SWIF’s predictions for the vTEC storm time response during the time interval 23–26 April 2012 for DOUR and USAL locations. The corresponding GNSS-derived vTEC estimates, monthly medians and IRI2012 output are also included for comparison purposes. The storm onset time was determined in the afternoon sector (23/04/2012, 14:00 UT), so that the vTEC response was forecasted 4 and 6 h before its onset at DOUR and USAL, respectively. In this example, the IRI2012 tends to reproduce only the median variation.