Question for weather geeks

The forecast from 11:25am this morning says southern New England will be hit by a strong, fast-moving storm that will develop off the coast of New Jersey this evening. It could bring gusts to 40mph and an inch of rain in 4 hours, causing localized flooding.

Can anyone tell me what makes that prediction possible? I find it kind of amazing that the behavior of a storm can be predicted before it's even formed.

( Here's the bulletin )