Second practice for the U.S. Grand Prix was cancelled after heavy rain and lightning prevented the cars going on track and eventually grounded the medical helicopter.
The teams were limited to wet running in the morning practice session, but the arrival of a storm over lunch ruled out any additional track time in the afternoon. Ahead of the scheduled start of the session, the FIA notified teams that FP2 would be delayed indefinitely because the proximity of lightning posed a threat to the safety of marshals. A further notification followed an hour later saying the session would be cancelled because the medical helicopter was not able to operate in the conditions.
The bad weather is expected to continue throughout the weekend, with flash flood warnings in place until Sunday morning when more heavy rain is forecast. Without a break in the conditions the cars will not be able to run, creating concerns for the weekend's schedule.
In recent years qualifying sessions have been postponed until Sunday morning when weather has ruled out running on Saturday afternoon, but races have always gone ahead, albeit sometimes shortened or postponed.