hrm current_observation wasn't in the fields and therefore won't be part of data. You could add a mapping of current_observation.temp_f.

However it sounds like this isn't even part of a recordset and you don't need a store.

Have you checked out Ext.data.JsonP.request?
http://docs.sencha.com/touch/2-0/#!/api/Ext.data.JsonP

It's funny that the example there is also to retrieve the weather