North-central US 24-hour precipitation from 00Z May 30 (7 pm CDT May 29) to 00Z May 31 (7 pm CDT May 30). Note that this would not include the Spencer supercell. Two prominent streaks of precipitation are mapped near the NE/SD border. The western one, across the Pine Ridge area near Chadron NE, is associated with passage of several thunderstorms during the evening of the 29th. [Those storms evolved from low-precipitation (LP) supercells which formed near the dryline, near the WY/NE border, on May 29.] Wet ground in this area appears to have temporarily hampered the vertical mixing and eastward progress of a small part of the dryline on May 30. This is evident in the much higher surface dew points at CDR and IEN sites at 17Z (non CDT), after the westerly wind shift elsewhere associated with the dryline.

The eastern streak was created by the morning/early afternoon supercell across northeast NE and IA. This storm produced an outflow boundary which may have influenced mesoscale conditions leading to the Spencer tornado. Rainfall data and map scale courtesy NASA.