What is the Most Rain to Ever Fall in One Minute or One Hour?

What is the Most Rain to Ever Fall in One Minute or One Hour?

As a follow up to my last blog I thought I would write up a short summary of the most intense short-term rainfalls on record in the United States. Wiley Post Airport in Oklahoma City picked up 3.10" in 1 hour and 2.00" in 35 minutes during the tornado and flood event on May 31st.

This spring has seen some phenomenal heavy rain events (as outlined in my previous blog). Some of these included very intense short-term precipitation totals such as the 2.00” in 35 minutes in Oklahoma City on May 31st and 0.69” in 3 minutes (5:26 p.m.-5:29 p.m.) at Houston’s Hobby Airport on April 27th and 0.71" in 13 minutes at Paducah, Kentucky the morning of Saturday, June 1st (4:53 am.-5:06 am.). NOTE: The 5.64" 24-hour rainfall on May 31st in Oklahoma City brought the May total to 14.52", a monthly record and just short of the all-time wettest month (any month) on record of 14.66" set in June 1989.

How do these stack up compared to the all-time U.S. records for short-term rainfalls?

Below is a table of the most intense rainfalls on record over various periods of time:



Table reproduced from my book 'Extreme Weather: A Guide and Record Book'.

One-minute rainfalls

It is hard to imagine how an accurate measurement of precipitation over a 60-second period can be made. However, the U.S. Weather Bureau conducted several exhaustive studies of the Unionville, Maryland claim to 1.23” in one minute on July 4, 1956 and determined it to be accurate (see for the Monthly Weather Review summary). A recording rain gauge: A Friez Universal Type with a 12-inch capacity, dual traverse pen, and 24-hour clock gear on a chart drum was used to make the measurement. It had good exposure and measured a storm total of 3.60” between 2:50 p.m and 11:30 p.m. with 2.84” of this falling in a 50-minute period between 2:50-3:40 p.m. The minute that ostensibly measured the 1.23” total occurred around 3:22-3:23 p.m. Here is a copy of the trace:



Although invisible in this reproduction, there is apparently a faint line on the original trace which is when the excessive rainfall occurred. From ‘Monthly Weather Review’, August 1959 p. 304.

A number of experiments were made on the gauge to determine its calibration and accuracy. The gauge passed all the tests to which it was subjected. Anecdotal information also played a part in the record’s certification. Extreme flash flooding and erosion were reported in the surrounding area and the sky became so dark that residents were forced to turn lights on in spite of being mid-afternoon. Roof gutters were overwhelmed, with water reported flowing off roofs “like Niagara Falls”.

The Unionville figure is generally considered not only the U.S. record for one minute but also the world record for such. A much-quoted 1.50” at Barot, Guadeloupe on November 26, 1970 cannot be verified (in fact, there appears to be no such location on the Caribbean island).

Other significant one-minute rainfalls include 0.69” at Jefferson, Iowa on July 10, 1955 and 0.65” at Opid’s Camp, California on April 5, 1926. At Jefferson 1.00” fell in 109 seconds.

Other amazing short duration rainfalls

The officially recognized U.S. rainfall for a 5 minute period was the 2.03” reported at Alamogordo Creek, New Mexico on June 5, 1960 (3.09” fell in 15 minutes). Fort McPherson, Nebraska reported 1.50” in 5 minutes on May 27, 1868 as did Pensacola, Florida on May 2, 1937. Patterson, New Jersey measured 1.64” in 8 minutes on July 13, 1880. Tyler, Texas reported 2.00” in 10 minutes on April 29, 1905. At Cambridge, Ohio a reputed 7.00" of rain fell in just 30 minutes on July 16, 1914. William G. Hoyt relates in his classic book Floods (Princeton Univ. Press, 1955) that "some persons caught in the open during the storm were actually alarmed for fear they would suffocate, the rain was falling so fast that they had to shield their noses so they could breathe". (p.25). Guinea, Virginia in Caroline County 8 miles southeast of Fredericksburg, reported 9.25" of rain in 40 minutes on August 24, 1906. The validity of the above measurements is open to question.

What is the greatest one-hour rainfall in the U.S. or World?

This is a matter of debate. There appear to be no ‘official’ records for such since each of the proposed cases has its own flaws. The most commonly cited figure is 12.00” at Kilauea Plantation, Kauai, Hawaii on January 24, 1956. This was part of a state record 24-hour rainfall that totaled at Least 38.00” on January 24-25 that year. However, the observer noted that the gauge was overflowing at several times during his observation cycle so the actual figures may have been considerably higher. The storm total was 43.5" apparently all within a 24-hour period but confusion reigns as to the details concerning this event.

Another contender is the 12" in 42 minutes at Holt, Missouri on June 22, 1947. This report has the details.. This is also accepted as a world record for the time span of one hour. Not too far away from Holt, Plainville, Illinois picked up 8.40" in 40 minutes between 6:05 and 6:45 p.m. on May 22, 1941.

A severe thunderstorm at Catskill, New York was reported to have dropped at least 10” of rain in one hour on July 16, 1819 according to bucket surveys.

The most intriguing candidate, however, is for a location (not identified with a specific town) in central West Virginia on the night of August 4-5, 1943. A U.S. Army Corps of Engineers storm report claims that 13.80” of rain fell in one hour between 11:30 p.m and 12:30 a.m. the night of August 4-5. This was measured by a remote gauge simply named No. 86. It appears to have been located near the town of MacFarlon (which measured 5” in 1 hour) in Ritchie County. Another gauge identified as No. 48 picked up 14.6” of rain in about one and a half hours. Curiously, a U.S. record of 19.00" in 2 hours and 30 minutes was measured at Rockport, West Virginia (Wood County) on July 18, 1889 which is only 15 miles west of MacFarlon. This begs the question of just why this part of the state seems prone to extreme precipitation events.

Woodward Ranch, Texas (about 17 miles northwest of D'Hanis, which is 40 miles west of San Antonio) measured an amazing 10.00" in 1 hour, 15.00" in 2 hours and a world record 22.00" in 2 hours and 45 minutes on May 31, 1935 between 3:00-6:00 a.m. This was an especially interesting event since, just hours earlier, a rain gauge in eastern Colorado (at a location 25 miles northeast of Colorado Springs) measured 24" of rain in a 6-hour period between noon-6:00 p.m. May 30th (22.80" of which fell in just 4 hours). Several other gauges in eastern Colorado also measured amounts close to this, lending creditability to the figure. The ensuing floods drowned 21 in this sparsely populated part of the state and the $100 million (adjusted for inflation) flood damage remains one of the costliest natural disasters in Colorado history. Naturally, extreme flooding ensued. The Weather Bureau’s Climatological Data, Colorado Section, May, 1935, reported the following in the Monthly Review:

"On the 30th, excessive local downpours occurred in the vicinity of Colorado Springs and along the northern slope of the Arkansas-Platte Divide...At the height of the flood, skies over extreme eastern counties (where the phenomenal rainfalls were recorded) were a chocolate brown. This was due to a most unusual situation. Along the Colorado-Kansas border there was a heavy dust storm. Clouds of dust could be seen for miles, while to the west torrents of floodwater roared, and at Bovina, hailstones, some as large as baseballs, were reported to have fallen. The coppery-hued sky cast a brown shadow, giving the scene a weird appearance."



The Monument and Fountain Creeks in Colorado Springs flooded portions of the city sweeping away homes and residents. This was the worst flood in Colorado Spring's history. Photo from Colorado Springs Pikes Peak Library District archives.

Yet another extreme rainfall worthy of mention is that of August 12, 1891 when the mountain town of Campo, California (located just a few miles north of the Mexico-U.S. border at an elevation of 2,800') reported an amazing 11.50" in 1 hour and 20 minutes (and a 16.10" storm total). The average annual precipitation here is just 14.82" and in August a mere 0.51". The observer, a Mr. Gaskill, who was considered reliable, stated that much more rain actually fell but he was unable to empty the gauge fast enough as it overflowed although flood waters undermined the gauge and tilted it a bit. This record has been of great interest to California climatologists since it may represent the most intense rainfall that has ever occurred in the state. The California Department of Water Resources gave it a return period of once in 270,000 years!

World 1-hour precipitation record

A purported 15.78" of rain fell at Shangdi (Sahngdu) Nei Monggol (Inner Mongolia), China on July 3, 1975 in just one hour. This record, along with many from China in the mid 1970's, has been difficult to verify. In a future blog I will look at some of the world point short-duration rainfall records, including those of China which has claim to some of the most amazing statistics in this regard.



The isohyetal map for the storm of August 4-5, 1943 in West Virginia that may have dropped 13.8 in one hour. Page from ‘Storm Studies’, Dept. of the Army, Corps of Engineers.

Another fascinating rainstorm, and perhaps of the greatest intensity on record for anywhere in the world, is the famous Smethport, Pennsylvania event of July 18, 1942. During this event an amazing 34.30” of rain reportedly fell (according to several bucket surveys) in a 12-hour period between midnight August 17th and noon August 18th. Of this, 28.50” fell in 3 hours (9.a.m-noon) and 30.70” in 6 hours. Mountainsides were reported to have their vegetation stripped to bedrock and 11 people drowned in the valley around the town. Other precipitation measurements made in the area included 20.4” in 12 hours at Emporium, PA and 18.5” at Mt. Jewett. An official USWB gauge in Salamanca, New York received 6.7” in 3 hours.



The isohyetal map for the storm of July 17-18, 1942 in Pennsylvania and New York that may have dropped 28.5” in three hours, a world record. Page from ‘Storm Studies’, Dept. of the Army, Corps of Engineers.

You may find it curios that so many of these records seemed to have happened in the 1940s and 1950s. This is not a coincidence. During the 1940s and 1950s the U.S. Government (via the Army Corps of Engineers) was very interested in determining what the maximum possible rainfall rates for any given location might be in order to engineer the massive infrastructure improvements that were being made across the country those decades.



The map above is an approximation of what the absolute maximum amount of rainfall that could possibly fall in a 24-hour period across the U.S. east of the Rockies might be, according to research by the Army Corps of Engineers. The actual measured maximum amounts by state are indicated by the blue dots. These are not official figures but 24-hour precipitation amounts that the USACE observed while investigating their ‘Storm Rainfall’ reports which resulted in a massive 1000-page plus compendium of intense short-duration rainfall observations. Map reproduced from ‘Extreme Weather: A Guide and Record Book’, based upon Army Corps of Engineers data.

Summary

Below is a table summarizing all of the precipitation records mentioned in this blog. Keep in mind that this is a mix of 'official' and unofficial measurements made by various U.S. government agencies.



P.S. This blog has almost exclusively focused on U.S. records. In the weeks to come I will take a look at extreme point rainfall records from around the world and also on those that hold the 24-hour record (a much wider net).

Christopher C. Burt
Weather Historian