Why were Saturday's tornadoes so strong?

FRANKLIN, Texas (KWTX) An upper-level low is the culprit for Saturday morning's severe storms across Central Texas. Most of Central Texas saw heavy rain and gusty winds, a few areas saw hail up to the size of golf-balls, but a long stretch of damage occurred from at least one tornado in Robertson and Leon County.

Photo by Christy Soto

The National Weather Service in Fort Worth will be conducting damage surveys across Robertson and Leon County this weekend and will be able to determine how strong Saturday's tornadoes were and if the damage is consistent with multiple tornadoes or one long-lived tornado.

Early damage indicators suggest the tornado that ripped through Franklin could have been the strongest tornado in Robertson County since 1959. We won't know for a few days exactly how strong the tornado was, but we know the atmosphere was primed for severe storms.

Saturday's storms was driven by a strengthening upper-level trough traversing West Texas. The orientation of the trough was northwest-to-southeast, known as a 'negatively-tilted trough'.

When a trough becomes negatively-tilted, atmospheric wind-shear increases, cold-air moves into the middle atmosphere (which increases atmospheric instability), and upper-level winds diverge. Upper-level divergence helps to create surface convergence and rising air.

The storm that moved through Milam, Robertson, and Leon County was riding along an axis of warm air. As warm air on the south-side of the storm flowed into the storm unimpeded, warm and humid air flowed freely into the storm.

The rising air tapped into abundant instability and wind shear to create a rotating thunderstorm, known as a supercell. The storm began to mature and rotate at the low-levels and eventually produced a tornado. Since this storm had abundant instability, wind shear, and warm and humid air to work with, it maintained severe storm intensity for nearly two hours.

Despite the storm being wrapped in rain and shrouding the tornado, Doppler Radar was able to see the strong rotation within the storm. A strong 'couplet', which are quickly rotating winds within a storm that's picked up on radar, was evident with this storm and a tornado warning was issued.

The couplet, a good indicator of a storm being capable of producing or actually producing a tornado, remained strong from eastern Milam County all the way to northern Leon County.

The rotational couplet only started to weaken once the storm was swept into an approaching cold front.

Although a strong rotation signature was visible on radar for nearly two full hours, stretching for nearly 80 miles across Central Texas, we won't know if a tornado was on the ground continuously throughout that time for a few days.