There have been some advancements in predicting when and where severe weather will occur. Unfortunately, many of the factors that contribute to tornado development are hard to predict, and forecasts change frequently, as forecast models change. There are certain "ingredients" that must be in place for tornadoes to develop. One of those ingredients is instability. Instability refers to warm and humid air in the lower atmosphere, with cooler air higher in the atmosphere. This contributes for the upward rising of warm air into the cooler air aloft, which can form thunderstorms. Many times instability is prime along cold or warm fronts, where 2 air masses are converging. The other main ingredient is wind shear, which refers to different wind directions and speeds at different levels of the atmosphere. Forecasters rely heavily on forecast models. There are many different models that forecasters use, and some times these forecast models differentiate if and where these ingredients will be in place. Forecasters look for trends and where models "agree" to make their forecasts. Today, there are many weather spotters and storm chasers that work with local media and NWS offices to relay information from under storms to advance warning times. Often before a tornado forms, a wall cloud forms under the base of the storm. These wall clouds are often visible in the Southern most part of a super cell thunderstorm, and gives spotters and chasers a good idea of when and where a tornado will form. This information is then relayed to the NWS and local media networks, and advanced warnings can be issued, often with confirmation or assistance from radar indicated rotation. I hope his helps you out and addresses your question. Let me know if you want any more information.