Page created and maintained by Roger Edwards, Storm Prediction Center
Supercells -- rotating thunderstorms which sometimes produce tornadoes -- are not only a U.S. phenomenon. They can happen anywhere the ingredients are right: moisture, instability, lift, and vertical shear (change in wind with height). The supercell in this picture formed after dark on the Serranias del Burro (a.k.a. Sierra del Huacha), a range of Mexican mountains west of Del Rio, Texas, and part of the northern Sierra Madre Oriental. [See the topographic image for geography of the area.]
The round "ball" of reflectivity in the hook is a rare feature, sometimes seen when a radar picks up tornado debris being lofted into a thunderstorm. [Tornado debris was the cause of a similar signature on May 3, 1999 in Oklahoma.] Unlike in the developed areas of central Oklahoma, there is apparently little which a tornado could hit in that part of Mexico to generate large debris -- unless the radar was detecting sagebrush leaves, broken mesquite branches and shredded chunks of prickly pear cactus. Maps of the area show no towns or villages, and only one dirt road winding west out of Ciudad Acuna into the Serranias del Burro mountains.
Because of the remote desert terrain west of Ciudad Acuna, the lack of population, and the lack of tornado verification in Mexico, we may never know if there was truly a tornado there. In the U.S., however, low level mesocyclones this strong are not usually seen even in many storms which do produce tornadoes. Based on all the radar signatures, the best we can say for now is that this supercell probably produced a significant tornado in Mexico. Later, as it crossed the border over Amistad Reservoir (a large man-made lake on the Rio Grande), it had two hook echoes, with a mesocyclone in each one, prompting this tornado warning for the Amistad Reservoir and Del Rio area:
According to the local storm report, a rancher sighted a funnel cloud with debris -- by definition, a tornado -- a mile and a half southeast of the Amistad Dam, 10 minutes later at 12:40 a.m. Tornado damage to ranch equipment was reported 20 minutes later, 15 miles north of Del Rio. After moving across the border, this supercell produced at least one tornado and hail the size of baseballs...and it wasn't as intense on radar as it was earlier in Mexico!
For several decades, forecasters at SPC (formerly NSSFC/SELS) have noticed large, apparently severe thunderstorms forming in the Serranias del Burro during spring and summer, often far removed from those on the same days in the Great Plains. Most stay in Mexico; but a few will cross the Rio Grande and produce large hail and heavy rain in the U.S. There has been almost no scientific study of thunderstrms in this area, however, because of its remoteness and the tendency for the storms to stay south of the border.
For more info on supercells and tornadoes, check out the new Online Tornado FAQ at SPC.