I was on the flip side of that several years ago in Oudenaarde, Belgium with 16,000 riders starting at different distances for the amateur ride before the pros riding the Tour of Flanders. A backdoor warm front was unexpected. We started at dawn it was 48 degrees at the start. The front moved in and it started to rain. You rode fast and hard just to keep warm. 2 hours in I was getting wet through thin, cheap "waterproof" paints and jacket. About 5 miles/20 mins the rain stopped, sun came out and it got hot quick near 70 degrees (by Belgium standards) as we approached the beautiful town of Ronse, last rest area, before our last two major cobblestone climbs. At the rest stop people were getting rid of clothes like they were on fire. The organizer were ready for it. They had pre-printed signs for pants/leggings, top/jackets. Socks and gloves went in a dumpsters. I ditched a plastic cheap rain pants and two, throw away long sleeve shirts. But looking at the huge piles I could see a lot very expensive gear, thousands dollars worth high end gear and I was in the top 20% of finishers. I can't imagine how big those piles became at the end of day. The next morning it was back in the high 40s.!Was at a outdoor Bday party last night. Everyone was enjoying themselves and seemingly within 5 minutes everyone was cold and went inside. I don't remember a temp drop like that.