A Flashy Flood Regime: Pockets of Torrential Rain Scattered Across U.S.

June 19, 2018, 4:22 PM EDT

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Above: Mark Pickett (left) and Ryan Craig (right) work to rescue Bruce Salley, who was trapped in his car by flood waters in a supermarket parking lot in Rockford, Illinois, on Monday, June 18, 2018. An evening thunderstorm brought heavy rains across the Rock River Valley of northern Illinois, causing vehicles to get stuck in flood waters and stranding motorists. Image credit: Arturo Fernandez/Rockford Register Star via AP.

Flash flood watches were strewn from northern Montana to the Texas Gulf Coast on Tuesday, the result of unusually rich moisture stranded across the United States beneath lackadaisical upper-level winds. It’s a classic summer setup for generating the kind of slow-moving showers and thunderstorms that can lead to huge rainfall totals and serious flash flooding. Unlike river flooding, flash floods tend to be localized in both time and space, which means people at risk need to be prepared to act quickly.

The most pressing concern on Tuesday for the largest number of people is the risk of flash flooding across southeast Texas, including the Houston area. The NOAA/NWS Weather Prediction Center has flagged most of the Texas coastal region as being in a moderate risk of excessive rainfall on Tuesday (this is separate from the Storm Prediction Center’s risk areas for severe weather).

The main trigger for the rains across coastal Texas is a nearly stationary disturbance with tropical origins that traveled from the western Caribbean into the western Gulf late last week, then stalled out near the Texas coast. Rich moisture continues to circulate from the Caribbean and Gulf around the low into Texas. The radiosonde launch at Brownsville, Texas, on Tuesday morning showed 2.49” of precipitable water (the amount of water vapor in the air above a given spot), which is close to a record value for June.

Infrared satellite image of the disturbance centered near the Texas coast at 1527Z (10:27 am CDT) Tuesday, June 19, 2018
Figure 1. Infrared satellite image of the disturbance centered near the Texas coast at 1527Z (10:27 am CDT) Tuesday, June 19, 2018. Image credit: NASA/MSFC Earth Science Branch.

Fortunately, the rains from this system will be nowhere near as widespread or as heavy as the 30-60” totals observed in catastrophic Hurricane Harvey, and the impacts are thus expected to be much more localized. However, there is still plenty of uncertainty in exactly where the pockets of heaviest rain will set up. In situations like this, the heaviest cells often lodge atop small-scale (mesoscale) boundaries that may not be in place until a few hours beforehand. The prediction challenge was plain to see in a forecast discussion issued by the NWS office in Houston on Monday afternoon: “Models are still having a very very very very very difficult time indicating where [the heaviest rainbands] could possibly happen, with different locations and amounts evident on nearly every run that comes out.”

Multi-day amounts over southeast Texas will end up well above 10” in some spots. Already, weather.com reports that Alice, Texas, racked up 9” of rain in 24 hours as of 10 am Tuesday morning. At least 11" fell between Monday and Tuesday in a rain gauge that overflowed about seven miles southwest of Premont (between Corpus Christi and Laredo), according to the NWS office in Corpus Christi.

Update: Major flash flooding was under way Wednesday morning in south Texas, including cities along the lower Rio Grande Valley, where a flash flood emergency was in effect from McAllen to Harlingen. Close to 13" of rain had fallen in the Corpus Christi area.

Road washout in northeast Minnesota, 6/17/2018
Figure 2. Heavy rains washed out part of Highway 23 in northeast Minnesota, several miles south of Jay Cooke State Park, as pictured here on Sunday, June 17. Image credit: Minnesota Department of Transportation.

Major flood woes in Upper Michigan

On Sunday and Monday, mid- and upper-level moisture from ex-Hurricane Bud combined with lower-level moisture from the disturbance now over Texas to yield an exceptionally water-rich air mass along a stationary front in the Upper Midwest. Dozens of sinkholes opened up across the region, according to weather.com. The highest-impact flooding was along a belt running from northern Minnesota into the Upper Peninsula of Michigan. Two U.P. counties, Houghton and Menominee, were declared disaster areas by Governor Rick Snyder on Monday. At least one flood-related death was reported, a 75-year-old man found near his truck in Wisconsin on Sunday.

On Monday night, it was eastern Colorado’s turn for storms that dumped heavy rain and very large hail from the northern Denver metro area onto the High Plains. A 3-inch-diameter hailstone fell in Superior, between Boulder and Denver, and reports of 2-2.5” hail (golf ball to tennis ball size) were widespread in the northern urban corridor.

“Yesterday's Denver-area hail storms certainly have the feel of a big cost event,” tweeted Bryan Wood, a meteorologist and storm damage analyst at Assurant, on Tuesday morning.

5-day precipitation outlook, 6/19/2018
Figure 3. 5-day precipitation forecast from 8 am EDT Tuesday, June 19, 2018, through Sunday, June 24. Image credit: NOAA/NWS/WPC.

The longer-term outlook

The stationary front across the Midwest will be sagging south during the week, allowing for storm complexes to develop on the central and northern Great Plains and translate eastward through the Corn Belt. By this weekend, the zone of heaviest storms will shift even further south, into a belt from Kansas to the Ohio Valley. Flash flooding threats could pop up anywhere along this quasi-stationary zone of rich moisture over the next few days, depending on subtle upper-level features and on where the previous day’s showers and thunderstorms end up.

As of Tuesday morning, there were no areas of tropical cyclone development being flagged by the NOAA/NWS National Hurricane Center in either the Atlantic or the East Pacific for the next five days.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

Bob Henson is a meteorologist and writer at weather.com, where he co-produces the Category 6 news site at Weather Underground. He spent many years at the National Center for Atmospheric Research and is the author of “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”


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