A stalled frontal boundary and deep tropical moisture from an upper-level low are driving scattered to widespread showers and thunderstorms across the eastern U.S. on September 21, 2026, according to National Weather Service and NOAA forecasts. The Northeast, including New York and Boston, faces the highest likelihood of measurable precipitation (≥0.01 inches) from lingering clouds and rain bands, with highs in the mid-60s to low-70s. South Florida sees repeated rounds of heavy downpours and storms fueled by southwesterly flow, elevating flood risk through midweek. In contrast, drier high pressure dominates the West and parts of the interior, keeping probabilities low for cities like Los Angeles. NWS daily climate reports will provide official resolution data, with model consensus favoring continued unsettled conditions in the East before gradual improvement Wednesday.