To be honest I don't think it's more likely to have plumes in summer than cold blasts in winter( speaking from an imby point of view) I really think it depends on location and altitute when it comes to cold and snow in winter, winter 09-10, 10-11, 12-13 and 17-18 brought plenty of snow here in the valleys heck even winter just gone there were 2 snow events and one of those was in april! Plus in the past 5 years there has been 2 red warnings for snow here in south wales. Last year brought the first heatwave that I can remember since 2006! No doubt there has been warm spells in summers gone since then but last summer was truly exceptional which I can't see being repeated for a long time.