When its storming in the Sooner State, there's something strange in
the air.  Dig up a fistful of Oklahoma rain.

Hello.

"Oklahoma rain" is slang for a sandstorm or dust storm.

sources:

"Oklahoma rain (sandstorm)"
From: Words from the Western Frontier, hosted by creighton.edu
http://puffin.creighton.edu/fapa/aikin/Web-files/WEST/words_from_the_western_frontier.htm

"Oklahoma rain - Sandstorm"
From: ePodunk.com: Americanisms
http://www.epodunk.com/americanisms.html

"Oklahoma rain = a dust storm"
From: Page 229, "The Cowboy Life: A Saddlebag Guide for Dudes,
Tenderfeet, and Cow Punchers Everywhere"
Read using the Search inside feature at Amazon.com:
http://www.amazon.com/exec/obidos/tg/detail/-/0671866826/

----------

Thus, a "fistful of Oklahoma rain" would be a handful of sand.

search strategy: 
"oklahoma rain", slang
"oklahoma rain", sandstorm
amazon.com: "oklahoma rain"

I hope this helps.