In the February 11, 2021 SCA External Letter (of registrations of names, devices, & badges) for the Kingdom of An Tir, there was a new badge entry (for one Vémundr Syvursson) that depicts a bank of clouds with lightning coming out of it. It’s intriguing, because the blazon is “a thundercloud argent”, whereas when people have wanted to register this kind of images, they’ve had to do it separately — the cloud, with a lightning bolt, or a sunburst inverted. In other words, the charge does not have its own unique name, one that encompasses the idea (in this case: a bank of clouds with three lightning bolts, but wavy like a kris knife, descending from them). So if this blazon is accepted for registration, it would be the defining instance of the “thunderbolt”, and would open up the charge for other folks to incorporate into their devices and badges as well (although if this is the default image, then people would need to be specific if they wanted a number of bolts other than three, for the bolts to be a color other than gold, etc., since the extant example of the charge is as below).
The source for this charge appears to be from a single person or family, “Donnersperg” (or “Donnersberg” — “Thunder Mountain”), and first appears in Bernhard Stiber’s Stammbuch (blatt 54), 1588-1597. Note that in the source itself, this page is marked as “unbekannt” (unknown). This isn’t the only Stammbuch in which these arms are depicted, but this is the one I like the most.
I did a traced version of this image for The Book of Traceable Heraldic Art, in anticipation that at least something would be registered, if not the term “thunderbolt” per se (it’ll be interesting to see if the Powers That Be accept this, or require that the blazon be rewritten). You can find it over at heraldicart.com on the “Cloud” page — enjoy! (And Mathghamhain also put up a version without the rays, if you’re just looking for a cloud.)