Giant Hail, Heavy Rain, Strong Winds, July 31 to August 1, 2024

Several waves of strong to severe thunderstorms produced some of the largest hailstones on record in Minnesota, along with pockets of very heavy rains and wind damage.

The storms formed as a warm and humid air mass intensified over the state, particularly in southern Minnesota. Temperatures rose into the upper 80s and low 90s F across much of the state, with dew points well into the 70s F in many locations from the Twin Cities on south and west.

The storms formed in multiple intersecting waves. The first entered  western Minnesota during the early afternoon, damaging trees, power lines, and small structures from near the border with South and North Dakota, east-northeastward past the Brainerd Lakes area and in towards Duluth. 

The rain-cooled air from these storms moved towards the extremely unstable building just to the south, and helped ignite several large and intense thunderstorms later in the afternoon and into the early evening, with radars estimating thunderstorm heights of up to 60,000 feet at times. Most of the cells developed south of the earlier storms, and moved eastward or southeastward rather slowly, affecting areas near the upper Minnesota River, the Twin Cities area, Faribault, Red Wing, and Wabasha. These large storms produced a over an inch of rain fall at the Twin Cities airport in under 30 minutes, and went on to produce 2-4 inches of rain in parts of Goodhue and Wabasha counties.

A couple storms developing with this second wave, originated very near areas of western Minnesota affected by the earlier wave, and these storms moved east-northeastward and became extremely powerful, producing historically massive hailstones. Three separate reports were received of hail in excess of 4 inches in diameter, and two separate locations in Stevens County near Chokio documented some of the largest hail ever reported in Minnesota. One hailstone, confirmed by photograph measured approximately six inches across, and another, in a different location was five inches across. The six-inch diameter stone is, at a minimum, the largest hail documented and confirmed in Minnesota in at least 38 years. Even the five-inch-diameter stone has few peers historically. Record-keeping for hailstone sizes is complicated by measurement delays (for safety reasons, among others), by melting, and by difficulty confirming and comparing historical reports.  

Around sunset, a third wave of thunderstorms formed across the southern quarter of Minnesota, producing 1-2-inch diameter hail, more spotty wind damage, and more intense rains--particularly near Waseca and Owatonna. A fourth wave of thunderstorms formed from northeastern South Dakota into western Minnesota again, and eventually these two waves merged into a single "MCS" or "mesoscale convective system," which is a large complex of thunderstorms and rain that often persists for hours as a nearly continuous mass. Steady rains and thunderstorms associated with this complex continued throughout the night across much of central and southern Minnesota, with light to moderate rain persisting well into Thursday in parts of the state.

Rainfall totals of 3-4 inches, with isolated reports of 5 inches or more, were common near Gaylord, Wells, Waseca, Owatonna, Faribault, Cannon Falls, and  and Gaylord. The National Weather Service cooperative observer at Theilman, in Wabasha County, reported 4.10 inches of rain. The observer at Wells reported 3.11 inches, bringing the station's annual total of August 1 to 40.07 inches. It's exceedingly rare for any Minnesota station to reach 40 inches this early, and many Minnesota locations have never recorded that much precipitation in any year.    

Was it a record?

It's hard to say! The extreme hail reported near Chokio on July 31 is among the largest for which we have official records in Minnesota and is the largest we can verify with existing evidence. After looking through the primary database for archiving hail and other weather hazards (here), and cross-referencing the largest reported hailstones with supplementary data, photographs, and historical documents, the stone that was approximately six inches across may be the largest hailstone on record for the state, though it has reasonable competition from at least four historical events. Confirming the record-holder is difficult because of the following considerations:

• Many hail reports are "eyeballed," with the size compared to those of various coins, balls, or other spherical or rounded objects. Most hail reports are not based on measurements, but on these sorts of reference comparisons. "Softball-sized hail," therefore, certainly implies someone observed massive hailstones, but rarely means that hail was exactly the size of a softball. (Moreover, slow-pitch softballs are about 9% wider than those used for fast-pitch!)
• When hail is actually measured, it is (understandably) done with whatever is available, leading to different methods and different results. Some historical accounts report only the weight of the hail, which does not translate readily to other size measurements. An observer who has a stiff tape measure or ruler is likely to measure diameter (or longest distance across) only. An observer with a flexible tailor's tape may be able to measure the circumference. Many historical accounts include only one type of measurement, and we cannot make inferences about what the other measurement would be because hail is rarely a true sphere (see next). 
• Although many hailstones are roughly ball-shaped, they rarely if ever are true spheres, which means we cannot assume a measurement of diameter corresponds to a certain circumference, or vice versa. This issue gets even more complicated by "aggregate" hail, which is the type of giant hail documented on July 31, 2024 (see picture above). Aggregate hail is often bumpy or "spiky," and even oblong, because it is made from the collision of multiple fast-moving hailstones that get glazed together as they travel through the storm cloud.   

 

Very large hail statistics 

Many thunderstorms in Minnesota produce hail, and in a typical year, we may have 40 days or more where it hails somewhere in the state. Most of that hail is relatively small and non-damaging. As hail size increases however, so does its volume, mass, and maximum potential velocity. As a result, the larger a hailstone, the greater its potential to cause damage or even injuries. Hail with a diameter of about an inch is the entry level for it being potentially dangerous, so the National Weather Service issues Severe Thunderstorm Warnings any time hail that large or larger is detected or suspected to be occurring. There may be 30-35 days in a typical year with one-inch hail.

Here are some basic frequency statistics for larger hail sizes in Minnesota:

• Two-inch diameter or greater: reported somewhere within the state 7-8 days per year, on average
• Three-inch diameter or greater: reported somewhere within the state 2-3 days per year, on average
• Four-inch diameter or greater: reported somewhere within the state about 5-6 days per decade, on average (39 days out of 70 years). This type of hail is quite extreme, and thankfully, somewhat rare (see image). 

 

Historical "contenders" for the record hail size in Minnesota

The Storm Events Database and other official resources contain four previous hail events that could vie for largest on record in Minnesota.

1. June 13, 1956, 5" diameter hail in Pine County. A form submitted by the former State Climatologist and member of the US Weather Bureau at the time indicated that the hail destroyed "vehicle windshields and roofs." No measurements were indicated.
2. October 14, 1966, Dodge County. This is unusually late for hail in Minnesota, let alone very large hail, but this day did feature an outbreak of tornadoes in Minnesota and Iowa (along with a snowstorm in northern Minnesota). A hailstone that crashed through a truck window in Dodge County was reported to have had a circumference of 16 inches and was called out in an official US Weather Bureau report to be an "all-time Minnesota hail size record." For a sphere, this would translate to a diameter of over five inches, but this may also have been aggregate hailstone, which would have made it highly non-spherical and potentially well over five inches across.   
3. July 4, 1968, 6" diameter hail in Murray County. The official report from that day indicates that hail was generally 1.25 inches in diameter but "some 6-inch hail reported with fields white with hail 4 inches deep." No further documentation found (yet).
4. July 28, 1986, 6" diameter hail in Nobles County. The official report includes reference to hail the size of grapefruits and a circumference measurement of 13 inches. For a sphere that would translate to a diameter of just over four inches, but this may also have been aggregate hail, which would have made it highly non-spherical and potentially well over four inches across.

The giant hail reported on July 31, 2024 in Stevens County was the largest reported in at least 38 years (since the 1986 storm) and the largest for which we have verifiable documentary evidence. Whether it was actually larger than any of the other giant hailstones on record in Minnesota is a matter of speculation. All we know is that this hail was extremely large and extremely rare in Minnesota. 

August 29, 2024

KAB