The severity of a storm (usually) has very little bearing on the delays, etc. The duration, size, and path of the storm are much bigger issues. It is well documented and known that the New York airspace is tight and congested. Throw in a thunderstorm in the area and it's the proverbial recipe for trouble. Think about a run of the mill example: a line of thunderstorms running north to south from upstate New York down past the Washington area moving west to east. This cuts off the Lendy arrivals into JFK
, the Milton arrivals into LGA
, and the Williamsport arrivals into EWR
. You now have dozens and dozens of flights either trying to outrun the storm to the north or south (but those arrivals corridors are already full) or entering holds as far east as Cleveland Center airspace. Depending on the length of the storm the flights continue to stack up and then, invariably, the storm hits the field which will stop all departures and arrivals. Once the storm clears the field and they start accepting arrivals again it's a mad dash for a slot. Well, with fuel being a finite resource something has to give and the diversions begin. Should come as no surprise to anyone and I don't think that this year is any worse than others in terms of thunderstorms or diversions, etc.
The one exception to this is that with the cost of tuel, airlines are not carrying the same amount of extra fuel as they did in the past. Certainly they're carrying enough to be safe, but not enough to hold for an hour or more and wait out the storm. I think because of that you are seeing slightly more diversions.