Hi jg,
I'm guessing the issue is one of two things - either it picked up some rubbish data during the transition and the stats has used default values OR there was a gap in the data too big for WeatherCat to deal with.
Either way, quit WeatherCat and open the data file for this month in a text editor (TextWrangler works well).
Open the data file for September (found at ~/Library/WeatherCatData/Location1/2012/9_WeatherCatData.cat where ~ is your home directory), scroll down to the time of the transition between stations (each line in the file has the time and date at the start of the line - looks like 37035 t:261715 T:12.89 Ti:20.72... the 't:' field is ddhhmm - in this example 17:15 on the 26th) and check either for a line with the bad data in it (and delete it or edit the values) or look for a gap bigger than half an hour. If it's a gap, copy the preceding line and paste it in updating the time field so there's data there.
Finally, delete the stats file for september (9_WeatherCatData.cat.hrs) then run WeatherCat - should clear it up.
