A weekly update on U.S. drought conditions shows the nation's worst drought in decades slightly worsened in the lower 48 states with the Midwest corn harvest in full swing.

The U.S. Drought Monitor's new map posted Thursday shows 65.5% of the contiguous U.S. was experiencing some form of drought as of Tuesday, up from 64.8% a week earlier.

It shows 21.5% of that area was in extreme or exceptional drought - the two worst classifications. That's up three-quarters of a percentage point.

The area of Iowa, the biggest corn producer, deemed to be in exceptional drought rose from 2.4% last week to 2.5%. That most severe classification rose 2.3%age points in Nebraska, to 73.25%.

Conditions in Kansas, Missouri, Illinois and Indiana remained unchanged.