National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: February 2018

Prepared: March 6, 2018

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

State: [Text data table for rain gages]

A shift of the large scale atmospheric wave pattern in the North Pacific during late January placed the main Hawaiian Islands within a wet pattern that continued into February. This pattern remained generally stuck through the month, producing several low pressure systems that passed over or near the state. This, in turn, resulted in one of the wettest Februarys across the island chain in over 10 years. The first heavy rain event occurred from February 4 through February 6 as a cold front supported by a strong upper level trough passed over the state. A few spots on Kauai recorded over 7 inches of rainfall which produced some minor flooding problems but no reports of significant damage. Much smaller amounts, mainly less than 2 inches, fell on the rest of the island chain. A weaker cold front moved across the state on February 8 and brought cool north winds in its wake but produced little significant rainfall.

After a few days of cool, dry north to northeast winds, a kona low developed a few hundred miles north of the main Hawaiian Islands on February 13 and shifted the winds to moisture-laden southerlies. Brief periods of moderate to heavy showers through February 15 produced wet conditions on Oahu and Maui County but did not result in any reports of significant flooding issues.

As the kona low moved away to the west on February 16, a surface low pressure trough moved in from the east. The trough moved over the Big Island and Maui on February 17 in conjunction with a strong upper level trough approaching the state from the northwest. Heavy rainfall developed along the windward slopes of the Koolau Range on Oahu during the evening of February 17. Automated rain gages from Maunawili to Punaluu recorded short term rain rates of 1 to 3 inches per hour with a peak rate of more than 5 inches per hour reported by the Ahuimanu gage at around 10:20 PM HST. At around this time, Kamehameha Highway was closed at Waikane due to the overflow of Waikane Stream. Other windward Koolau streams such Waiahole Stream, Kahana Stream, and Punaluu Stream also had large spikes in water levels during this time. Rainfall eased around 2 AM HST on February 18 but resumed before noon with similar resultant stream level increases which once again closed Kamehameha Highway at Waikane Stream. The highway was also severely inundated near Kualoa Ranch. Several low-lying properties received flood damage with the most significant impacts in the Waiahole and Waikane areas. A few motorists were also rescued when their vehicles stalled on the flooded roads.

The mid-day outbreak of heavy rainfall also affected Maui County with the windward slopes of the West Maui Mountains getting hit particularly hard on February 18. Preliminary data from the U.S. Geological Survey (USGS) indicated that Waihee River had its highest discharge in over 15 years. Flooding in the river damaged several homes and structures along its banks and two individuals were temporarily stranded by flood waters. Fortunately there were no injuries reported. A couple of miles to the south, Waiehu Stream also overflowed which briefly closed Lower Waiehu Beach Road. As the upper level trough moved eastward, unstable conditions shifted to the Big Island by the late afternoon and early evening hours. Severe thunderstorms along the Kona slopes produced heavy rainfall but the storm cells moved too fast to produce flooding problems. However, the outflow winds damaged structures and caused one death. A strong thunderstorm subsequently developed over the windward Kohala slopes and moved slowly southeastward to Honokaa. Intense rainfall from this thunderstorm produced flooding in Honokaa that damaged a business. The upper level trough lifted to the northeast of the Big Island on February 19 allowing more stable conditions to settle in over the island chain through February 21.

By February 22, yet another strong low pressure system developed over the region, this time to the west of the main Hawaiian Islands. This final round of heavy rainfall for the month mainly affected Kauai and Oahu. During the afternoon of February 22, several waves of heavy rainfall developed over central and north Kauai. As usually happens in these scenarios, Hanalei River overflowed its banks and inundated Kuhio Highway near the Hanalei Bridge for several hours. The saturated conditions also destabilized the slopes in some areas resulting in a few landslides, the most significant of which closed Kuhio Highway between Wainiha and Lumahai on February 23. Heavy showers continued on February 25 over Oahu with 2 to 5 inches of rainfall occurring over the windward slopes of the Koolau Range. Although the rainfall totals were less than half  of the previous week’s storm totals, the still saturated soils managed to produce about the same level or greater flood response in several windward Oahu streams. As was the case during the previous Sunday, Kamehameha Highway was closed at Waikane Stream for several hours. After this event, trade winds and more stable conditions filled into the area from the east to close out a very wet month.

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

Wet conditions affected the entire island of Kauai and all of the gages reported above average monthly rainfall totals. The USGS’ gage on Mount Waialeale posted the highest monthly total of 51.78 inches (212 percent of average) and the highest daily total of 9.42 inches on February 26. The Hanapepe, Hanalei, Kokee, and Wailua gages recorded their highest February totals in a data record going back to the early 1990s. Mount Waialeale and Lihue Airport had their highest February totals since 1989.

Locations along the northwest, north, and northeast areas of Kauai had rainfall totals in the near to above average range for 2018 through the end of February. Nearly all sites located in the east, south, and west sides had above average year-to-date totals. Mount Waialeale had the highest 2018 year-to-date total of 76.95 inches (156 percent of average).

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

Most of the gages on Oahu had monthly totals in the above average range for the month of February. The USGS’ Poamoho Rain Gage No. 1 had the highest monthly total of 32.65 inches (207 percent of average). The highest daily total (midnight to midnight) of 10.38 inches came from the Hakipuu Mauka gage on February 18. However, the highest 24-hour total of 11.97 inches occurred at Ahuimanu during the period from 8 PM HST on February 17 through 8 PM HST on February 18, which encompassed the double-header flash flood events along the windward Koolaus. The gages at Hakipuu Mauka, Hawaii Kai Golf Course, Kahuku, Lualualei, Maunawili, Olomana Fire Station, and Poamoho all recorded their highest February totals in data records going back as far as 1991. The USGS’ Poamoho Rain Gage No. 1 had its highest February total since 1979.

Almost all of the gages along the Koolau Range had totals for 2018 through the end of February in the above average range. Sites along the south shore and Waianae Range had totals in the near to below average range with several sites having to recover from significant January deficits. The USGS’ Poamoho Rain Gage No. 1 had the highest year-to-date total of 44.19 inches (130 percent of average).

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

Above average monthly rainfall totals were posted by most of the gages across Maui County. The USGS’ gage on top of Puu Kukui had the highest monthly total of 21.55 inches (82 percent of average) and the highest daily total of 5.71 inches on February 15. The Kaunakakai Mauka gage posted its highest February total in a data record going back to 1992. Kula Branch Station, Ulupalakua Ranch, and Kahului Airport had their highest February totals since 1982, 1989, and 1990, respectively.

A majority of the gages across Maui County had rainfall totals for 2018 through the end of February in the near to above average range. There were several sites with totals still in the below average range due to substantial dryness in January. The Puu Kukui gage had the highest year-to-date total of 38.31 inches (67 percent of average).

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

Most of the gages on the Big Island logged above average monthly totals for February. The USGS’ Saddle Road Quarry gage had the highest monthly total of 38.90 inches (375 percent of average), of which 30.30 inches occurred during the last 7 days of the month. The highest daily total was 16.74 inches from the National Park Service’s Pali 2 rain gage on February 15. This was an odd heavy rain event that primarily occurred over the lower elevations of the Kau District with rainfall pouring onto mostly porous lava flows with no known flooding impacts. The Pahala rain gage had its highest February total in a data record going back to 1991, and the Kapapala Ranch gage posted its highest February total since 1979.

Rainfall totals for 2018 through the end of February were in the near to above average range at most of the gages in the North Hilo, South Hilo, Puna, and Kau Districts. Many of the sites in the North Kona, South Kona, and South Kohala Districts had year-to-date totals remaining in the below average range due to significant January dryness. The Saddle Road Quarry gage had the highest year-to-date total of 69.82 inches (335 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