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Modified Growing Degree Days are similar to Growing Degree Days with several temperature adjustments. If the daily maximum temperature is above 86°F, it is reset to 86°F. If the daily minimum temperature is below 50°F, it is reset to 50°F. Once the max/min temperatures have been modified (if needed), the average temperature for the day is computed and compared with a base temperature, which is usually 50°F for corn and soybeans. Modified Growing Degree Days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86°F or falls below 50°F. For example, if the high for the day was 92°F and the low 68°F, the average for use in the Modified GDD calculation would be 86 + 68 = 154 / 2 = 77.

These maps are generated daily at approximately 11:30 a.m. Central Time during the growing season.They are intended to provide a general picture of the seasonal MGDD and departures from average in the Midwest. MGDD accumulations and departures calculated for a particular station for the same period as the map will likely differ from the same point picked on the map due to the gridding and resultant smoothing process.

These maps are generated from gridded temperature fields. The maps are interpolated from a 2/3 degree latitude by 1 degree longitude grid (approximately 75 km x 75 km). A substantial amount of preliminary data (data subject to further quality control) is used to generate these maps through the current date. The maps should be considered preliminary and subject to correction.

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