Taanit, Chapter Three, Mishnah One
This mishnah teaches that sometimes we skip the order of fasts that was taught in the previous two chapters and we proceed straight to the last set of fasts, when we blow the shofar and fast for the entire day.
1) The order of public fasts mentioned above is enacted because of [lack of] the first rain, but if the crops have undergone [an unusual] change they sound a blast immediately.
2) Similarly, if the rain has stopped for forty days between one rainfall and the next, they sound a blast immediately, because it is a plague of drought.
Section one: If it doesnt rain during the first season in which rain should come, then we begin the series of fasts that was described in the previous two chapters. However, if the crops start to look as if they are going bad, then the situation is obviously more serious. In such a case we skip the first two sets of fasts and go right to the third set, the set where we blast the shofar. The change of the crops appearance is far more serious and therefore it calls for an immediate sounding of the alarm.
Section two: Another case in which we skip immediately to blowing the shofar and fasting for the entire day is a situation in which it began to rain but then stopped raining for forty days. Such a situation foreshadows drought and therefore it is especially dangerous.