https://phys.org/news/2018-09-nasa-florence-stretched-out-strongest-storms.html

At 2:35 a.m. EDT (0635) on Sept. 18, the Moderate Resolution Imaging Spectroradiometer or MODIS instrument aboard NASA's Aqua satellite analyzed Post-Tropical Cyclone Florence in infrared light. MODIS found several areas of coldest cloud top temperatures and strongest storms located over several U.S. states: Maryland, Delaware, northeastern upstate New York, central Vermont and New Hampshire.

Those cloud top temperatures were as cold as or colder than minus 63 degrees Fahrenheit (minus 53 degrees Celsius).  NASA research has found that cloud top temperatures as cold as or colder than the 63F/53C threshold have the capability to generate heavy rainfall.