|Above: Mt. Waialeale, on Hawaii’s Kauai Island, holds the U.S. historical record for wettest single month, when 148.83” of rain was measured during the month of March 1982. Last year, Waipa Gardens, located about 10 miles north of Mt. Waialeale, measured the greatest rainfall in any 24-hour period anywhere in the U.S. when 49.69” of rainfall was measured on April 14-15, 2018. Image credit: Courtesy Paul Chesley (from my book Extreme Weather; A Guide and Record Book).|
The 12-month period ending in June 2019 was the wettest year-long period on record for the contiguous United States (see Bob Henson's post about this). Contributing to this was the very wet calendar year of 2018. It was the third wettest on record (since 1895) for the contiguous U.S., surpassed only by 1973 (#1) and 1983 (#2). Eight states and the District of Columbia saw at least one site break the respective state record for most precipitation measured in a calendar year (see table further down in this post).
|Figure 1. The 12-month period of July 2018-June 2019 was (by far) the wettest July-to-June period on record for the contiguous U.S. at least since 1895, with 37.93” beating the 35.11” from July 1982 to June 1983. This was also the wettest of any 12-month span on record, just ahead of the 37.82” recorded on August 2018–July 2019. Image credit: NOAA/NCEI.|
Some recent state precipitation records (2018-19)
Most recently Arkansas broke its all-time state 24-hour precipitation record when 16.17” of rain deluged the town of Dierks on July 15-16, 2019. Kansas surpassed its wettest month record in May 2019, when 26.73” of rain was measured at a site 3 miles southwest of Rock. The greatest 24-hour rainfall on record anywhere in the United States occurred on April 14-15, 2018, when 49.69” flooded Waipa Garden on the island of Kauai, Hawaii. And then, of course, there have been the tremendous rainfalls associated with tropical storms the past few years which broke state precipitation records for any tropical cyclone in Arkansas, Hawaii, North Carolina, South Carolina, and Texas.
However, the only state monthly precipitation record officially broken in recent years was in Texas, where Harvey contributed to most of Port Arthur’s August 2017 total of 54.74”.
Below is a table showing results of my research into the precipitation records for each state. The data was originally compiled by weather historian David M. Ludlum and published in his 1970 booklet “Weather Record Book: U.S. and Canada” (a Weatherwise magazine publication) and since updated by myself. Not all of these are official records recognized by the SCEC (State Climate Extremes Committee). You can find their list (but almost exclusively just for each state’s 24-hour record) here. The reason(s) their list and my list do not completely jibe is that I include some 24-hour records that were not officially measured at a sanctioned COOP or other U.S. weather bureau site.
However, there is inconsistency in the SCEC list in this regard. For instance, the contiguous U.S. 24-hour record of 42.00” at Alvin Texas on July 25-26, 1979, was not measured at an official weather site. Also on their list is the annual precipitation record for Maryland (84.56” at Catonsville in 2018) which was measured at a CoCoRaHS site (NOAA’s official Climatological Data Summary for 2018 published the state maximum to be 80.78” at Mechanicsville). In my opinion, this is fine if the apparent record is investigated and confirmed, as was the case in this instance. However, it begs the question of why such investigations are not also done for other likely reliable records broken at unofficial sites such as CoCoRahs stations elsewhere in the country. For instance, a CoCoRaHS site near Cross Plains, Wisconsin (just west of Madison) measured 15.33” of rain during a storm on August 21, 2018. Several other sites in the Madison area also measured rainfall totals in the 12”-15” range, all of which fell in about a 10-hour period. Yet the “official” Wisconsin 24-hour rainfall record remains the 11.72” measured four miles northeast of Mellen on June 24, 1946.
It should be noted that verifying the older precipitation records that are not vetted by the SCEC is an almost impossible task. Unlike verifying older temperature extremes by employing reanalyisis, where broader atmospheric conditions can be determined, extreme precipitation events are often too localized to be confirmed in this way. To some degree, we can verify monthly and annual precipitation records by comparing the figures to what other nearby sites reported and excluding any anomalous figures.
|Figure 2. Above are tables that summarize summarizing each state’s precipitation records for (top) wettest 24-hour and single-month periods and (bottom) wettest and driest calendar-year periods. The tables include my updates to data originally researched by David M. Ludlum and published in 1970 by Weatherwise magazine. Note: There are three cases (Pennsylvania, Virginia, and Maine) where the listed 24-hour record is greater than the listed monthly record. This is because the sites did not report their monthly totals for that month when the 24-hour measurements occurred, although the actual 24-hour figures have been well investigated.|
Other candidates for state records
The state 24-hour maxima marked with an asterisk above are larger than the official records. These maxima were published by the Army Corps of Engineers in their report “Storm Rainfall 1887-1955”, published in 1962. Details for each maximum are included below.
COLORADO: 24.00” on May 30, 1935, at a gauge located 25 miles northeast of Colorado Springs (near Cherry Creek) and also a gauge just northeast of Burlington (near the Kansas border) on May 30-31. The rainfall reportedly occurred over just a six-hour period!
|Figure 3. An example of the “Storm Studies 1887-1955” reports prepared by the Army Corps of Engineers and published in 1962. In this case the first page of the investigation concerned the phenomenal rainfall reported on May 30-31, 1935, on Colorado’s eastern plains. Flooding in Colorado, Kansas, and Nebraska resulted in more than 113 fatalities. You can read more about this extraordinary event in a WU blog entry I posted six years ago.|
IOWA: 24.00” at two locations on September 17-18, 1926, near Boyden and Maurice
KENTUCKY: 20.00” on July 4-5, 1939, at Simpson
LOUISIANA: 23.80” on August 7-8, 1940, at Miller’s Island
MICHIGAN: 12.60” on August 31-September 1, 1914, at Cooper
MONTANA: 13.30” on June 19-20, 1921, at Springbrook
NEBRASKA: 15.80” on June 10-11, 1944, near Stanton
NEW HAMPSHIRE: 12.00” on November 3-4, 1927, at Kinsman Notch
NEW JERSEY: 24.00” on September 1, 1940, at Ewan
NORTH DAKOTA: 8.50” on June 26-27, 1914, at Hazleton
OKLAHOMA: 23.00” on April 3, 1934, near Cheyenne. Also listed (but noted as “unofficial”) was a 24.00” total on September 3-4, 1940, near Hallett
WYOMING: 9.70” on September 28-29, 1923, at Savageton
|Figure 4. The map above was reproduced in the Army Corps of Engineers study “Storm Rainfall” published in 1962 (and colorized for use in my book Extreme Weather). It depicts what hydrologists at that time estimated to be the maximum possible amount of rain that can fall in a 24-hour period east of the Rocky Mountains given the physics of the atmosphere. It is a “broad stroke” idealization and does not take into account the effect mountainous terrain may have on the rainfall intensity. It is likely that at this time, some 55 years later, the figures are in need of revision, especially with a warming atmosphere raising the odds of extreme precipitation events.|
Extreme precipitation events likely to become more frequent due to climate change
Many studies have concluded that the frequency and intensity of extreme precipitation events are likely to become more frequent as a result of climate change, and that such changes are already occurring in many locations. A report in the online journal Eos last June noted that for every 1°C rise in temperature the Earth’s atmosphere can hold 7% more water (see this summary of the report).
As precipitation events become more extreme, we may also see starker juxtapositions between record-high and record-low precipitation events. One anecdotal example: the summer of 2019 was the second warmest on record for the state of Alaska and also the driest for portions of the south-central and southeast (like Anchorage). Yet Nome managed to observe its greatest 24-hour rainfall event on record when 2.43” of rain were measured on August 1-2. Nome’s period of record (POR) goes back to 1906.
Christopher C. Burt