The Texas electric grid could suffer a massive shortfall in generating capacity in a winter deep freeze, potentially triggering outages similar to those in February, according to a report on Thursday by an electric reliability authority. The assessment by the North American Electric Reliability Corp (NERC), a nonprofit regulatory authority, comes as Texas lawmakers and regulators continue to investigate ways to bolster the grid to avert a repeat of last winter"s blackouts, which left 4.5 million customers without power in a deep freeze that killed more than 200 people. In normal winter conditions, the anticipated reserve margin - a cushion of extra capacity versus demand - is comfortable at nearly 42% for the Electric Reliability Council of Texas (ERCOT), the grid operator for about 90% of the state. But if severe frigid weather hits, it could disrupt pipeline natural gas supplies and power stations, leaving a capacity deficit as high as 37%, NERC said. The Texas grid is mostly isolated from other U.S. grids, with a limited ability to import electricity when a local shortfall materializes. ERCOT also operates the only major U.S. grid that does not have a capacity market - a system that provides payments to operators to be on standby to supply power during severe weather events. ERCOT said in a statement that it had made significant progress since last winter. "The electric grid will be able to perform significantly better this coming winter than in the past," it said.