In most places, save for areas of the central Plains, and across the southwestern U.S., parts of the deep South and southeastern U.S., drought conditions began and rapidly intensified during the Summer of 2012.
After beneficial and timely rainfall during the early to mid-Spring season, a rapid shift to much drier conditions, exacerbated by extended spells of record or near-record setting daily as well as monthly temperatures, set the stage for rapid drought development.
Analyses comparing the current drought with those of the 1930s are ongoing. However, across much of the central U.S., the current drought onset is similar to the drought of summer, 1988.
La Niña (cooling of oceanic water temperatures in the central Pacific) was largely responsible for the multi-year drought and intensification of conditions on the Southern and Central Plains. In 2011, the worst one-year drought on record in Texas occurred during a La Niña event that began in the previous year and ended by summer. Spells of historic heat-wave conditions exacerbated drought severity, too.
Blame El Nino
The return to neutral ocean conditions in the equatorial Pacific, or "normal" water temperatures, and over the summer season the arrival of weak El Nino conditions (warming of those same waters) suggests little change to sensible weather throughout much of the Heartland over the next several weeks, if not longer.
Summer and early autumn arriving El Nino's typically generate a much drier, hotter-than-usual pattern. However, signs of a shift to above normal rainfall are seen across much of the southern tier of the U.S. due in part to a strengthening sub-tropical current of air aloft. And, while tropical activity usually lessens in El Nino years due to that same strengthening of winds aloft, the current set-up may not be strong enough to fully impact the tropics. So there may still be a fair number of features that may increase rain systems through the southern and perhaps the eastern U.S. Some may recall that it was the remnant features of a couple tropical systems that started the end demise to the Great Drought of 1988. Additionally, expect drier conditions in the Ohio Valley, much of the Midwest, as well as the northern and central Plains.