The problems is with your external temperature sensor. Here is what is states on page 38 of the WeatherCat Manual:
If there are a lot of sensor errors then the problem lies between the weather station and its sensors ? if wireless try repositioning the station, if wired, check the connections. In all cases it?s worth checking any batteries. (Note, for Davis Vantage owners, WeatherCat can warn you when batteries and or power supplies are out of specification ? see ?Hardware Options? from the ?Tools? menu.)