followed vs. followed by

A storm followed.

随之而来的是一场暴风雨。

Lightning is usually followed by thunder.

在闪电之后通常有雷声.

Literally, I believe "followed by" is more logical. Since this sentence is written by a Reuters journalist, we may guess other possibilities. One possibility, I guess, is that "by" is ommitted, because there is no other understanding about which city is the most stressed one.

Interested readers are refered to the following link.

http://www.reuters.com/article/2009/10/21/us-usa-cities-stress-idUSTRE59K0V520091021