When you combine a potent upper level disturbance, a very moist atmosphere and little to no wind to push thunderstorms along, you get what we had yesterday.
Copious amounts of rain fell throughout the day, causing some serious flash flooding in parts of the Finger Lakes. Central Seneca County was particularly hard hit, as was the Branchport area, Ithaca, and areas between Skaneateles and Syracuse.
Radar estimates a large area of over 1" of rain (shown in yellow), with numerous pockets of 3 (pink) or even 4+ (white) inches of rain yesterday. For many of these areas, the majority of that rain came in a short time of an hour or less.
The good news is that this disturbance is now to our east, and we should have increasing amounts of sunshine today. Some more rain is possible Tuesday, Wednesday and Thursday, but these days shouldn't be anything close to what happened yesterday. High pressure will build in on Friday and keep us dry into next week.