In sharp contrast to October, which had a below-average number of trade wind days, November featured a well above average amount of trade wind days across the Hawaiian Islands. The dominance of trade winds produced above-average rainfall along the northeast-facing windward slopes, and generally below-average amounts over leeward areas.
Several low-pressure systems aloft helped destabilize the atmosphere at times, resulting in periods of enhanced rainfall embedded within the trade wind flow. The most significant of these occurred during the period from Nov. 22 through Nov. 26 during a period of fresh to strong trade winds.
On Nov. 22, a roughly east-west oriented band of clouds and showers embedded within the trades moved over the Big Island, producing a prolonged period of enhanced rainfall along the windward slopes. A large area of 5 to 10 inches of rainfall covered most of the North Hilo, South Hilo, and Puna Districts. Peak rainfall amounts of more than 10 inches occurred over the slopes above Hilo north of the Saddle Road.
Fortunately, sufficient gaps between showers mitigated excessive runoff and there were no reports of significant flood-related damage. Periods of heavy rainfall occurred again on November 24, this time focusing on the windward slopes of Maui, O‘ahu, and Kaua‘i. Although stream levels became elevated, there were no reports of significant flooding problems from any of these areas.
On Kaua‘i, the heavy rainfall on Nov. 24 saturated the ground and set the stage for flooding the next day. An area of heavy rainfall developed over the windward slopes during the morning of Nov. 25 and resulted in a water level jump of 8 feet in just under an hour in the Hanalei River. Floodwaters quickly inundated Kuhio Highway near the Hanalei River Bridge and the road remained closed through most of the day.
The final episode of heavy rainfall during this wet period occurred on Nov. 26 along the slopes of the Big Island’s South Kona District. Heavy rainfall developed during the mid-afternoon and persisted after sunset with totals of 3 to 8 inches producing minor flooding within the normally dry drainages in the area. While afternoon showers are not uncommon along the Kona slopes, unstable conditions aloft helped ratchet up the intensities of these showers during this event.
While the windward slopes of the state received ample amounts of rainfall, many of the leeward areas remained dry through November. Portions of Maui County and the South Kohala District of the Big Island remain locked in extreme drought (the D3 category on the US Drought Monitor map), with vegetation reported to be in very poor condition.
Kaua‘i, November 2020
Windward rain gages picked up mostly near to above average rainfall for the month of November. Leeward gages had mostly below average monthly totals. The Us Geological Survey’s (USGS) rain gage on Mount Waialeale had the highest monthly total of 59.23 inches (158% of average) and the highest daily total of 9.30 inches on Nov. 24. This gage recorded another 7.42 inches on Nov. 25 for a TWO-day total of 16.72 inches. The monthly rainfall totals at Mount Waialeale, Omao, and Wailua UH Experiment Station were the highest November amounts since 2009.
All of the Kaua‘i rainfall totals for 2020 through the end of November were near to above average. The Mount Waialeale gage had the highest year-to-date total of 396.67 inches (109% of average) and should end the year with more than 400 inches for just the second time in the last 10 years.