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October Rainfall Summary

November 5, 2020, 4:12 PM HST
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“The first month of the October through April Hawaiian Islands wet season produced a below-average amount of trade wind days over the island chain. This was largely due to a strong atmospheric ‘blocking pattern’ of high pressure that dominated conditions over the northeastern Pacific during the second half of the month.

To the west of the ‘block,’ a Kona low developed on Oct. 16 and affected the west half of the state on Oct. 17 and 18. Rain bands on the eastern periphery of the low’s circulation moved over Kaua‘i from the south, providing one to three inches of much-needed rainfall along the south-facing slopes of the island. Fortunately, the areas of rain did not persist over specific locations, thus limiting impacts to just minor flooding.

Although the Kona low moved northward on Oct. 19 and 20, a low-pressure trough lingered west of Kaua‘i for another week, keeping the island chain under persistent and moist southeasterly to southerly low-level winds. With more atmospheric instability over the west half of the state, brief periods of heavy rainfall occurred daily over portions of O‘ahu and Kaua‘i from Oct. 20 through Oct. 25. The short duration of these rain events prevented significant flooding problems.

On Oct. 26, a strong upper tropospheric disturbance dropped in northwest of the state. The slightly more progressive nature of this system pushed a cold front, the first of the wet season, across Kaua‘i and O‘ahu on Oct. 28 before it dissipated over Maui County. Rainfall along and ahead of the front produced some minor flooding on Kaua‘i, O‘ahu, and Maui Counties. Following the front’s passage, cooler and drier westerly to northwesterly winds closed out the remainder of the month.

While all of this activity was occurring, the Big Island was frequently under southeasterly low-level winds. A few shower areas moved mainly over the southeastern side of the island, but the east- and northeast-facing slopes were generally dry during the month. Showers during several afternoons along the Kona slopes and interior sections produced near to above-average October totals.

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Due to the lack of trade winds, windward Big Island rainfall totals for October were mostly below 50 percent of average. Many of the sites along the Kona slopes and in the Pohakuloa region had near to above-average monthly totals. The high frequency of low-level southeasterlies resulted in the highest October total coming from the Pali 2 rain gage in the Ka‘ū District. This gauge logged 10.90 inches (250 percent of average) for the month and had the highest daily total of 5.58 inches on Oct. 26. The Kamuela Upper, Pi‘ihonua, and Waiakea Uka rain gages all broke records for the lowest October rainfall total, and the Mountain View gage had its lowest October total since 2003.

Rainfall totals for 2020 through the end of October remained near to above average at most of the gages. The USGS’ rain gage at Kawainui Stream had the highest year-to-date total of 139.84 inches (123 percent of average).”

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