These messages indicate that the skin temperature at some grid point(s) exceeds 340 K, which is physically possible in hot desert regions in the summer (e.g., Sahara and Arabian deserts). If part of your model domain covers one of these deserts and you are running WRF in the summer season, it is normal for these messages to occur in the course of a successful run. They do not necessarily indicate that the model is becoming unstable. if the reported values frequently exceed 345 K, you may want to increase the frequency of radiation calls (set radt to a smaller value). If you see these messages during a run in which skin temperature above 340 K is implausible, then something is wrong.