Weather Underground is having some issues with its API--it is occasionally returning bad data for forecasts (there might be issues in other areas, too). One user saw bad high/low data and bad dates for 10-day forecast devices. At first I thought that this was only a condition when the language preference was set to Dutch, but now I'm not so sure. Today, I woke up to a forecast high temperature for tomorrow of -17F and a low of -6. I then asked for a weather data refresh from the plugin menu and the data were accurate again. This was with a language preference of English.
Note that the plugin does no calculations on these data and the language preference is handled on the WU side--I simply post the data provided by the API.