Energy markets are closely watching for signs of damage at Gulf Coast refineries, many of which had shut down to prepare for rainfall and flooding from Tropical Storm Harvey.
The damage could mean a loss of more than 1 million barrels per day in refining capacity just in the Houston and Galveston, Texas, areas — that's not including hundreds of thousands of more barrels in the Corpus Christi, Texas, area.
The Houston area has seen 24-34 inches of rainfall. Flash flood watches are in effect for more than 11 million people, and another 10-20 inches of rainfall is possible over the next 72 hours.
Analysts say the refinery outages could last for weeks or months if workers find significant damage due to flooding at the facilities. While refineries in Corpus Christi appear to have emerged relatively unscathed after Harvey made landfall as a hurricane on Friday, the picture is not yet clear for the Houston area, said Tom Kloza, global head of energy analysis at Oil Price Information Service.
"I'm very, very worried about Houston, Beaumont, Bay City, the triangle where so much refining is located. It's hard to forecast because, as I say, it's unprecedented and the outcomes are completely unknown," he told CNBC on Monday.