There are numerous synonyms for the phrase "become less severe". Here are a few examples to consider: mitigate, alleviate, ease up, lighten up, abate, lessen, reduce, diminish, temper, and moderate. When used correctly, these synonyms can add variety and clarity to your language while conveying the same meaning. For instance, "the severity of the storm mitigated" can be rephrased as "the storm lessened in severity" or "the severity of the storm eased up". Regardless of which synonym you choose to use, be sure to read over your sentence to ensure that it makes sense in its context.