Tornadoes form in unusually violent thunderstorms when there is sufficient (1) instability and (2) wind shear present in the lower atmosphere. Instability refers to unusually warm and humid conditions in the lower atmosphere, and possibly cooler than usual conditions in the upper atmosphere.
According to the Fleming's left handrule we know that when a positive charge moves with velocity v perpendicular to the direction of magnetic field the it experiences a magnetic force perpendicular to the the plane in which the velocity vector and the magnetic field vector lies.
As a result all the three vector are mutually perpendicular in such a case.
Here as per given condition we have a negative charge moving along x-axis which experiences the deflection in -y direction so the magnetic field will be along the negative z axis i.e. into the screen.
