Day 209: Overview of Thunderstorm Producing Record Hailstone
During the evening of 23 July 2010, Vivian, South Dakota experienced a severe thunderstorm no homeowner would enjoy; the thunderstorm packed winds estimated to be in the 110-120 miles per hour range. If that wasn’t problem enough, the thunderstorm was also producing hail that measured over 8″ in diameter and 18.5″ in circumference. (Anecdotal reports indicate that the hail was closer to 11″ in diameter when it fell!) Photographs from the region show holes in roofs. Not damaged roofs, but actual holes greater than 5″ in diameter! At least one hailstone was preserved and it officially weighed in at 1.9 pounds — even after substantial melting! You can read more about this thunderstorm, and see pictures of the hailstone(s) here! (Note, I would have blogged about this sooner, but I was waiting for official word that this hailstone is the world record holder, however that will take a while longer.)
If this hail stone is verified as legitimate, and I have no reason to believe it won’t, it will smash the previous record for largest hail stone. That hail stone measured 7″ in diameter and weighed closer to 1.6 pounds. It fell in Aurora, Nebraska on 22 June 2003. Prior to the Aurora, NE hailstone, the world record hailstone was from Coffeyville, Kansas.
Our current understanding of thunderstorms indicates that in order for a thunderstorm to generate a hailstone of this size, several things must come together. First, we need to have a lot of atmospheric instability. This allows for thunderstorms to develop very strong updrafts, which are necessary to suspend the hailstones aloft. Next, we need the freezing level to be relatively close to the ground. When the freezing level is close to the ground, more of the thunderstorm is below freezing. This means that most of the thunderstorm’s moisture is located in a part of the atmosphere that is conducive for ice formation. Lastly, we need the thunderstorm to rotate. This is because a rotating thunderstorm actually acts to make the updraft stronger than it would be if the updraft was not rotating. This allows for the thunderstorm updraft to suspend hailstones even longer, allowing them to become larger.
The radar images above clearly depicts a very strong thunderstorm, known as a supercell, shortly before the town of Vivian, SD experienced the potentially record hailstone. (Vivian is located in the “hook-like” appendage in the bottom-middle of the thunderstorm.) The two images above are both evaluating a height of 15,000 feet above the radar. At this height we can clearly see in the right image that this supercell thunderstorm had very strong rotation!
If we look at around 27,000 feet, we can see what is known as a Bounded Weak Echo Region (BWER) signature in the reflectivity (above/below left). (It looks like a donut-hole. If you are unfamiliar with this signature, the image below is annotated.) This signature is indicative of an extreme updraft. If we examine the velocity image, we see that the thunderstorm is still rotating, albeit, it is more difficult to discern.
The last set of images are a bit different from what I tend to show. Normal radar images depict a quasi-horizontal cross-section through a thunderstorm. These images are actually a vertical cross-section through the thunderstorm (beneath the vertical cross section is a typical radar image!). I’ve annotated a couple of important parts of the thunderstorm in the image below. This thunderstorm had a very strong forward-flank downdraft (which is where most of the heavy rain falls), a very strong rear-flank downdraft (strong winds, moderate rain, and hail1 are typically found here), and an extremely strong updraft (the largest hail fails around the updraft). At the time of the cross section, Vivian, SD was located almost directly beneath the strong updraft, which helps explain why the largest hail fell there.
Nice article Patrick! Can you display a sounding or some upperlevel analysis to this amazing event? Dr.Jeff Basara once commented on the height of the EL and the production of large hailstones; I can’t quote him accurately, but he said something like, “If the tropopause were high enough basketball hail would be possible!” I’ve always suspected hailstones, larger than those recorded in Aurora,NE, were landing across the plains. Most stormchasers, me included, usually position for tornadoes or ‘beauty shots’ of storms. About once out of every 5 chases I’ll ‘sample’ the hail a storm might produce.
Hi Simon, I looked at observed soundings in the vicinity, but none of them were close enough to have captured what I presume the storm environment was like. I’ll attempt to get some RUC soundings this afternoon and add them today.
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Great post! Regarding your note on what Chuck said about hail in the RFD, I had some golfball-sized hail fall on my car three weeks ago from an RFD (or at least due to the winds of the RFD wrapping around the storm).
I was up in northwest Kansas, driving east towards a nice little cell that was starting to weaken, and based on the location of the core of the storm, I figured I could drive another mile before dropping south to get ahead of it. Suddenly, chunky, nearly spherical hail of about 2 inches in diameter started blowing towards us!
I should have seen it coming, but there was practically blue sky above us while the hail was being blown towards us at an angle, thanks to the RFD.
Somehow I managed to get away without even a dent or a window chip (I assume it was mostly because the hail was not falling straight down so much as flying at us at an angle).