The extreme heat and drought across the Central U.S. this summer is courtesy of a La Nina atmospheric pattern that has been in place for almost two years. Characterized by relatively cold waters across the tropical eastern Pacific and high pressure over that region, storm systems are shunted across the northern latitudes of North America, reducing snowpack in the western mountains (except for the Pacific Northwest) and bringing drought conditions to the central and southern Plains.
Having developed late in 2010, this La Nina strengthened by late 2011, producing spring-like conditions last winter and summer warmth that began by mid spring across the American Heartland. Since the eastern Pacific high has diverted the jet stream across Canada, few Pacific storm systems have penetrated the central U.S., allowing a high pressure ridge to settle over our region; beneath this ridge, sinking air has produced hot, dry conditions and a devastating drought while torrential rains and floods have occurred around its rim (across the Upper Great Lakes, mid Atlantic and northern Gulf Coast). This prolonged La Nina is also responsible for the intense tropical storms in Japan and China this summer, as warm Pacific waters are displaced to the west.
La Nina patterns generally develop every five years or so and back-to-back La Ninas occur 50% of the time. The severity of their impact is likely to increase with global warming and the current two-year drought across the Southern Plains may be a harbinger of even worse events in the future. On the positive side, the waters of the eastern tropical Pacific are beginning to warm and this prolonged La Nina is expected to break down by December. That forecast, if accurate, suggests little relief from our long, hot, dry summer in the coming months.
