Grain can be traded in the futures market before harvesting. If grain production is predicted to be insufficient, grain futures prices will rise; if grain harvests are predicted, grain futures prices will fall. This morning, meteorologists predicted that from tomorrow there will be much-needed rainfall in the grain-producing areas. 
Tomorrow's rainfall predicted by meteorologists is likely to extend beyond the grain-producing areas
 since sufficient moisture is very important to the survival of current grains, the price of grain futures today will fall sharply.