Rainfall in Wales is mostly as a result of the arrival of Atlantic low pressure systems and is heaviest between October and January over the whole country. The driest months are usually April, May and June, and Wales experiences fewer summer thunderstorms than England. Rainfall varies across the country with the highest records being from the greatest elevations. Snowdonia experiences total annual rainfalls exceeding whereas coastal regions of Wales and the English border may have less than . The combination of mountainous areas and Atlantic lows can produce large quantities of rain and sometimes results in flooding. The amount of snowfall varies with altitude and enormously from year to year. In the lowlands, the number of days with lying snow may vary from zero to thirty or more, with an average of about twenty in Snowdonia.