Pilger, Nebraska Twin Tornadoes: How They Happened and How Unusual They Were

From that loop, it appears there were three separate tornadoes:

• Tornado A first formed west of Stanton, Nebraska, then soon weakened north of Stanton as it curled toward the north.
• Tornado B then formed east of Stanton and appears to have been the one that hit Pilger, Nebraska.
• Tornado C then formed just southeast of the Pilger tornado, then wrapped northward tracking close to, or interacting with, tornado B.

According to tornado warnings and statements from the National Weather Service near Omaha, tornadoes from this supercell were sighted for at least one hour and 10 minutes from 3:46 p.m until 4:56 p.m. Central time, and continued for 30 minutes after the end of the radar loop above.

Schematic of a supercell's rear-flank downdraft and updraft.

The process of tornadogenesis in a supercell remains an active area of research, with the supercell's forward-flank downdraft (rain-cooled air), rear-flank downdraft (a surge of less cool, dry air at the western edge of the supercell) and low-level mesocyclone/updraft all possibly playing roles.

When the rear-flank downdraft surges and completely surrounds the updraft, cutting off warm, moist air, the first tornado will slowly fizzle and curl toward the left (for an east- or northeast-moving supercell). 

However, just to the southeast of the weakening tornado, a second tornado may form where the two downdrafts and the updraft meet. In this manner, you can have multiple tornadoes simultaneously form from the same storm: one typically weakening, while the other is maturing or strengthening.

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