The 2000 CDC growth charts are used extensively in clinical practice to assess growth or patterns of growth and, in the US, to define obesity. Obesity is defined as a sex- and age-specific body mass index (BMI) at or above the 95th percentile. The growth charts are based on national data collected between 1963 and 1994 and include a set of selected percentiles between the 3rd and 97th and LMS parameters which can be used to obtain other percentiles and associated z-scores. Extrapolating beyond the 97th percentile is not recommended and leads to compressed z-score values. The prevalence of severe obesity among youth has increased, however, and it has become necessary to track children with extreme BMI values above the 97th percentile. For this study additional national data from children with BMIs at or above the 95th percentile in NHANES between 1995 and 2016 were used to fit a half normal model and to project percentiles and corresponding z-scores beyond the existing 95 percentile. Benefits of using this BMI metric to complement the CDC BMI growth charts in tracking extreme BMI are demonstrated.