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Statewide precipitation summary for May

MEDIA RELEASE

The end of April normally signals the end of the Hawaiian wet season. However, the atmosphere decided otherwise and kept the state under unstable and wet conditions through the first half of May, said Kevin R. Kodama, senior service hydrologist at NOAA/NWS Weather Forecast Office Honolulu.

From May 1 through 4, an upper level trough brought very unstable conditions statewide. The heaviest rainfall occurred during the afternoon of May 2 when a line of strong thunderstorms developed just offshore of Honolulu.

The western portion of the line generated twin waterspouts just outside of Honolulu Harbor then dissipated as the heavy rainfall moved onshore.

Rain rates as high as 4 inches per hour quickly produced urban flooding in the Waikiki and Makiki areas and several homes were flooded along Nanea St.

A few days later, another upper level system dropped into the area around the main Hawaiian Islands bringing a new round of unstable conditions. Brief but intense thunderstorms over the slopes of the South Kona District of the Big Island resulted in the closing of the Belt Highway near Kainaliu during the afternoon of May 6.

The low pressure system strengthened far west of Kauai on May 8 and maintained a very unstable airmass over the west half of the state through May 12.

Heavy rainfall over windward Oahu caused Waikane Stream to overflow and closed Kamehameha Highway briefly during the early morning hours of May 8.

At around the same time, heavy rainfall developed over north Kauai with multiple bursts of heavy rain producing 10- to 14-inch daily totals over north and east Kauai and 3 significant flow peaks in Hanalei River. This forced the closing of Kuhio Highway at the Hanalei River Bridge for more than 24-hours.

A more localized Kauai event flared up on the afternoon of May 12 when the gage at Kapahi recorded over 5 inches of rainfall in a 2-hour period. Flash flooding in Moalepe Stream briefly closed Kahuna Road in the upper Kawaihau area of east Kauai.

The upper level low pressure system moved away from the state on May 14 allowing a more stable trade wind pattern to settle in over the island chain. Most of the remaining days of the month included moderate to fresh trade winds with showers primarily over the east-facing slopes.

April and May rainfall over the leeward areas of the Big Island have helped ease the prolonged agricultural drought conditions. Extreme drought, which has been affecting portions of the Big Island continuously since July 7, 2009, was finally downgraded to the severe drought category.

Island of Kauai:

All of the gages on Kauai recorded above normal rainfall totals. By May 10, the rainfall accumulations at most locations had exceeded the monthly normal. The 59.64 inches recorded by the U.S. Geological Survey’s (USGS) Mount Waialeale gage marked the state’s highest monthly total and the highest May total at that site since 2002.

Mount Waialeale also had the highest daily total of 14.38 inches during the flash flood event on May 8. The USGS’ Hanalei River and North Wailua Ditch gages also recorded daily totals greater than 10 inches during the same event.

Lihue Airport had 9.08 inches of rain, which made it the wettest May since 1997.

Most of the gages on Kauai had totals in the near to above normal range for 2011 through the end of May. Mount Waialeale had the state’s highest year-to-date total of 161.74 inches (89 percent of normal).

Island of Oahu:

Most of the gages on Oahu reported above normal rainfall for the month of May. Only the gages at Waimanalo and Bellows logged below normal totals. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage posted the highest monthly total of 30.99 inches (178 percent of normal) and the highest daily total of 6.43 inches during the flash flood event on May 8.

Of this total, over 3 inches occurred between midnight and 3 AM HST. The 3.09 inches recorded at Honolulu Airport made it the wettest May since 1978 (3.39 inches).

Most of the gages on Oahu had near to above normal rainfall totals for 2011 through the end of May. The Oahu Forest NWR gage had the highest year-to-date total of 81.54 inches (90 percent of normal) and was third highest statewide.

Maui County:

Most of the gages on the slopes of Haleakala had near to above normal rainfall totals for the month of April. In contrast, most of the west Maui gages and all of the Molokai gages had below normal rainfall. The USGS’ Puu Kukui gage had the highest monthly total of 17.25 inches (49 percent of normal) and the highest daily total of 3.83 inches on May 5.

Rainfall totals in Maui County were not as noteworthy as on Kauai and Oahu but it was still the wettest May at Kahului Airport since 2004.

Most of the rain gages in Maui County had below normal rainfall totals for 2011 through the end of May. The Puu Kukui gage had the highest year-to-date total of 73.38 inches (42 percent of normal) and dropped a notch to fourth highest in the state.

Island of Hawaii:

Most of the gages across the Big Island posted near to above normal rainfall totals for the month of May. Glenwood had the highest monthly total of 20.21 inches (168 percent of normal). This gage received 11.44 inches during a 4-day period from May 4 through 7 and had measurable rainfall (at least 0.01 inches) every day during the month.

The mid-afternoon flash flood event on May 6 in the South Kona District included a 2-hour rainfall total of 2.73 inches at the Kealakekua gage. The monthly total of 9.09 inches (118 percent of normal) at this site set a new record for the wettest May with data going back to 1991.

Despite the wet conditions, most of the rainfall totals on the Big Island for 2011 through the end of May were still below normal.

Glenwood continued to have the highest total of 57.61 inches (70 percent of normal). Most of Big Island totals were in the range of 40 to 70 percent of normal.

Data Sources:

Data used in this report are largely from National Weather Service sources including climate network weather observation stations at Lihue, Honolulu, Kahului, and Hilo, the Hydronet state network of automated rain gages, and selected Cooperative Observer sites.

Additional data come from automated rain gages operated by the state Department of Land and Natural Resources, the US Geological Survey, the US Bureau of Land Management, the US National Park Service, the Department of Defense, and the US Fish and Wildlife Service. Data presented here are not certified and should be used for information purposes only.

For more information on the drought, please refer to the latest Drought Information Statement at: www.weather.gov/data/HFO/DGTHF…

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