The supercell that simultaneously spawned a pair of large tornadoes in Pilger, Nebraska, on Monday was a frightening sight. Let's delve into the science behind this situation and how often it occurs.
(MORE: Pilger Latest News | Photos)
One Supercell: A Family of Tornadoes
Below is a radar loop of Monday's storm from the National Weather Service Doppler radar near Omaha, Nebraska. At left, the storm is shown with conventional reflectivity, with areas of rain and hail, whilestorm-relative velocity is shown at right, with areas of strong rotation indicated by areas of green shading very close to areas of red shading.
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.
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.
This is like the evolution of a frontal system you'll see on weather maps on a much smaller scale.
Namely, a cold front, warm front and intensifying surface low (the first tornado) transitions to an occluded front, with an intense low slowly weakening (like the rear-flank downdraft choking off the warm inflow to the first tornado), followed by a new area of low pressure forming at the junction of the cold and warm fronts, where the next tornado forms.
In this manner, a single, powerful supercell can produce multiple tornadoes over an hour or longer. These are known as tornado families.
What's Unusual About the Pilger Twins?
Multiple tornadoes simultaneously from the same thunderstorm are not as unusual as they sound.
Multi-vortex tornadoes contain so-called suction vortices rotating around the main tornado. These suction vortices are typically thinner than the main tornado, but don't be fooled by that. They can cause intense damage, because their winds combine with the forward speed of the parent tornado.
What was impressive and unusual about the Pilger tornadic supercell was the appearance of two distinct large, strong, long-lasting tornadoes at once, shown live on The Weather Channel.
"In all other cases I have seen, one tornado may last for a little while fairly close to another, but nothing like what happened (Monday)," said The Weather Channel senior meteorologist and veteran storm chaser Matt Crowther. "This is unique in my experience."
There is little doubt the research community will heavily examine this case.
"The atmospheric parameters for instability and supercell potential were extreme," said Stu Ostro, senior meteorologist at The Weather Channel. "That doesn't always result in strong tornadoes, as it's just one factor, but it did Monday, and something about that supercell resulted in the remarkable twin twisters."
Severe weather expert Dr. Greg Forbes has a hypothesis: incomplete cycling.
"The rear-flank downdraft (RFD) surge probably eased off and didn't choke off the Pilger tornado, yet the next tornado of the cycle formed south of Pilger," says Forbes. "The lack of a surging cold RFD allowed the Pilger tornado to keep going strong in tandem with the new one of the cycle to its south."
This brings to mind one of the most infamous tornado photos of all-time pictured above at right; the April 11, 1965 Palm Sunday double tornado, which was snapped by a photographer from The Elkhart Truth.
One more recent example of a tornado family (or cyclical tornadogenesis) was the supercell spawning the Greensburg, Kansas EF5 tornado on May 4, 2007. The Greensburg tornado was only one of several that night in western Kansas.
Other examples include a May 15, 2003 Texas Panhandle event sampled by the Center for Severe Weather Research, and the March 13, 1990 Hesston and Goessel, Kansas tornado event.
Finally, did you know one supercell complex once produced seven tornadoes on one evening in one city? Coincidentally, this was also in the Cornhusker State, the "Night of the Twisters" in Grand Island, Nebraska on June 3, 1980.
website: http://www.weather.com/news/tornado-central/pilger-nebraska-twin-tornadoes-how-they-happened-20140617
