NOAA National Weather Service Weather Forecast Office

250mb winds

A very strong upper jet came in from the Pacific, across the southern Plains, and into the outbreak region.

500mb winds

Strong winds aloft can again be seen here blowing directly into the region where the outbreak occurred.

500mb heights

A deep trough asserted itself over the Rockies and Plains, which translated eastward and helped spark storms in the outbreak. East of the trough southwest flow brought warm air northward from the southern Plains and Gulf of Mexico.

700mb relative humidity

Dry mid-level air punched into the outbreak area, assisting in the development of severe thunderstorms.

850mb winds

A low-level jet speared northeast into the mid-Mississippi Valley.

Surface temperatures

Plenty of warm air was available.  The high temperature in South Bend that day was 66 degrees (normal is 54 ) and Fort Wayne reached 69 degrees (normal is 55 ).

Surface relative humidity

In addition to very humid air available over the eastern United States, a pronounced "dry line" (a sharp boundary between dry and moist air) moved from the border of Texas and Louisiana that morning eastward to a line from Indiana to Louisiana by late evening.  While dry lines are common in the High Plains of Texas, it is very unusual to see them make it as far east as the one on April 3, 1974 did.

Sea level pressure

Deep low pressure moved from Kansas on the morning of the third to Michigan by the morning of the 4th.  High pressure off the East Coast helped pump warm, moist, unstable air north from the Gulf of Mexico into the eastern United States.