Your Questions Answered: Why do many thunderstorms develop overnight?

By Justin Horne - Weather Authority Meteorologist/Reporter

SAN ANTONIO - KSAT viewer Elijah submitted a question for our KSAT meteorologists.

Why do most of our more potent or severe thunderstorms happen late at night to early in the morning before sunrise?

KSAT Meteorologist Justin Horne had this answer for him:

There is something to storms arriving overnight and early in the morning in South Texas.  Part of it, of course, is just coincidence and timing of storm systems. 

But, one thing we occasionally see here (and up and down the Plains during the spring and summer) are what are called MCS’s, or Mesoscale Convective Systems.  They are big clusters of showers and storms.  Typically, they form to our west and northwest and race east or southeast toward South Texas.  MCS’s are most active at night, because nighttime is when a low level jet can form.  That low level jet pumps in fuel (moisture) from the Gulf of Mexico.  They feed off the jet and reach their peak overnight. 

In addition to that, geography plays a role, too.  We lie just to the east of a large mountain range in Mexico.  Many times, a combination of afternoon instability and orographic lifting (mountains helping to lift the air) is enough to fire up storms, even if chances aren’t great here.  If they move east, it is not until the nighttime hours that they reach San Antonio and surrounding areas. 

These are just some of the factors and sometimes it is a combination of many factors.  Remember, there are also always exceptions to the rule!

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