You should be able to create a variable and post external data, however, you'd have to pull the data yourself and then push it to Ubidots.
Here's a Python example of a similar integration. It's the actual code of how we pull the number of users we have from the Hubspot API (the marketing platform we use), and then save this value to Ubidots:
In the case of WunderGround API, you should be able to use this code by changing the name of the variable "users" for "temperature" and also changing the HTTP GET request line, with the line to query the WU API:
Then you can play with the JSON response to make sure it's parsed right.