I could be wrong (and probably am) because I have next to no meteorological knowledge, but I think that the 'wintry showers' are possible because although the temperature is quite high, the dew points are fairly low. For instance, it's 7.6C here, but the dew point is just 1C. So I guess any precip here could have a wintry element to it if it were to arrive now. I'm probably speaking rubbish though,thinking I understand it when I don't!