What happened to the heavy rain?

For two days prior to June 11, 2 or more inches of rain was forecast to fall over portions of southeast South Dakota, southwest Minnesota, northwest Iowa, and northeast Nebraska.  The heavy rain never materialized in most of this area.  Instead heavy rainfall fell over portions of eastern Nebraska and into southwest Iowa.  So why didn't heavy rain fall over where we forecast it to occur the previous couple of days?  We believe that it has to do with the evolution of thunderstorms during the late afternoon and evening on June 10 across Nebraska.  

Late in the afternoon of June 10, thunderstorms began to develop over western Nebraska.  By the evening, the storms had expanded and extended across central Nebraska from the South Dakota border to the Kansas border.  Warm and moist air continued to flow northward ahead of this line of storms into eastern Nebraska and western Iowa.  This was going to be the moisture that would allow the thunderstorms and heavy rain to develop over southeast South Dakota and southwest Minnesota on June 11.

Over the next 3 hours, the storms continued to grow in area as the moved to the east.  By 1030 pm on June 10, a line of thunderstorms extended  from Sioux City, Iowa into north central Kansas.  As a result of the large complex of thunderstorms, we believe that the moisture which had been flowing northward toward eastern South Dakota and southwestern Minnesota was diverted to the northeast into Iowa.

Additional storms were beginning to develop over northeastern Colorado and the Nebraska panhandle as well.  It was these storms that were expected to move northeast and affect southeast South Dakota, northwest Iowa and southwest Miinnesota during the morning and early afternoon of June 11.

Three hours later the storms that had been over eastern Nebraska had moved little and continued to extend from northwest Iowa into north central Kansas.  The heaviest rainfall at this time was from the Omaha area into Kansas.  Because the large complex of thunderstorms remained, much of the moisture continued to be diverted to the northeast into central Iowa. 

Meanwhile the thunderstorms that had developed in northeast Colorado around 10 pm were not able to expand in area and produce widespread heavy rainfall. With much of the moisture being diverted into Iowa, there was probably not enough moisture and instability to allow these thunderstorms expand in area as had happened with the storms over eastern Nebraska earlier in the night.

At 424 am, the large complex of thunderstorms had finally begun to weaken over southeast Nebraska and western Iowa.  Nonetheless, the presence of this complex of storms had already diverted much of the moisture into central Iowa.  As the complex began to weaken some moisture would be able to flow back into eastern Nebraska and southeast South Dakota but it was far less than was expected.

The second area of thunderstorms over western Nebraska continued to move east and was approaching the South Dakota border.  Because moisture and instability was limited, rainfall with these storms was not heavy with maximum rainfall rates less than half an inch per hour.   Therefore, instead of getting 2 to 4 inches of rain with these storms, rainfall is expected to be less than an inch over much of the area today.