National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: July 2018

Prepared: August 17, 2018

Note:  This summary uses the arithmetic mean, or average, for “normal” rainfall values.

State: [Text data table for rain gages]

The entire month of July consisted of large scale trade wind conditions near the surface. An unusual aspect of the weather was that most of the heavy rain events were associated with the Fissure 8 eruption in the Lower East Rift Zone of Kilauea Volcano. On several instances, an unstable and anomalously moist trade wind air mass combined with the heat and moisture inputs from Fissure 8 to produce persistent, localized heavy rainfall over the Leilani Estates and surrounding areas. Occasionally the winds aloft shifted to a southeasterly direction which  allowed rainfall to occur northwest of Fissure 8 and produced minor flooding in and around Pahoa. The peak Fissure 8-induced rainfall day was July 2. Environmental conditions on this day were destabilized by an upper tropospheric low pressure system northeast of the Big Island combined with a passing area of enhanced low level tropical moisture. These conditions were locally enhanced by the Fissure 8 eruption to produce a long-lived, nearly stationary thunderstorm with frequent lightning over the Leilani Estates area. A volunteer observer with the CoCoRaHS network in the western portion of Leilani Estates recorded 13.97 inches from 7 AM HST, July 2 to 7 AM HST, July 3. Fortunately this heavy rainfall area was highly localized and occurred over a porous soil area with no streams so there was no significant flooding.

Elsewhere across the state, many of the windward areas received near to above average rainfall but leeward areas had drier than average conditions. As a result, drought impacts returned to Maui County by mid-July, then Oahu and the Big Island in the latter half of the month. Kauai County has had adequate rainfall and remained drought-free so far this summer.

Island of Kauai : [July 2018 map] [year-to-date map]

Most of the gages across Kauai recorded near to above average rainfall for the month of July. The U.S. Geological Survey’s (USGS) gage on Mount Waialeale had the highest monthly total of 48.11 inches (124 percent of average) and the highest daily total of 5.01 inches on July 14. Waialeale’s monthly total was its highest July rainfall value since 2000.

Rainfall totals for 2018 through the end of July remained above average at most of the rain gages on Kauai. The highest year-to-date total was 326.24 inches (144 percent of average) at Mount Waialeale.

Island of Oahu: [July 2018 map] [year-to-date map]

July rainfall totals were near to below average at most of the gages on Oahu. Locations along the leeward slopes of the Waianae Range were especially dry with monthly totals at less than 50 percent of average at all of the sites. The USGS’ Poamoho Rain Gage No. 1 had the highest monthly total of 12.54 inches (69 percent of average) and the highest daily total of 1.53 inches on July 14. The Hakipuu Mauka gage posted its lowest July total since 2000, and Wheeler Army Airfield had its driest July since 2004.

Despite recent dryness, rainfall totals for 2018 through the end of July remained near to above average at most of the gages on Oahu. A few of the gages along the lower leeward slopes of the Waianae Range had below average totals. The Poamoho Rain Gage No. 1 had the highest year-to-date total of 141.10 inches (109 percent of average).

Maui County: [Maui July 2018 map] [year-to-date map] [Molokai/Lanai July 2018 map] [year-to-date map]

Gages in areas exposed to trade wind rainfall recorded near to above average rainfall totals. However, leeward locations on Maui, especially along the lower elevations, had very dry conditions with several sites indicating less than 0.10 inches for the entire month of July. The  USGS’ Puu Kukui gage had the highest monthly total of 31.33 inches (95 percent of average), which was the highest July total at this site since 2005. The Puu Kukui gage also recorded the highest daily total of 5.18 inches on July 6. This was part of a 4-day period from July 5 through July 8 which produced 12.73 inches of rain. Under trade wind conditions the West Maui Mountains have an incredible rainfall gradient that is clearly shown in the July totals. Puu Kukui’s 31.33 inches contrasts sharply with the 0.05 inches at Lahainaluna about 4.75 miles away. This is a change of 6.6 inches of rain per mile.

Maui County rainfall totals for 2018 through the end of July were near to above average at most of the gages. The USGS’ West Wailuaiki gage had the highest year-to-date total of 185.28 inches (132 percent of average).

Island of Hawaii: [July 2018 map] [year-to-date map]

Most of the rain gages on the Big Island posted below average rainfall totals for the month of July. Leeward areas were very dry with most of the monthly totals at less than 50 percent of average. The Kona slopes region, which should be in its summer wet season, continued to have below average rainfall. As mentioned above, the Fissure 8 eruption in the Lower East Rift Zone has clearly boosted rainfall in Leilani Estates and the surrounding areas. The same CoCoRaHS observer that recorded 13.97 inches on July 2 also posted a monthly total of 43.29 inches, making it the wettest spot on the Big Island and the second wettest spot in the state. The Rainfall Atlas of Hawaii shows that this area averages about 10 inches of rain in July. Since Fissure 8-induced rainfall occasionally drifted to the northwest, this helped boost rainfall at Pahoa to its highest total since 1997.

Big Island rainfall totals for 2018 through the end of July were near to above average at most of the rain gages. The Saddle Road Quarry gage had the highest year-to-date total of 217.52 inches (265 percent of average).

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 of Hawaii Department of Land and Natural Resources, the U.S. Geological Survey, the U.S. Bureau of Land Management, the U.S. National Park Service, the Department of Defense, the U.S. Fish and Wildlife Service, and the U.S. Department of Agriculture.  Averages come from the National Climatic Data Center (1981-2010 series) and the Rainfall Atlas of Hawaii (http://rainfall.geography.hawaii.edu/).  Data presented here are not certified and should be used for informational purposes only.

Kevin R. Kodama
Senior Service Hydrologist
NOAA/NWS Weather Forecast Office Honolulu