On live parliament TV (active at the time of writing) MPs repeatedly refer to that situation with the terms you have given.A first thing to keep in mind is this Withdrawal Agreement is just that. It is after its ratification that the future agreements can be undertaken, because the EU has refused to give the UK a priority status otherwise. First an exit, then a trade deal as a third country, essentially.In this case, overnight, and worryingly soon, the UK exits the EU with no replacement deals abroad (yes, I know there are a handful, but none of great significance). Essentially, while the EU has said it would continue as is for a period (for its own benefit, allowing them to adapt), this would leave the UK with no provisions for advantageous trade, which in turn can affect prices and potentially employment.The catastrophe we talk of, therefore:is the potential that British exports become unattractive in markets we export to, therefore firms might consider layoffs as sales declinethat foreign companies will no longer consider the UK as a hub for their activities for a time, because they won't have access to other markets they want, also harming employmentNot having the two years transition period, allowing the UK to ratify deals with further afield, as well as a future trade agreement with the EULeaving the island of Ireland and Northern Ireland in a highly confusing and unsettling set of circumstancesLeaving the UK to fall back on u201cWTO rulesu201d with everyone else. Now, Jacob Rees-Mogg might use that term optimistically, but note that no one else operates this way, because it's crap (that may as well be the technical term). WTO terms are primitive, and again, leave the EU forced to impose the same tariffs on the UK as any other country, until a future deal is negotiatedNow, it's all very well slashing tariffs on imports, as the government has said it will do in the case of a no-deal Brexit, but it doesn't actually protect domestic produce, and it certainly doesn't guarantee that other countries won't impose tariffs on ours (and then we go back to the business uncertainty).To arrive at this point essentially overnight, rather than a 2 year transition granted by the WA, is obviously much more volatile and uncertain for all involved.Even if it is ratified, we still have another 2 years of negotiations with the EU to come.Enjoy.