To improve the range of atmospheric conditions used in consequence assessments, we use an unsupervised machine learning technique based upon Kohonen neural networks (Self Organizing Maps, SOMs) to identify the atmospheric patterns in the historical record. The historical record is generated with a high-resolution limited area atmospheric model that re-analyzes the past 30 years based upon global data and local observations. The SOM is trained using daily patterns of multivariate output from the atmospheric model re-analysis. A good SOM analysis typically overestimates the total number of patterns resulting in many patterns that bear close similarity to each other, which can make interpretation of the results difficult. In order to overcome this limitation we apply an optimization stage based upon hierarchical trees to identify the minimum number of unique atmospheric patterns. We can then provide the analyst with a reasonable number of cases, along with the frequency of occurrence, that represent the full range of possible atmospheric conditions.