A thunderstorm forecasting decision tree

One can distinguish various kinds of thunderstorm - air-mass, multi-cell or supercell storms (Section 9.5) (1). Dynamically and morphologically, these storms are very different, although they may evolve from one type to the other, and many storms appear to be some hybrid. Air-mass storms are most common but do little damage. Supercell storms are least common, but longest-lived and most harmful.

From a purely operational perspective, one can distinguish between normal and severe thunderstorms. Severe storms are associated with flash floods, strong winds, hail or a tornado.

Which kind will occur can be predicted with useful accuracy by means of a ‘decision tree’, a diagram which links possible circumstances and consequences of a number of decisions (2, 3, 4). These are based on prior information, considered in turn, on the conditional instability at the surface, wind shear in the lower troposphere, lifting indicated by divergence at 300 hPa, and the relative humidity of the mid-troposphere, etc.

An example of a simple decision tree to issue a thunderstorm warning at an airport is shown in Fig 1.

Fig 1. Computer-based logic tree for the issuance of a thunderstorm warning near an airport (ltg=lightning, VIL=vertically integrated liquid, or column reflectivity, derived from a ground-based radar) (5).

1. NOAA technical memorandum SR-185, 1997: Techniques for issuing severe thunderstorm and tornado warnings with the WSR-88D Doppler radar.
2. Colquhoun, J.R. 1987. A decision tree method of forecasting thunderstorms, severe thunderstorms and tornadoes. Weather & Forecasting 1, 337-45.
3. Colquhoun, J.R. 1996. A decision tree method of forecasting thunderstorms, severe thunderstorms and tornadoes revisited. Preprint for 5th Severe Thunderstorm Conference (Aust. Bur. of Meteor.), Avoca Beach, 7pp.
4. Colquhoun, J.R. and G.A. Mills 1998. Thunderstorm and severe thunderstorm prediction using a decision tree and mesoscale NWP model. Preprints for 19th Amer. Meteor. Soc. conference on Severe Local Storms, p607-10.
5. Churma, M.E. and S.B. Smith, 1998: Evaluation of the AWIPS thunderstorm product. Preprints 16th Conf. on Weather Analysis and Forecasting, Phoenix, Amer. Meteor. Soc.