Learn French: L'orage a éclaté. La météo l'a prédit. - The storm broke out. The weather forecast predicted it.

It stands for the entire previous sentence: L'orage a éclaté (The storm broke out). This is the neuter object pronoun le, which we use to replace whole ideas or clauses rather than just a single noun.

Because l' comes before the verb, shouldn't there be past participle agreement on prédit?

There is, but it is invisible! When a direct object comes before the verb in the passé composé, the past participle must agree with it. However, because the neuter pronoun le replaces a whole idea, it is always treated as masculine singular. Therefore, prédit stays in its default masculine singular form.

Why use éclater for a storm? Doesn't it mean to explode?

Literally, éclater means to burst or explode. However, it is the standard, idiomatic verb to use in French when a storm, a war, or a riot suddenly begins. It translates perfectly to the English phrase 'broke out'.

Is la météo the general word for 'the weather'?

Not exactly. La météo is short for la météorologie. It is the everyday word for 'the weather forecast' or 'the weather report'. If you want to talk about the actual weather conditions outside, you use the word le temps.