Jan 24 (Reuters) - At least one chemical plant and an oil refinery were scrambling to recover from operational upsets on Tuesday after severe weather tore through an oil and gas refining hub outside Houston. Shell (SHEL.L) said it was experiencing an incident at its Deer Park chemicals facility following severe weather, according to a company tweet. Petroleos Mexicanos also reported operational upsets due to weather at its neighboring oil refinery, according to a company alert. The National Weather Service issued a tornado warning for the Houston area on Tuesday afternoon. Exxon Mobil (XOM.N) said operations at its Baytown, Texas, plant were stable following the severe weather, with no injuries reported.

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