NOAA Storm Prediction Center (SPC) meteorologists issue daily forecasts, or convective outlooks, for severe thunderstorms over the U.S. based upon current weather observations and forecast models (detailed further below). Meteorologists closely monitor areas they think are at higher risk for tornadoes.
If conditions develop that are favourable for tornadoes, SPC forecasters will issue a severe thunderstorm or tornado watch which usually lasts 4 to 6 hours. Local forecast offices, emergency managers, storm spotters, and the general public are alerted to the possibility of severe weather.
Tornado warnings are issued by the local National Weather Service Forecast Office when either a tornado has been spotted or indicated on water radar. If you’re in the warning area, seek appropriate shelter straight away.
Computer forecast models
Meteorologists tend to rely on large and intelligent computer programs clued numerical weather prediction models to help them decide if conditions will be right for the development of tornadoes.
These particular models are designed to calculate what the atmosphere will do at certain points over a large area, from the ground to the top of the atmosphere. Data is collected from weather balloons launched around the world twice a day, as well as measurements from satellites, aircraft and more.
Computer forecast models start with these current weather observations and attempt to predict feature weather, including supercells, using physics and dynamics to mathematically describe the behaviour of the atmosphere. The predictions are usually output in text and graphics (mainly maps).
Computer models tend to work great if the weather follows the rules meteorologists at SPC have set. When the weather breaks the set rules the predictions have issues as well.
Another technique being developed is the concept of ensemble forecasting. Instead of using just one model, a supercomputer runs several models at once – an ensemble essentially. If each run (of the model) looks the same, then it can be assumed that the weather will follow the set rules. If the runs look different in different places, then it’s fair to say something in the atmosphere is causing the weather to misbehave.
Interpreting the output of the model is key, and takes a fair amount of practice. Forecasters use their experience, knowledge, persistence, and eyes to fine-tune their forecasts. An incredible advancement has been made in model displays, the output used to be on black and white maps. Forecasters can now look at the output on their computers and use different colours to understand what’s happening more clearly.