Bonjour Kenneth,

To supplement Chris's answer:

Once upon a time, there were a few French expressions "en cas que, au cas que" which used Le Mode Subjonctif, although they've now become archaic. These expressions have been entirely substituted with "au cas où" which is always followed by Le Mode Conditionnel. See here: https://www.academie-francaise.fr/au-cas-ou-tu-seras

Also, Le Mode Subjonctif implies an uncertainty (doubt / hypothesis etc) while le Mode Conditionnel also implies a hypothesis (hence, an uncertainty too).  Therefore, it would be naturally to assume that Le Mode Subjonctif would be used after "au cas où". Unfortunately, it isn't the case. 

I hope this is helpful.

Bonne journée !