Timothy made the model shown below to represent the steps in the formation of a tornado. The diagram has four boxes. The first box on the top left has the symbol '?' in it, an arrow points from it, over to the left to a box with the text 'Severe thunderstorm'; this box has an arrow pointing down from it to a box with the text 'High winds push clouds'; and this box has an arrow pointing to the left to a box with the text 'Rotation within thunderstorm'.

Which of these best fits the empty cell in Timothy's model?