As you are probably aware, catastrophic flash flooding hit Ellicott City, Maryland on Saturday night. Ellicott City is several miles west of Baltimore.
According to the National Weather Service, 5.52” of rain fell in 90 minutes there, with more than half of it (3.16”) coming in the first 30 minutes. The storm total rainfall was 6.50". The early analysis from the NWS suggests this rainfall rate has less than a 0.1% chance of occurring in any given year. Put another way, this is a rainfall rate that statistically occurs there less than once in a thousand years.
Flash Flooding also occurred in central New Jersey Saturday, with 4-7” of rain leading to dozens of disabled vehicles requiring water rescues.
From a climate perspective...
According to the 2014 National Climate Assessment, there was an increase of 71% in the amount of precipitation in the heaviest events in the Northeast (including Maryland) between 1958 and 2012.
Below are links to graphics to help you put the event in a climate perspective on the air or online.