June 2, 2015 Tugging the C.A.P.E.

The Liberty Gazette

June 2, 2015

Ely Air Lines
By Mike Ely and Linda Street-Ely

Linda: How ‘bout this weather? We blinked and went from drought-induced burn bans to flood rescue scenes. One of the nation’s top aviation weather scientists, Scott Dennstaedt, enlightens us on the sudden departure from parched, to drenched.

Scott: 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 usually received throughout the entire year. Rainfall totals reported to exceed 20 inches have been pretty common. And to cap it all off, Monday last week more than 10 inches of rain fell in Houston causing widespread flash flooding in the city. So what caused this extreme rainfall event?

The phenomenon that was responsible for this deluge of rain 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.

You’re probably accustomed to thunderstorms occurring in the afternoon. That’s usually the way it happens 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 last week was born early that morning in western Texas and began as a pair of MCSs. Throughout the morning the two systems tracked east and eventually merged 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.

Linda: Other data Scott analyzes include the index of Convective Available Potential Energy, which last week gave signs of high convective rainfall rates that could produce local flash flooding, which is exactly what happened.

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