For context I'm building a post apocalypse world that was in modern age before but storms and tsunamis wiped almost all civilizations off the planet.
The question is how huge should civilization wiping storms be.
Your storm will have to be tremendously huge if you want to wipe out nearly all civilization. Most big and average cities have means to let people survive for an extended time while safely remaining indoors. If you want to effectively wipe out civilization, you need to make sure that pretty much everyone dies, right? Therefore, houses and buildings need to be destroyed so that nobody can hide. You could inspire yourself from some films like The day after Tomorrow or 2012, where huge storms and tsunamis are happening. They could give you a frame of reference for the scale of storm required.
Otherwise, you'd need tsunamis in order to flood islands and kill people there. Storms will need a hell of a lot of power to destroy civilization's installations AND potential hidden survivors. It all depends on where you plan to make society revive. Survivors will hide underground, perhaps Nowadays most famous hideouts for huge catastrophes are underground bunkers. I do not know where they are located but this is (or was) where we take refuge when in big trouble. Considering that survivors will hide there, you would need a planet-sized storm to wipe out our beautiful humanity's constructions.
So you're on the verge of Humanity's death with this one. In the first place, we must hope there are enough people, men and women, to repopulate. It is likely that equality between men and women will be strained, or even "temporarily" disposed of altogether. Women will likely be protected/sheltered in order to provide means to repopulate Earth, because without strong birthrates there will be no repopulation.
Depending on the situation, there may or may not be power sources, factories, or places of shelter remaining after the destruction has ended. There is no way that food production will remain at pre-cataclysm levels, and the storm will have totally destroyed your ecosystem. Fish would be highly unhappy with the tsunamis, and mammals would be pretty much extinct. Any survivors would need to find a way to produce food and potable water. There might be canned food that survived the apocalypse, but such supplies would be limited and unsustainable.
Survivors may or may not be able to find usable ruins of surviving utilities. Needless to say, Internet and connection systems will all be down. Unless you have a group of experts, repairs will take a while to set in motion. The same goes for basic, yet complex stuff like power, heating your newly built shelter ... Oh, wait, if you do not want to live in a bunker, you will need to either find a remaining house or build it yourself. Survivors will need time to recover before even thinking about growing up again and gaining technical knowledge, establishing a new hierachy or political system, or rebuilding an economy.
As for social interactions between groups, there are various cases. There might be one single group, multiple groups dispersed across the globe, or multiple groups within a more limited geographic area. Regardless of the disperal, there are two realistic options: You can stay within your own group at the expense of other groups, with consequent theft and looting, or you can be considerate of Humanity in general, and gather a new set of people to start a new civilization.
As for simulations of the groups, consider:
If you search for materials about post-apocalyptic worlds, look into zombies. I know it is not an example of realism, but The Walking Dead or even games like DayZ might give you some inspiration on how humans interact in a heavy crisis case. The Walking Dead is more than just a TV series, so dig for books as well. I think that is pretty much it. Good luck with your survival scenario !
This is not absolute, but I really think that if you plan a massive civilization wipeout, no current creature could survive. Ever. Birds, mammals, cats, bears, dears, cows, everything would be dead after the wrath.
This is based on my personal knowledge, thinking process and experience about post-apocalyptic scenarios. You can pick up inspiration from it, and I invite other users to beautify my answer with more suggestions or sources.
Yes, all the links are from Wikipedia.
Are you in hurry? A mega tornado can lift enough material to cause an extinction level event. A similar effect to a nuclear winter can occur. All the dust raised from the ground blocks the sun and destroys the ozone layer. This will not kill of humanity. But crops will fail, farm animals will die and massive wars would be started. So your world will collapse within decades.
After the event, most of the society would be stuck inside: hiding from the UV damage. As long as there is power, plants can be raised inside. Thus you may go with a subterranean society relying on geothermal energy to maintain pockets of civilization.
You do not need so much of a storm, as long as it is wide-spread and long-lasting.
Your storm seasons just need to wipe out food crops for long enough that most people starve. It needs to come on suddenly to prevent widespread preparation and avoidance.
Imagine some ten years of crop destroying storms world-wide. What will be left? Such storms would stop air-planes, boats and much of transport over land. Only local resources will be available. That alone will be bad enough in densely populated area's.
The first year will see many survivors, but every following year will see fewer. Survival gear will eventually wear out. Buildings and other infrastructure will become unusable from all the battering and precipitation without much maintenance. Cities become death traps as weeds grow over everything and floods cover many lower area's and buildings start to fall apart. Rats, feral dogs and cockroaches become rampant and spoil any reachable food resources.
Begins to look like a really uncomfortable, slow and stinking Armageddon already.
I honestly don't think a storm needs to be big to tear down humanity. If 90% of power dies in the northern hemisphere 50% will die in the next 3 months.
No more power means:
this leads to
Within a year only a handful of people will be alive in the northern hemisphere.
Within 2 years the rest of society would break down including the 10% of power supply that still is running.
But tings are not all grim
Most of Africa and other now considered undeveloped places would survive. It will take another 2000 years to get back to our level of civilization(?) we are now before it just happens again.
The rise and fall of civilizations
Read some history you'll see the light (rather the darkness). We are doomed it's not a matter of if, it is a matter of how and when.
Your doom prophet