April 2017 Precipitation Summary

Monthly Precipitation Summary

State of Hawaii

Month: April 2017

Prepared: May 3, 2017

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

State: [Text data table for rain gages]

The October 2016 through April 2017 Hawaiian Islands wet season concluded in dramatic fashion with the passage of a subtropical cyclone, or kona low, near the state. As mentioned in an earlier summary, “kona” is not capitalized here because in this context it is used as the Hawaiian adjective for “leeward” rather than the location “Kona”.  The presence of this type of weather system near the island chain in mid-spring is rather unusual with less than 10 percent of all kona lows occurring in April and May. While unusual, it fits perfectly into what has been quite an erratic wet season across the State of Hawaii. Initial impacts occurred during the system’s development phase as a precursor cold front moved across the state on April 28. Thunderstorms over the heavily populated areas of south Oahu formed during the afternoon hours and threatened to produce significant urban flooding near the Honolulu International Airport and surrounding light industrial areas. Fortunately the most intense rainfall propagated offshore before sufficient runoff accumulated to cause problems. The front subsequently moved east, stalled over Maui, then its remnant moisture became the focus of rainfall during the next 48 hours. By 8 AM HST, April 30, roughly 4 to 8 inches of rain had fallen over Maui with the highest amounts along the slopes of Haleakala. Road flooding occurred on South Kihei Road which was apparently exacerbated by a combination of high tide and surf conditions. A portion of Honoapiilani Highway west of Maalaea also had water flowing across the road but it remained open to traffic. Piilani Highway at Pahihi Gulch was closed due to water and debris across the road. Farther east on Piilani Highway, a sinkhole reopened near Kalepa Point, likely as a result of the enhanced rainfall. Oahu and the Big Island both received about 1 to 4 inches of rain spread out over many hours which mitigated significant flooding problems.

While the late April kona low had the most significant flooding impacts, other weather systems during the month also produced periods of enhanced rainfall. From April 13 through April 15, an upper level low pressure trough to the northwest of Kauai helped boost rainfall within the moderate trade wind flow. Windward areas of Kauai, Oahu, and the Big Island received about 1 to 3 inches of rain but no significant flooding problems were noted. This was followed by a weak cold front which moved across the state on April 16 but produced only a small increase in rainfall. However, its remnant moisture embedded within the trade winds generated 2-day rainfall totals of 5 to 10 inches on the windward slopes of Maui (April 17 and 18) and the Big Island (April 18 and 19). The rain totals were significant but the windward areas of both islands can handle a lot of rain, especially when it is spread out over a couple of days. Another weak cold front reached the island chain on April 20 and stalled over Maui on April 21. Rain gages on Maui recorded 1 to 2 inches of rain which was enough to produce some minor flooding issues but no significant damage.

Island of Kauai : [April 2017 map] [year-to-date map]

Rainfall totals for the month of April were in the near to above average range at many of the gages on Kauai. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 26.57 inches (70 percent of average) and the highest daily total of 6.49 inches on April 22. The 4.42 inches recorded at the Hanapepe gage broke the record for the wettest April at this site. On the north side of the island, the Hanalei gage posted its highest April total since 1997.

All of the rain gages on Kauai have recorded near to below average rainfall totals for 2017 through the end of April. The highest year-to-date total of 72.27 inches (58 percent of average) was from the Mount Waialeale gage.

Island of Oahu: [April 2017 map] [year-to-date map]

April was very wet across Oahu with most of the gages posting above average monthly rainfall totals. The USGS’ Poamoho No. 1 gage had the highest monthly total of 18.60 inches (85 percent of average) but the gage at Waimanalo had the highest daily total of 6.09 inches on April 29 as part of the late month kona low event. Records for the wettest April were broken at Ahuimanu Loop, Aloha Tower, Hakipuu Mauka, Hawaii Kai Golf Course, Kahuku, Lualualei, Punaluu Pump, Waianae, Waihee Pump, and Waimanalo. Additionally, gages at Honolulu Airport, Niu Valley, and Palolo Fire Station recorded the highest April totals since 1974.

The wet April conditions, record-breaking at several locations, have pushed rainfall totals into the near to above average range for 2017 through the end of April at most of the gages on Oahu. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 56.43 inches (73 percent of average).

Maui County: [Maui April 2017 map] [year-to-date map] [Molokai/Lanai April 2017 map] [year-to-date map]

Most of the gages across Maui County logged above average rainfall totals for the month of April. Records for the wettest April were broken at Haiku, Kaunakakai Mauka, Pukalani, Lahainaluna, and Waikapu. The gages at Kahului Airport, Kula Branch Station, Molokai Airport, and Ulupalakua Ranch posted their highest totals since 1989. The USGS’ gage at West Wailuaiki Stream had the highest monthly total of 22.14 inches (88 percent of average) and the highest daily total of 7.40 inches on April 17.

Rainfall totals for 2017 through the end of April were in the near to above average range at most of the gages across Maui County. One of the notable exceptions was the USGS’ Puu Kukui gage which has recorded 42.38 inches, or just 32 percent of average. The USGS’ West Wailuaiki gage had the highest year-to-date total of 62.87 inches (71 percent of average).

Island of Hawaii: [April 2017 map] [year-to-date map]

April rainfall totals were in the near to below average range at many of the Big Island gages. The main exceptions were in the North Kona and South Kona Districts where most of the gages posted above average totals. The highest monthly total was 15.33 inches (99 percent of average) from the USGS’ rain gage at Kawainui Stream. Piihonua had the highest daily total of 5.97 inches on April 18.

Below average rainfall totals for 2017 through the end of April were posted by almost all of the gages on the Big Island. The USGS’ gage at Kawainui Stream had the highest available year-to-date total of 49.47 inches (90 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.

Reference for kona low frequency:
Otkin, J. A., and J. E. Martin, 2004: A synoptic climatology of the subtropical kona storm. Mon. Wea. Rev., 132, 1502-1517.

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