* Drought exceptional, worsening in Plains wheat states

* Some improvement noted in Midwest corn, soybean area

* Meteorologist sees minor tweaking of drought

* U.S. remains gripped by worst drought in half century (Adds supporting statements from National Drought Mitigation Center official)

By Sam Nelson

CHICAGO, Aug 16 (Reuters) - The worst U.S. drought in a half century kept a tight hold on top farm states over the past week with a few improvements, but some areas experienced more serious degradation, a weekly report from climate experts showed on Thursday.

Rain provided some relief to parched farmland stretching from Iowa through Ohio, but other areas including the Southern and Central Plains remained scorched.

"This week we saw a decline in drought in the lower 48 states for the third week in a row but the overall decline was small," said Brian Fuchs, climatologist for the National Drought Mitigation Center at the University of Nebraska.

"Last week drought covered 62.46 percent of the area, and this week 61.77 percent was in drought."

Climatologists reported drought affecting 87 percent of the U.S. corn crop, 85 percent of soybeans, 63 percent of hay and 72 percent of cattle.

"It's a one-in-50-year drought event, based on the Drought Monitor," Fuchs said.

More than half of the corn and soybean areas were experiencing extreme to exceptional drought, which has led to reduced yields and earlier harvests for those crops, according to the weekly Drought Monitor report.

"The total extent of the drought hasn't changed that much, just some tweaking of the areas affected," said Kyle Tapley, meteorologist for MDA EarthSat Weather, a private weather forecaster that uses the Drought Monitor in compiling its data.

The biggest improvement occurred in the eastern Midwest, in particular northern Indiana, Tapley said.

He said the drought map showed expansion and worsening of the drought especially in the western Midwest and Central Plains states, creating a tenuous situation for hard red winter wheat farmers in Kansas, Nebraska, Oklahoma, Colorado and Texas.

"It doesn't look good for fall planting of wheat," Tapley said.

The report showed large swaths of exceptional drought, the most extreme category reported by the Drought Monitor, in Oklahoma. Texas experienced a minor deterioration of conditions.

"It was intensifying in areas such as Oklahoma, Kansas, western Nebraska and western Missouri. Those areas continued to see no relief," Fuchs said.

Exceptional and extreme drought expanded in Colorado. In Idaho, moderate drought to abnormal dryness expanded and wildfires were on the rise.

Widespread rains in the Midwest helped alleviate some dryness in top corn and soybean states Iowa and Illinois, while North and South Dakota received beneficial precipitation.

Cooler temperatures have helped slow moisture usage in the U.S. crop region, some more light showers were falling this week and there may be more showers by next week.

But no drought-busting rainfall is expected.

"A cool front is moving through the Midwest, bringing light rain to most of the area. There should be a half inch of rain with some areas getting up to an inch or more," predicted Andy Karst, a meteorologist with World Weather Inc.

Temperatures should remain in the 70s to 80s Fahrenheit on Thursday through the middle of next week, then rise to the low 90s.

"There's another rain event at midweek (next week) that will be important to crops. If that fizzles out, there will be more stress on crops," he said.

Rains in the Southeast improved drought conditions there, and most of the region reported abnormal dryness rather than extreme or exceptional drought.

The Northeast and Mid-Atlantic maintained the status quo, with minor reductions in abnormal dryness in Maine. (Editing by Dale Hudson)