It would be a fairly simple change to achieve what you want. Simply change line #47 of the source to look something like this:

var current = (WeatherType)Function.Call<int>(GTA.Native.Hash._GET_CURRENT_WEATHER_TYPE);

WarnPlayer("", "Forecast", “Current weather - “ + current.ToString() + “\nExpected - “ + expectedWeather.ToString());

You can then save the file with a .cs extension and copy it to your scripts folder.