If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Gretna: The hottest months are usually August and July with an average temp of 83°F, while the coldest months are January and February with an average of 59°F. The rainiest months in Gretna are August, July, June, and September, with each month seeing an average of 7 inches of rainfall.