Most importantly, because of its large size, the intense central pressure of the storm is more typical of a Category 2 or 3 hurricane. An experimental product from the National Hurricane Center has rated Irene at 5.0 on a 6.0-point scale for its potential to create damaging storm surge and waves. These are values seen more typically in Category 4 hurricanes, meaning Irene is extremely dangerous.
That damaging potential, combined with Irene’s arrival at what might be exceptionally high tide, is a major reason for the mandatory evacuations of low-lying areas across the New York and New Jersey.
And a quick observation from both New York and across the Hudson River in New Jersey: Traffic since about 3 pm has been remarkably light, almost at a pre-dawn level. Many cars that are traveling are delivering food.