Tornado nowcasting

E. Linacre and B. Geerts


In meteorology nowcasting refers to short-term weather prediction solely by means of current information, without the aid of numerical models. Nowcasting is a potentially useful technique to anticipate tornadoes: the advancement of a tornado warning by just a few minutes can save many lives. Tornadoes usually catch forecasters by surprise, even today when high-resolution satellite and radar imagery is available. The reason for the poor predictability of tornadoes is twofold:

  1. Thunderstorm development, intensity and evolution cannot be predicted with the current generation of numerical weather prediction models, and it is unlikely that this will change in the next few decades. The problem is primarily due to lack of high-resolution data. However the environment in which (severe) thunderstorms can develop is fairly predictable.
  2. Current observing systems, such as satellite imagery and especially Doppler radar, do not readily discriminate between a tornadic and a non-tornadic thunderstorm. Of particular interest are early signs of a tornadogenic storm in the remote imagery.. There are some clues, but they only slightly increase the probability of remote detection.

Clearly, both numerical predictions and nowcasting of tornadoes benefits from higher-resolution observation platforms. In the USA, where violent tornadoes are more common than anywhere else, a dense network of Doppler radars has been installed during 1987-'97, in part to improve the predictability of severe weather. The following facts apply to the USA (1):

Fig 1. Typical evolution of the base height of a mesocyclone prior to touchdown (T) of a tornado. Time is expressed in terms of volume scans, which are completed in about 5 minutes. The rapid downward extension of the mesocyclone base height, typically about 15 minutes prior to tornado touchdown, is used as an argument to issue a tornado warning. (Source: Operational Support Facility, National Weather Service)


Fig 2 Supercell storms in Minnesota, as seen by Doppler radar. The black contours outline the counties. The picture represents an area of about 120 by 150 km. The radar is located in the southeastern quadrant, in the center of some ground clutter seen in the reflectivity image (a). On the right (b), green colours indicate flow towards the radar, red means flow away from the radar. (Source: Operational Support Facility, National Weather Service)


(1) AMS 1997. Policy statement on tornado forecasting and warning. Bull. Amer. Meteor. Soc., 78, 2659-62.