During the afternoon hours of July 27, 2017, an extreme hailstorm struck the most populated city of Turkey, Istanbul. This rapidly growing supercell which produces large hail led to numerous injuries and damaged automobiles, houses, aircrafts, crops, and infrastructure of the city. As a result of the movement of the cut-off cyclone from Middle East to western Turkey, warm air advection penetrated over Marmara, and the land surface temperature of Istanbul reached 34 degrees C (5 degrees C above the mean). The transport of significant amount of moisture, which was caused by excessive heating of Marmara Sea surface temperatures (24.9 degrees C), to the low levels of the atmosphere by strong southwesterly winds enabled the increase of low-level moisture convergence. Both abnormal temperature differences between land and 500-hPa level (41.5 degrees C) and excessive wind shear values (20.3m/s) between surface and 6km above ground level (AGL) increased the thermal instability and updraft conditions of the baroclinic atmosphere. This condition resulted in thunderstorms, stormy and gale wind gusts (31.9m/s 15:20 UTC), extreme lightning activity (totally 2696 cloud-to-ground and 5791 intracloud), large hailstones between 3-6cm diameter and high hourly precipitation amounts (38.7, 36.2, 29.8, and 27.2mm in Sisli, Kadikoy, Uskudar, and Fatih regions, respectively, between 14:00 and 15:00 UTC) in the urban settlements of the city. The stability indices, Showalter, K, Total of totals, SWEAT, and CAPE also showed the high probability of severe thunderstorm occurrence over Istanbul. Based on a comparison among these five indices, the SWEAT index is most appropriate to represent the atmospheric conditions over the city owing to low-level moisture, warm air advection, and low and mid-level wind speed terms in its equation.