There are various reasons why some strong thunderstorms produce tornadoes while others do not. Below are some reasons why a storm may not produce a tornado:
1. Instability is too weak. Updraft is too weak to stretch and twist the inflow into the updraft.
2. Directional shear in the lower troposphere is too weak. Weak directional shear will prevent the updraft from rotating. Spinning inflow will more likely spin up a tornado than non-spinning inflow. Weak directional shear will usually not allow supercells to develop.
3. Speed shear in the troposphere is too weak. This will cause the downdraft to cut-off the updraft.
4. PBL is too dry. If the LCL (Lifted Condensation Level) is high above the earth's surface, the region that +CAPE begins will also be high above the earth's surface. This reduces the amount of updraft stretching near the earth's surface. High based storms produce tornadoes less frequently.
5. Not a good balance between CAPE and shear. If CAPE overwhelms the shear, then the storm is less likely to rotate. If the shear overwhelms the CAPE, the storm will be torn apart.
6. Not a good balance between the RFD (Read Flank Downdraft) and FFD (Forward Flank Downdraft). This can occur when a storm moves too slowly (low shear environment). The RFD and FFD act as mini fronts that enhance the inflow into the updraft.
7. Not enough enhancement of low level streamwise vorticity and Helicity by mesoscale influences.
8. Inflow (low level wind toward updraft) is too weak. Inflow generally needs to be greater than 20 knots.