Due to climate change, intensive storms and severe precipitation will continue to happen, causing destructive flooding. In July 2021, a series of storms with prolonged rain episodes took place in Europe. Several countries were affected by severe floods following that rainfall, causing many deaths and material damage. Thus, a good understanding and forecasting of such events are of uttermost importance. This study highlights the interest of multi-GNSS tomography for the 3D modelling of the neutral atmosphere refractivity. The tropospheric parameters have been retrieved for the July 2021 flood in Germany from two tomographic solutions with different constraining options using either GPS-only or multi-GNSS estimates. Our investigations show that the stand-alone solution (especially the multi-GNSS) is producing more patterns of refractivity, and is temporally more stable. We compare the tomographic results with external observations such as radiosondes and GNSS radio-occultations from Metop-A & -B satellites. The results show that tomography is producing wetter conditions than the reference. However, we can see the precursor information of the initiation of deep convection in the ground-based GNSS technique.