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*These probability values were estimated from a 30-year period of severe weather reports from 1982-2011. The procedure to create the maps is as follows:

  1. Reports for each day are put onto a grid 80 km x 80 km.
  2. If one or more reports occur in a grid box, that box is assigned the value "1" for the day. If no reports occur, it's a zero.
  3. The raw frequency for each day at each grid location is found for the period (number of "1" values divided by number of years) to get a raw annual cycle.
  4. The raw annual cycle at each point is smoothed in time, using a Gaussian filter with a standard deviation of 15 days.
  5. The smoothed time series are then smoothed in space with a 2-D Gaussian filter (SD = 120 km in each direction).