A new index that characterizes a typical length of horizontal and vertical transport is proposed to predict meteorological conditions that are unfavorable for the dispersion of air pollution. This index can be easily computed from the model output of a mesoscale NWP model and can be easily interpreted by a forecaster. It is shown that, for situations of low wind speed and stable atmosphere when atmospheric transport is weak, this index is sufficient to make reliable predictions of peaks of extreme concentrations of some pollutants whose concentrations are predominantly determined by a constant emission. The relation between this index computed from the output of the Aire Limite´e Adaption Dynamique De´veloppement International (ALADIN) NWP model and the extreme concentrations of the pollutant nitrogen dioxide (NO2) during winter in the Brussels, Belgium, capital area is discussed in detail. Although the concentrations of NO2 are also determined by chemical reaction of nitric oxide with ozone, the fact is used that, for strong pollution events during typical meteorological winter situations in Brussels (having low background concentrations of ozone), the high concentrations of NO2 are mostly determined by direct NO2 emission. It is also shown that by the same method some extreme concentrations of particulate matter with diameter ,10 mm (PM10) can be predicted to some extent.