At Butler Center for Arkansas Studies:

http://arstudies.contentdm.oclc.org/cdm/singleitem/collection/civilwar/id/145/rec/2

Below are excerpts from this diary by a soldier of the 9th Wisconsin Infantry.

June 28-29, 1862 [from Baxter Springs KS] March to Hudson's Crossing [On the Neosho 1 1/2 miles above the mouth of Spring River where it becomes Grand River]. March to Cowskin Prairie (Camp Round Grove) [Present Grove OK]. Weather very hot and dry...

July 8 Men, horses, and mules suffered greatly road being dry and dusty and no water fit to drink. Thermometer 100 degrees (Fahrenheit). [They were on the west side of the Grand River somewhere north of Flat Rock Creek -- between present Wagoner and Chouteau].

July 9 Were compelled to move camp to a place where water could be had. Went some miles further to Flat Stone Creek [sic Flat Rock Creek]. No rain. Camp very dusty. Weather hot. Thermometer on one day 118 degrees in the shade (in Dr. Loehr's tent).

Just sounds like typical Oklahoma weather to me. ;-)