KTRK tweeted this picture, taken from a monitor turned to NASA TV.
NASA says the lightning came within a third of a mile of the launch pad. NASspaceflight.com is keeping a running account of preparations, and it notes the lightning strike "was VERY powerful at nearly 70,000 amps (realize your house uses about 200 amps and most lightning strikes are 20-30,000 amps.)"
Chris Bergin, managing editor of the site, says "Confirmed lightning strike at the pad, which requires data reviews by all affected systems -- engineering. Already in work."
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Even if the reviews go well, the bad weather is predicted to still be around the area on Friday.