Federal weather forecasters issued flood alerts affecting nearly 39 million people late Saturday into Sunday as a line of thunderstorms developed across the East Coast, stretching from Washington, D.C., to New Haven, Connecticut. These storms brought heavy rainfall and the potential for flash flooding, prompting emergency management officials to take proactive steps to protect vulnerable residents. In New York City, officials opened 51 weather shelters to provide safe refuge for those at risk, particularly those living in basement apartments where flooding can be especially dangerous. The National Weather Service issued multiple flood alerts, including flood watches, across several states late Saturday and into early Sunday. These alerts covered parts of Maryland, Pennsylvania, Delaware, and New Jersey. In Washington, D.C., the flood watch expired at 1 a.m. Sunday, but areas around Philadelphia and its surrounding counties remained under a flood watch through the morning hours. New York City was expected to enter a flood watch starting at 4 a.m. Sunday, lasting until noon, as forecasters predicted heavy rainfall across the region. According to New York City Emergency Management, Nassau and Suffolk counties, as well as southern Connecticut, could receive 5 to 7 inches of rain. The city warned that eastern boroughs like Brooklyn and Queens could also face heavy downpours. A slight shift in the storm’s path could increase the risk of flash flooding in these areas, particularly for residents living in basements or below ground level. Officials urged these individuals to move to higher ground to avoid potential danger. The thunderstorms are forming due to a trough of cold air high in the atmosphere moving east and colliding with warmer air masses. This clash creates conditions for intense rainfall and severe thunderstorms, according to the National Weather Service. In a late Saturday forecast discussion, the service noted that the combination of atmospheric instability, deep moisture, and the forcing mechanism from the colliding air masses could lead to an increased threat of excessive rainfall and severe weather across the affected regions.