Aside from a passing shower, rainfall of any consequence is rare in Hawaii’s capital in the summer months. Yet on Tuesday, a torrent of historic proportions dumped 4.2 inches of rain on Honolulu, a record for the month of June.
In a single June day, Honolulu received as much rain as it normally would in 16 June months.
The rainfall was 30 times the prior record of 0.14 inches for June 25 (set in 1994). It ranked among the top 25 heaviest daily rain events for any month of the year, and the only one occurring outside the October-to-April rainy season.
The downpours triggered flash flood warnings for the entire island of Oahu.
The rain originated from an unusual low-pressure system in the region, much more common in the heart of the winter wet season.
The weather system was preceded by several days of hot and humid weather that was also out of character for the Hawaiian islands, with temperatures hitting at least 90 on 12 of 14 days in Honolulu from June 7 to 20. Weather.com reported Honolulu broke daily record highs on 9 of the first 18 days in June.
Dew points, a measure of humidity, swelled into the stifling mid-70s. Trade winds typically keep island weather comfortable for about 80 percent of the year, with dew points rarely climbing out of the 60s.
The cold front associated with the unusual low-pressure zone triggered thunderstorms and heavy rain. The rainfall was probably increased by warmer-than-normal ocean temperatures surrounding the islands, which were also a contributor to the recent record-warm air temperatures. The warm ocean “raises the ceiling on precipitation intensity,” tweeted Daniel Swain, a climate scientist at the University of California at Los Angeles.
In the wake of the storms, high temperatures dropped 10 degrees — the normal spread between summer and winter in the islands.
Typically, summers in Hawaii are quite dry in leeward and lowland areas; the dry season runs from May until sometime in October.
Every few years, a decaying tropical system draws near, bringing increased humidity and rainfall during the dry season, but this more often occurs in August or September, when ocean temperatures have warmed. And this unusually timed rain had nothing to do with a dying hurricane, but rather a cold high-altitude, low-pressure system typical of winter storms that affect Hawaii most often from November through March.
June is by far the driest month in Honolulu; until this year, the most it had ever rained in the entire month was 2.46 inches. Its average June rainfall is just 0.26 inches. With another 1.3 inches falling in the morning hours of June 26 (another daily record), the running total for June 2019 is already up to 5.66 inches, more than double the previous record set in 1971, and 22 times the average.
The heavy rains were not limited to the capital city; locations across the westernmost Hawaiian islands of Kauai to Maui logged very high daily totals this week.
Here are more totals:
Waimanalo — 7.11 inches. Average June rainfall: 1.15 inches
Niu Valley — 5.21 inches. Average June total: 1.32 inches
Aloha Tower in downtown Honolulu: 4.79 inches. Average June total: 0.96 inches
Waianae Boat Harbor: 2.12 inches. Average June total: 0.36 inches
Island of Molokai
Kaunakakai: 4.12 inches. Average June total: 0.16 inches
Lanai: 5.20 inches. Average June total: 0.82 inches
Forecasters predict more typical trade wind weather to return to the islands this weekend, but they indicate that another upper-level disturbance may once again disrupt the stable trade wind regime later next week, bringing increased humidity and the renewed chance for heavy rain and thunderstorms.
The lead author, Ryan Saunders, was born and raised in Hawaii and works as a freelance writer in Germany.