Austin - Torrential rain and dangerous electrical storms led to the cancellation of Friday's second free practice session and may threaten the running of this weekend's United States Grand Prix.
The session was initially postponed indefinitely because marshals at the Circuit of the Americas were unable to work because of the close proximity of the lightning.
Hopes of starting the session later were dashed when it was confirmed that practice had been abandoned because the circuit's medical helicopter would not be allowed to take off for at least two hours due to the conditions.
"@F1 @circuitamericas looks like my boat licence may help today....," tweeted Lotus's French driver Romain Grosjean.
If the conditions persist, as forecast, the race could be delayed or cancelled, many paddock observers said.
The opening free practice session, in the morning, was also hampered by rain and run in damp conditions.
Germany's Nico Rosberg, who is seeking a victory to keep alive his hopes of wrecking Mercedes team-mate and defending two-time champion Lewis Hamilton's Texan title party, was fastest in that session.
Hamilton was fifth and Sebastian Vettel of Ferrari, who is second in the title race, was sixth, but the session was of little value and gave few clues as to the relative competitive potential of the teams.
The cancellation of Friday's action was not unexpected following forecasts of storms associated with typhoons in the region throughout the weekend.
Hurricane Patricia, which has hit the coast of Mexico, is reported to be the strongest storm ever measured.
Continued bad weather could lead to the cancellation of practice and qualifying on Saturday as well as Sunday.
The last time qualifying took place on a Sunday was at the 2013 Australian Grand Prix when the Saturday session was postponed because of heavy rain.
In 2004, and in 2010, qualifying for the Japanese Grand Prix was abandoned on Saturday because of a typhoon and it took place on Sunday morning.
"There's nothing we can do," said the sport's commercial ring-master Bernie Ecclestone.
"It's alright for those of us who have got jobs and things to do, but I feel sorry for the public.
"They are here to enjoy some racing and all they are getting is soaked in the rain."