This figure shows the trajectories (represented by the white lines) of what were initially two storm-cells (cells 1 and 2 were initially one storm cell; cell 3 remained a single storm cell). The first cell approached Baltimore from the south (starting at 18:01, the first double colored circle at the bottom). This individual cell, when it reached the urban environment, split into two cells: cells 1 and 2 (notice that the circles showing the path of this cell become one color after 18:19, when the cell splits). Cell 1 moved toward the center of the city but didn't make it over the city and was deflected (at 18:31) toward the western part. Cell 2 got trapped at the southwest part of the city and remained stationary for almost half an hour. "We saw this pattern repeating several times during the storm based on radar observations," Ntelekos said. Cell 3 originated to the west of Baltimore and never made a pass over the city. Credit: Alexander Ntelekos, Princeton University School of Engineering