.

Several rain gages along the North Hilo, South Hilo, and Puna Districts of the Big Island recorded 6 to more than 10 inches of rainfall over the three-day period ending at midnight on Dec. 20. Due to the relatively strong low level winds, rain bands moved quickly, which helped prevent significant flooding problems.

In December, NWS reported the highest daily total, 5.77 inches, came from the Glenwood gage on December 18, followed closely by Mountain View’s 5.71 inches on the same day. The Mountain View gage had the highest monthly total of 19.33 inches (138% of average).

The USGS’ rain gage at Kawainui Stream had the highest annual total of 179.63 inches (133% of average). This was followed closely by Glenwood’s 179.21 inches (77% of average).

NWS noted an above average number of trade wind days across the main Hawaiian Islands for the month of December.

“The higher than average frequency of trade winds resulted in rainfall mainly occurring along the east-facing windward slopes, with leeward areas being considerably drier,” NWS reported.

The lack of significant cold fronts and the higher than normal frequency of trade winds in the past couple of months have contributed to the dry conditions across the leeward areas of the state.

“Extreme drought in Maui County has continued well into the wet season and has resulted in the death of numerous cattle and Axis deer on Molokai, brush fires in West Maui, and very poor pasture conditions for livestock,” NWS stated.