The IFFA is a manual technique used to estimate heavy precipitation amounts. This method is used as a backup to the Auto Estimator, especially for convective rainfall. The IFFA is still used, especially for winter storms and lake effect snow.
Analysts utilize a flow diagram or decision tree.
The technique has been modified to take into account warmer top convection by comparing the observed anvil temperature with the nearby radiosonde sounding derived equilibrium level. Cloud top temperatures equal to or colder than the equilibrium level would indicate heavier rainfall rates than clouds that were warmer. Three steps are used to estimate convective rainfall amounts. STEP 1 - Locate the active portion of the thunderstorm system. IR temperatures and gradients and bright, textured clouds in the visible are used for making this decision.