I've toured for years and have encountered many storms.
Also, because I have done the majority of my touring in the West in later years -
I have often been in open terrain far from any shelter.
So waddya do?
Instead of any footprint for my tent, I use a small 5x7 tarp with grommets.
It's readily accessible on the back rack - with two bungees and a fence line - a quick lean-to.
Most highways are fenced - so you use the top fence line for the bungees.
If you have time you can stake down the two bottom corners -
Otherwise hold one and use bike for other in a 2-minute case.
I've been caught out in hail a few times and it ain't fun.
Your helmet is essential for protection - heavy hail is dangerous.
Quickest, safest thing is to pull out your sleeping pad and cover yourself.
Both of these situations do not involve something like tornados.
(Although in the West you can have some darn high winds.)
I've ridden out many a storm in a culvert.
There are a lot more than you think if you are just a car user.
They may be a little groaty - but hey.
Barring that, flatten yourself in a low area - again use your sleeping pad for protection.
Storm in Thunder Basin, Wyoming
(The nice thing abour Western storms is that you can see 'em coming for miles.)