for April 30, 1967, at 1:00 a.m.
EST (06:00
UTC), the morning prior to the tornado outbreak. Beginning on April 30, 1967, a potent
mid-latitude cyclone generated severe weather, including
blizzards and
severe thunderstorms, across a broad region extending from the northernmost
High Plains and
Rocky Mountains to the
Mississippi and
Ohio River valleys. These conditions occurred within a seventy-two-hour-long span. By 1:00 p.m. CDT (12:00 p.m. CST; 18:00 UTC) on April 30, a
low-pressure area of at most was centered near
Pierre, South Dakota, with a
stationary front superimposed from north of
Sioux Falls to near
LaCrosse, Wisconsin. Nearby, a
warm front also attended from south of Sioux Falls to near
Des Moines, Iowa, and
St. Louis, Missouri. As the
warm sector advanced into northernmost Iowa and southern Minnesota,
surface air temperatures rose into the 60s and low 70s °
F, while
dew points reached the 60s °F. Winds at the surface ranged from out of the east-southeast. By 7:00 p.m. CDT (6:00 p.m. CST; 00:00 UTC), the warm front migrated to near the Minnesota-
Iowa border, while the mid-level
trough associated with the surface cyclone acquired a
negative tilt. At the same time, a strong
jet stream brought deep-layer
wind shear over portions of northern Iowa and southern Minnesota—indicating favorable conditions for
tornadogenesis. ==Daily statistics==