Over the last five or more years a drought of historic proportion has plagued much of Texas. In fact, the National Weather Service reported that 2011 was Texas’ driest year on record. Fast forward to 2015 and that’s hardly been the case over the last few weeks as a good portion of Texas has received more rain in the month of May than they usually receive throughout the entire year. Rainfall totals reported to exceed 20 inches have been pretty common. And to cap it all off, this past Monday a very significant rainfall event occurred throughout central and eastern Texas with more than 10 inches falling in Houston Monday night causing widespread flash flooding in the city. So what caused this extreme rainfall event?
The phenomenon that was responsible for this deluge of rain on Monday is called a Mesoscale Convective System or MCS. Similar to hurricanes, they are very seasonal. Occurring mostly east of the Continental Divide, they start out in the Southern Plains and Deep South during the month of May. As the jet stream moves north through the summer months of June and July, they tend to occur in the Central Plains, Middle Mississippi Valley as well as the Tennessee and Ohio Valleys. Finally, into July and August, they are seen more in the Northern Plains, Upper Mississippi Valley and Upper Great Lakes regions.
These systems are usually severe and can often produce a few tornadoes, dangerous lightning, large and damaging hail and strong straight-line winds. But perhaps the most devastating feature is the torrential rains that can fall from some of these storms since they are often long-lived weather systems. Nevertheless, these convective systems are absolutely necessary since they provide much of the needed rain for agriculture in the Midwest during the summer months.
Mesoscale Convective Systems are easy to spot on the color-enhanced infrared satellite found in the ForeFlight Imagery as shown above. When mature, they usually appear as a large circular or oval cloud shield that can cover one or more Midwest states with very cold cloud tops that show up on this image as purple and white. Under this cloud shield is usually a bow-shaped line of strong thunderstorms at the leading edge of the MCS as seen on this NEXRAD mosaic below.
You were probably taught that the early morning hours are the best time to fly to avoid thunderstorms. That’s usually sound advice unless you are dealing with an MCS that will often develop and mature in the overnight hours and persist into the next day. So they are often nocturnal beasts that almost seem to create their own environment to feed on.
In fact, the MCS that flooded Houston Monday night was born early that morning in western Texas and began as a pair of MCSs as shown above. Throughout the morning the two systems tracked east and eventually merged (below) into a single complex of storms setting the stage for a very wet evening in Houston.
This is a very common setting in the Plains where the unique geography of the region favors nocturnal and early morning thunderstorms. During the warm season, this setting promotes a strong flow of low-level moisture northward from the Gulf of Mexico, often referred to by meteorologists as a low-level jet stream. Moisture carried by the low-level jet helps to maintain these systems that often begin during daytime hours on the higher terrain in western Texas and Colorado. Because of the low-level supply of moisture, the MCS can mature and persist well into the nighttime hours.
The Skew-T Log (p) diagram for Houston Monday evening shows the low-level jet as a maximum wind speed at 6,000 feet. This moist, southerly flow keeps the surface dewpoint temperature in the low 70s to offer a good source of moisture for the MCS to ingest.
Last but not least, the Skew-T diagram shows the atmosphere was very unstable Monday evening with a lifted index of -6, Convective Available Potential Energy (CAPE) approaching 3,000 Joules/kg and a K-Index of 42. A K-Index this high is a good sign of high convective rainfall rates that can produce local flash flooding.