How Rare? Two Feet of Rain Near Pensacola, Florida!

I knew when I came into work this morning, when the satellite image over Florida looked almost exactly like yesterday morning's, that we were in for bad news. Indeed, flooding photos abounded on AccuWeather.com and NBC News.

Some incredible rain gauge readings posted on Twitter this morning showed as much as 24.4 inches of rain in just over 24 hours in Pensacola, Fla. It was the most incredible amount, but only one of many flooding stories told during this week's severe weather outbreak which has left a wide area with more than 5 inches of rain:

Here's a close-up of the Doppler-Estimated rainfall in the Pensacola area:

Two feet of rain is more than most hurricanes and tropical storms deliver! The last major flood I remember blogging about was Tropical Storm Fay in 2008, which delivered over 20 inches of rain to the east near Tallahassee (see below - with over 27 inches near Melbourne).

The National Weather Service in Mobile, Ala., also reported: "The Fish River near Silverhill will rise to 23 feet. This means the river will crest *ABOVE* the all-time highest historical crest of 22.78 feet on July 20, 1997, during Hurricane Danny!" In fact, the river did rise from 11 to 23.13 feet overnight, from its typical stage of 2 feet yesterday morning.

So how rare is this amount of rainfall for any one individual place? There are two tools that we can use to estimate this. The first is "Flash," a tool from the University of Oklahoma. This tool estimates (based on radar and rain gauge estimates for the last 24 hours) that this amount of rain would only happen every 50-75 years (a "50-year flood").

However, the PDFS tool indicates that it would have a much longer return period if it happened at the Pensacola airport - on the order of a 500- to 1000-year flood. Suffice to say: It's very rare and unlikely to happen in the same spot twice in a lifetime. Pretty impressive for an upper-level low pressure system spinning off thunderstorms, and not a tropical system!