Roanoke, Virginia just received one of the heaviest rainfalls I've ever seen on Doppler radar, with areas just north of the City of Roanoke receiving over 4 inches of rain in just one hour's time.
Here's the one hour rainfall product from the Blacksburg VA Doppler radar, showing the rain that fell between 2:50PM and 3:50PM Eastern Time (click to enlarge):
4"/hour rainfall rates aren't totally unheard of in parts of the country, especially in places along the Gulf Coast. But something like this happening in western Virginia? There has to be something going on here. Well, there is. It was a "perfect storm" of sorts.
For the last two weeks, the eastern United States has been wedged in between two big weather systems -- a Bermuda High, and a series of low pressure systems over the Midwest.
Most everyone is familiar with the Bermuda High -- a center of high pressure that sits over the Atlantic Ocean and gets particularly intense during the summer months. When we have a strong Bermuda High as we do now, the clockwise circulation of wind around the center of the system serves to pump warm, very moist tropical air up from the Caribbean and Gulf of Mexico and into the eastern 2/3rd of the United States.
This moisture pump is amplified when a low pressure sets itself up near the Mississippi River. Winds flow counter-clockwise around a low pressure center, so it creates an enhanced region of southerly flow over the eastern United States, aiding in the transport of very moist tropical air over this region of the country.
This enhanced ribbon of deep tropical moisture is aptly called an atmospheric river.
Here's an awful illustration I made using this morning's run of the GFS model. The thick red line approximately shows the axis of the atmospheric river as it moves across the eastern US. The black lines poorly denote approximate wind directions. Click to embiggen.
The amount of moisture in the atmosphere is evident by taking a look at observed dew points around the Mid-Atlantic region. The dew point at Roanoke Regional Airport at 3:53PM today was 70°F. That, too, isn't too unusual for the middle of July. Generally, dew points below 60°F are comfortable, 60°-70° is getting muggy, 70°-75° is uncomfortable, and any dew points above 75° are just downright disgusting.
The dew point isn't the whole story, though. Roanoke's dew point is actually one of the lowest in the area as of 4PM. This leads us to the next factor -- precipitable water.
One of the best ways to measure how much moisture exists in at any one point in the atmosphere is to look at the precipitable water map. Precipitable water (PWAT) shows how much rain would fall, in inches, if all the moisture at a specific point in the atmosphere were condensed at once and fell as precipitation. In other words, imagine all the atmospheric moisture above your head suddenly falling as rain. The amount that falls in inches is the PWAT value.
PWAT values over 2.0" are considered high, and they're indicative of deep tropical moisture. 2.0"+ PWAT values are common during the summer months, but the duration with which this atmospheric river has sustained itself (going on two weeks now) is unusual. Here's the current PWAT map from the RAP model. The legend is on the upper-left side of the map, in inches. Higher numbers indicate more moisture. Click to enlarge.
Warm, unstable air? Check. Deep tropical moisture? Check. Now, it's up to the thunderstorms to produce these massive rainfall amounts. The storm that hit just north of Roanoke has two distinctive features: 1) it wasn't moving very fast, and 2) it converged with another storm.
This map shows the RAP model's predicted storm motion as of early this afternoon. The little barbs point in the direction of the expected motion of the storms. As you can see over the southwestern Virginia area, there isn't much storm motion at all. There isn't enough wind through the atmosphere to move the storms that form over the Roanoke area, so they're just drifting very slowly off to the east.
Thunderstorms that move very slowly (or not at all) are dangerous, especially if they're producing heavy rain over areas that are already inundated from previous rains. The Roanoke area needs less than 1" of rain per hour in order for flash flooding to occur.
This thunderstorm was especially bad, though, because it consisted of two thunderstorms that merged into one. When two storms collide with each other, depending on how strong they are, there can be a huge upward thrust of air and moisture that combines into a single, strong updraft. This enhanced upward lift increases the amount of precipitation in the storm, and prolongs its duration over the area where the two storms converged. This was clearly the case on Doppler radar when the extreme precipitation amount occurred:
This extremely localized heavy rain event demonstrates the interconnectivity of our atmosphere. Features thousands of miles across and 7 miles deep can move into place and interact perfectly with one another to create the perfect conditions for a few dozen square miles of land in Virginia to get swamped with over 4 inches of rain in one hour.
It's important to remember that extreme rainfall and flash flooding like this is going to be one of the major impacts of climate change in most parts of the world, including the United States. Events like this could and likely will become more common in the future.
It's always cool to look at the meteorological background behind an event like this, but there's always a human toll. The pictures coming out of Roanoke at this hour are pretty awful to see.
5:05 PM PT: I posted this in the comments, but it's always worth an update in the diary itself. Back in 2006, Dr. Greg Forbes listed the world record rainfall rates on his TWC blog. Not sure if any of these have been busted in the 7 years since, but I'd hope not.
--1 minute: 1.50 inches at Barot, Guadeloupe (Nov 26, 1970); a rate of 90 inches per hour if it could have kept going at that pace!
--42 minutes: 12.00 inches at Holt, Missouri (June 22, 1947); 17.14 inches per hour rate
--1 hour: 15.78 inches in China (July 3, 1975)
--24 hours: 71.85 inches at La Reunion (January 7-8, 1966)