How to I Add In Degree Symbols to Temperatures in a Javascript String?

I have a string that was retued from an API, and I want to add in degree symbols from the temperatures that are included ("Tomorrow will be a high of 89F" → "Tomorrow will be a high of 89°F"). What would the Regex look like for my variable named forecastString?

This replace will work:

forecastString = forecastString.replace(/[0-9]{1,3}F/g, function addDegreeSymbol(x){retu x.replace("F", "°F");});
forecastString = forecastString.replace(/[0-9]{1,3}C/g, function addDegreeSymbol(x){retu x.replace("C", "°C");});

The Regex finds any numbers of 1-3 digits followed immediately by an F/C, and replaces the F/C with °F/°C