Heres the chart which bought thundersnow in Jan 2004
http://www.wetterzentrale.de/archive/ra/20...00120040128.gif
http://www.wetterzentrale.de/archive/ra/20...00220040128.gif
I remmeber this day very well.
It started raining heavily at around 1:30 and continued for about half an hour. Then all of a sudden there was lightining and thunder. After this the rain turned immediatly to snow. Snowflakes were so big it settled everywhere straight away. This snowed continued for about 2 hours and left about 4" of snow. Temperatures got down to -7C at night. Next day schools were cancelled . This was only the second time school were closed because of snow.