Here is an picture of a F5 tornado that struck Moore in 1997.
And here's daddy
That's right, you are looking at a supercell, gigantic behemoths of thunderstorms that extend as high as 70,000 ft, and the father of hail, torrential rain and (of course) tornadoes. Although you requested for an "EF0" tornado, I still described the F5 tornado. Because, tornadoes are notoriously difficult to produce.
Let's see how tornadoes are formed-
-Hot, humid air- In order to have even a decent garden-variety thunderstorm (forget supercells) you need massive amounts of hot, humid air. Sure, 43°C sounds pretty balmy, but there is a reason why you can't make tornadoes with a few flying creatures shedding off body heat.
Try out this activity at home.
Take some boiling water, and pour it into a bucket of water at ,let's say, 40°C). The bucket gets just lukewarm, or maybe not even warm up at all.
Heat does not just depend on energy, it also depends on the mass of the object. A bucket of water at 40°C contains more overall heat than a cup containing water at 100°. This also applies to gases (including air) as well.
When you get warm air, it creates updrafts, often containing moisture. This moisture, when it contacts the cooler, upper atmosphere, it condenses to form clouds. Just normal fluffy or wispy clouds. End of story.
But, if you have a lot, on the order of thousands or even millions of tons (yes, tons) of warm air, then as it cools down, it rapidly transfers its latent heat to the surrounding air, causing it to suck up even more air, and this air also releases its latent heat. This chain reaction goes till you end up with a rather tall and hefty anvil cloud. The latent heat released is immense, on the order of millions or even billions of joules.
-Wind shear- Wind speeds and directions can vary with altitude. I doubt whether your 40ft wyverns can change wind direction just by flying. Have you ever noticed (in an apartment) how the terrace is windy, whereas the air is still at ground level?
When you get two horizontal air currents that have different directions and speeds, they form a horizontal air vortex, which is basically the parent vortex of the tornado, or the mesocyclone.
Image of an mesocyclone:
-What goes up, must come down-Updrafts do not last forever. Especially if you lift air to freezing altitudes, then it will rapidly cool and sink back to the surface. These are what meteorologists call downdrafts, and these are the key features that drive microbursts and heat bursts (caused due to adiabatic heating of rapidly sinking air parcels)
Updrafts and downdrafts, when combined, have a notorious knack for rubbing hail against smaller ice pellets, this friction is basically what causes lightning in thunderstorms. (Of course, don't go to your refridgerator and rub ice cubes against frost to create electricity, you need a tad lot more to generate lightning)
So, your downdrafts are gonna tilt your horizontal air vortex downwards, and well, there you have it, a tornado is born.
So, coming back to the question:
-You need a lot more wyverns in your flock, think on the order of millions of wyverns. The best that a flock of 20 flying creatures can do it to stir up a dust devil (albeit very weak and short lived).
-If you want your wyverns to create a massive warm-air parcel, for a flock of 20 creatures, you would need to have body temperatures on the order of hundreds or even thousands of degrees (Rip wyvern).
The best case scenario I can think for your novel is that an entire kingdom of wyverns have gone fishing for a festival (use story license), and while hunting, they create a lot of hot air over the lake where they are fishing, and accidentally trigger a supercell. And maybe trigger wind shear from the sheer amount of wyverns that are fleeing in different directions and trigger a small tornado.
You just need a lot of wyverns to do the job.