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Rainfall Summary For November 2020

December 3, 2020, 1:19 PM HST
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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.

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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.

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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.

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After an October with windward sites recording less than half of the average monthly rainfall, November totals from these same sites were mostly above average. The slopes of the South Kona District also received well above average rainfall.

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Below average monthly totals were mostly in the Ka‘ū, South Kohala, and North Kona Districts, as well as the Pōhakuloa region in the interior of the island. The Mountain View rain gage had the highest monthly total of 28.44 inches (150% of average), followed closely by USGS’ Saddle Road Quarry gage with 28.36 inches (218% of average). The Saddle Road Quarry gage had the highest daily total of 11.93 inches during the above mentioned heavy rain event on Nov. 22. Among the windward Big Island sites, there were 11 with November totals greater than 20 inches. On the Kona slopes, the Kealakekua gage observed its highest November total on record.

Most of the Big Island rain gages had near to above-average rainfall totals for 2020 through the end of November. The USGS’ rain gage at Kawainui Stream had the highest year-to-date total of 166.56 inches (137% of average).

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