NOAA/NWS 1925 Tri-State Tornado Web Site--Weather Ingredients
Back in 1925, weather records were not nearly as detailed as they are today. With a lack of observing stations, data was sparser and forecasts were more vague. Therefore, the exact conditions that preceded the Great Tri-State Tornado are not well known. However, given what we now know about tornado development and using what records there were from 1925, we can surmise that March 18, 1925 undoubtedly would have been a candidate for a moderate or high risk of severe weather!
That morning, surface low pressure over Northwest Arkansas and Southwest Missouri tracked northeast across Southeast Missouri, Southern Illinois, and Southwest Indiana during the day, reaching Eastern Indiana that evening. Extending east from the low was a warm front, with a cold front trailing to the southwest. As the low tracked northeast during the day, its associated warm front advanced north, allowing warm moist air from the Gulf of Mexico to infiltrate the Tri-State area. In fact, temperatures that started out in the 50s during the morning reached the 60s over most of the tornado track by 1 p.m. and even the 70s in the vicinity of Cairo, Illinois by 4 p.m. So, we know that a lifting mechanism was in place and moisture was abundant with the Gulf opened for business.
We can also infer there must have been very good upper-level support. Given the fact that the tornado traveled at speeds of 60 to 70 mph along most of its path, we can safely assume that perhaps a 100-knot upper-level jet max was nosing into the area from the west/southwest. With veering winds (south at the surface becoming west/southwest aloft), wind shear was also present to help initiate the storm’s rotation. There must have been decent instability as well—with warm air advection at the surface and probably cold air advection in the upper levels.
What makes this tornado interesting, though, is that its occurrence was nearly coincidental with the track of the surface low. While other tornadoes in the warm sector of the low affected parts of mainly Tennessee and Kentucky that day, none were as massive, long lasting, or violent as the Tri-State Tornado. This goes against conventional thinking that while it is not uncommon for a tornado to occur in conjunction with the surface low, the most violent ones actually occur in the warm sector of the storm—well south and east of the low’s track.
A question that many scientists often pose is: Was the Tri-State Tornado actually ONE tornado or a family of tornadoes? Findings from modern weather records and research suggest that a tornado that endures as long as the Tri-State Tornado actually results from a cyclical supercell rather than one massive storm. In this theory, the storm continuously evolves, and the decay of one supercell leads to the development of another—and so forth. Each supercell may be responsible for parenting one or more tornadoes. Without a close examination of the storm and its damage path, it can appear to the "novice" observer that the damage resulted from ONE tornado, when in reality, a family of tornadoes generated by a cyclical supercell caused the destruction.
The only problem with applying this theory to the Tri-State Tornado is that a cyclical supercell tends to exhibit breaks in its damage path as the storm evolves. However, the Tri-State Tornado’s path of destruction was CONTINUOUS. Only twice in the storm’s path—near the onset and demise—did a slight decrease in the tornado’s damage suggest that the event may have been not one—but a family of tornadoes. But if the Tri-State Tornado was one massive storm, then why hasn’t such a storm been documented in the 75 years proceeding its occurrence? Could it be that the 1925 tornado was a rare event—occurring only once in several hundred years? Did it actually result from a cyclical supercell? Or could it be that we lack enough information to come to a definitive conclusion at this time? Despite all the uncertainties surrounding the nature of the 1925 Tri-State Tornado, one thing is for certain—a storm like it will happen again. The only question is: when and where?