We're in the Prime Time of Year For Flash Flooding in the U.S.

This was illustrated nicely by a graph NOAA's Weather Prediction Center tweeted, showing the daily average number of flash flood reports received by local National Weather Service offices from 2007 through 2017.

As the thin bars on the graph above illustrate, while flash flooding can occur any time of year, the number of daily flood reports rose rapidly starting in late-April, reaching a broad summer peak before tailing off into the fall.

NOAA/WPC noted 75 percent of all flash flood reports in the U.S. occurred from late April through mid-September, and 34 percent occurred from June 10 through Aug. 3.  

To be clear, by flash flooding, we mean only those short-fuse events triggered by heavy rain over a relatively small area, rather than longer-lived river flooding events.

Another way to look at this is by the number of flash flood warnings issued by NWS offices by month, as Daryl Herzmann from Iowa State University put together. You can also clearly see a May through summer peak in this visualization.

Flash Flood Factors

There are several reasons why the threat of flash flooding grows by late-spring and peaks in the summer. Some are rather obvious; others, a bit more subtle.

Warm, Humid Air Is More Widespread

With the sun's more direct rays shining on the Northern Hemisphere, warmer air returns to an increasingly larger fraction of the country as spring progresses. 

Since more water vapor can exist in warmer air, that increases the rainfall potential for both individual thunderstorms and larger-scale weather systems.

Jet Stream Slows, Moves North

The atmosphere's steering wheel for large-scale weather systems, the jet stream, both weakens and migrates into the northern U.S., even into southern Canada, by summer as the north-to-south temperature contrast weakens.

With generally lighter steering winds aloft, especially in the central and southern U.S. by summer, thunderstorms and clusters of storms move more slowly. The slower the movement, all other factors being equal, the greater the rainfall potential.

Another result of this jet stream migration and weakening by summer is the absence of cold fronts sweeping the warm and humid air away from the southern U.S. 

Thunderstorm Clusters

While some MCSs can move rapidly, producing widespread damaging winds, others can move very slowly if winds aloft are weak, triggering major flash flooding, particularly in the overnight and morning hours in the nation's midsection.

(MORE: Why These Thunderstorm Clusters Are Both Important and Dangerous)

The Southwest Monsoon

By summer, high pressure in the upper levels of the atmosphere usually sets up in the Plains, opening the door for increased moisture from the eastern Pacific Ocean and Gulf of Mexico to stream into the Desert Southwest.

Coupled with intense heating of the mountainous terrain, slow-moving thunderstorms can flare up over the high country of the Southwest and Rockies, then spread over lower elevations. 

If the atmospheric moisture is deep enough, the slow movement of these thunderstorms could dump torrential rain, flooding normally dry washes and arroyos, and triggering flash flooding in urbanized areas such as Phoenix and Las Vegas.

(MORE: 5 Things To Look For During the Southwest Summer Monsoon)

Tropical Cyclones

This is the elephant in the room regarding the peak in rainfall flooding in the U.S.

The key is that you don't need an intense landfalling hurricane, such as Category 4 Hurricane Harvey in August 2017, to produce flooding rainfall.

You just need a slow-moving tropical cyclone, whether a depression, storm, hurricane or remnant. 

It was Harvey's loafing near the Texas Gulf Coast for four to five days after its landfall that led to the rainfall flooding disaster in Houston, Port Arthur and other locations.

A much weaker tropical cyclone, Allison in June 2001, wasn't even technically still a tropical cyclone when it unleashed its final chapter of inundating rain that pushed flooding in Houston to a $5 billion disaster. Again, it was just a slow-moving remnant.

Incidentally, this isn't just a concern along the Gulf Coast and East Coast. Remnant eastern Pacific tropical cyclones have triggered some of the more severe flash flood events in the Desert Southwest over the years.

For example, in July 2015, torrential rain seeded from what had been Hurricane Dolores triggered flooding in the deserts of Southern California, washing out a section of Interstate 10 east of Palm Springs.