Areas south of Baltimore, Maryland got slammed with record-breaking rainfall this afternoon, with some areas just south of the city seeing a foot of rain in just one day. What's even more amazing is that while one town saw twelve inches of rain, areas just a few miles away got just one inch.
One of the amazing things about thunderstorms is their seemingly random nature. Most of the time meteorologists can tell the general area where they can form, but not exactly where. Sometimes it comes as a complete surprise that storms will produce such incredible amounts of rain. That was pretty much the case today; the areas that saw the heaviest rain in the D.C./Baltimore areas didn't see a flash flood watch. It caught people off guard.
The phenomenon that occurred is called "thunderstorm training." Sometimes, due to processes wonkier than you probably care about, thunderstorms can continuously redevelop and travel over the same areas for hours at a time. The movement of the thunderstorms resembles a train moving over railroad tracks, hence the term "training." This dumps an enormous amount of rain over the same areas while leaving neighboring vicinity dry, and that's exactly what happened today.
The radar image above shows the radar estimated storm total rainfall. Baltimore is just off the top of the image, centered in the cluster of red highways and orange state routes. The scale to the left shows the rainfall in inches. The distance separating the twelve inch bullseye and areas along the Bay that only saw one inch is just seven miles.
You simply cannot imagine what a foot of rain in one day looks like until you've lived it. I recently graduated from the University of South Alabama in Mobile, and about two weeks before graduation, we had a major severe weather outbreak just to our north. The storms traveled down to the northern Gulf Coast as a complex called a "mesoscale convective system" (MCS), which promptly stalled over us and created a prolonged training event. We got more than a foot of rain in Mobile in just over 24 hours, and a few dozen miles to our east, Pensacola, Florida got an incredible 24 inches of rain in the same period of time. The event led to historic flooding; some residents even claim it was worse than the flooding that occurs in major landfalling hurricanes.
I took the above video of the initial line of storms moving through, and if you'll excuse my painfully dorky narration, it was a sight to see. That first round of storms dropped three inches of rain in just 23 minutes. That's a rainfall rate of almost nine inches per hour!
The beauty of nature is unending, but its power is something to be reckoned with. Just like people who see a tornado or live through a hurricane, bearing witness to some of nature's most energetic storms is a humbling experience. In Mobile, we were somewhat used to incredibly heavy rainfall (but not quite on that scale), but this kind of event in a place like Maryland is very rare indeed. I'm sure local meteorologists and weather historians will be able to fill in the gaps, but I grew up in the D.C. area and I can't think of any comparable events that dropped so much rain in such a small period of time.
The single worst flooding disaster in the region by far was in Nelson County, Virginia during Hurricane Camille. The county saw more than two feet of rain in just a few hours and the resulting flash floods killed more than one hundred people.
Thankfully, it doesn't appear that today's flooding in Maryland resulted in any injuries or deaths. That's actually surprising given the amount of photos that came out on Twitter and Facebook of people driving through flooded roadways. As I said in my post earlier this afternoon, it only takes a couple of inches of swiftly moving water to disable a car or make it buoyant, requiring a swift-water rescue crew to come out and save the driver and passengers. No matter how highly you think of yourself, you can't tell how deep the water is and you can't judge if you're going to make it across. Please don't cross a flooded roadway. Your life isn't worth giving up for a brief moment of momentous stupidity, and getting where you need to go certainly isn't worth risking the lives of those who have to rescue you.
[Radar image via Gibson Ridge]