forecast.io format for tasker
You're close on the syntax, but there's an error based upon the .api's structure.  Instead of  precipProbability.hourly.data, you'll need to use hourly.data[0].precipProbability.  Replace the [0] with any value between 0-48 for the number of hours into the future you want the data.

Also,it's helpful to get a json viewer to help interpret the structure for selecting the correct values.  I use json viewer which allows you to enter your forecast.io api call direct and see your data live.  You can also use the timemachine functionality and test your parsing against different weather conditions (e.g. severe weather alerts).