I'm not sure there is a really strong geographic signal for giant hail. In other words it likely happens a lot more frequently than we think. The problem with the giant hail climatology, well...hail climatology in general, is that it's so dependent on StormData, which is pretty lackluster. That's of no fault of the NWS, because the documentation is dependent on the hail actually hitting residents, and those residents actually reporting the hail...which happens much less frequently than you might imagine, strangely enough.
For example, if you'll excuse the irresponsible story telling (since I heard this second-hand), the Vivian SD 8" stone was never reported to the NWS. From what I understand they were out surveying a weak tornado, and either the EM or a resident talked about his neighbor picking up a monster stone and preserving it. If that turns out to be untrue I'll redact the anecdote. But the point remains. I surveyed a giant-hail producing storm with Scott Blair back in Sept 2010 that hit Wichita. The findings of that survey can be found in this paper that we published in the EJOM (
http://www.slightrisk.net/yahoo_site_admin/assets/docs/EJOM_ICT_Hail.34700408.pdf ). The main discussion of the article was that while a 7-8" hail-producing storm went right through the Wichita metro, the NWS received around 30 reports. However a perusal of social media yielded several hundred reports. While social media will help with the future climatology of hail, it doesn't offer much for the historic documentation.
I worked with several fine Mets on a paper regarding giant hail climatology and detection, and if you're interested in learning how to use radar to try to interrogate 4+" hail in a supercell, which can come in handy for chasers, then I recommend the following piece by Blair et al. (
http://www.ejssm.org/ojs/index.php/ejssm/article/view/87/67 ).
One more piece to look at if interested is a presentation Blair et al. did at the 2014 SLS in Madison, WI, regarding the "true hail-fall characteristic" of a storm vs what we believe to be the hail-fall characteristic. He has completed 4 years of hi-res hail documentation and compared it to the historical documentation. The results will likely surprise you....or maybe they won't.
https://ams.confex.com/ams/27SLS/webprogram/Paper255707.html
The good news with the above recommended reading pieces is that they are very easy to read and digest. There isn't much in the way of technical meteorology in them. But they offer some pretty valuable insight into the reality of hail frequency and detection, which is very important for chasers to understand, especially when it comes to deciding which storms to mess with and which ones to leave alone. The general findings are that there are A LOT more of these giant-hail producing storms than anyone ever considered.
