I think this might just work... What you want to do here is a check to see if the swa object exists using the known fact that javascript objects are sparsely populated arrays.

There are several ways to do this.
var swa = null;
if (weather['swa'])
{
swa = weather.swa.a[0].t;
}

OR

if (weather.hasOwnProperty('swa'))
{
swa = weather.swa.a[0].t;
}

The 2nd one, you can read about on google or the mozilla site as well as in Flanagan.