Details on the rain coming to Long Island:
It looks like around 1 to 2 inches of rain from Wednesday afternoon to Thursday afternoon with it coming down moderate to occasionally heavy at times as precipitable water values will be will approach 1.7 or 1.8 inches, which is very high for this time of year. It will also be humid with dew points in the low 60's.
From the National Weather Service:
At the surface, a frontal system approaches with central low pressure moving in a northeast direction within Southeast Canada Wednesday. Its associated cold front approaches during the day and moves in Wednesday night into early Thursday. A wave of low pressure will develop along the front stalling it just south of Long Island. The low pressure wave moves farther south taking the front farther south as well Thursday afternoon into Thursday night. Rain showers remain in the forecast with highest chances along the coast.
This is what the Euro is showing from 3 PM Wednesday to 3 PM Thursday: