Latest Seasonal Assessment Climate anomalies attributed to the Summer 2010 - Spring 2011 La Nina event promoted widespread drought development and intensification across the southern tier of the U.S. In many locations, significant deterioration occured, with the southern Great Plains experiencing some of the worst impacts. Apart from the relief afforded by isolated thunderstorms, or perhaps a tropical system, drought conditions are anticipated to persist over the south-central states through the August-October period.
During the past few weeks, a persistent ridge of high pressure maintained hot, dry conditions across the core drought areas of the southern Great Plains. Heavy rain (2-3 inches) fell across portions of the central and northeastern Gulf Coast, bringing substantial relief, while daily wet-season thunderstorms throughout the Florida Peninsula continued to erode entrenched drought conditions. For most of the Southeast, substantial improvement is forecast for the August-October 2011 season. Additional drought improvement is anticipated across Arizona, western New Mexico, and southern Colorado, due to a fairly healthy monsoon circulation across the Southwest. However, eastern New Mexico and western Texas have yet to see comparably widespread, significant rainfall, and prospects for this area remain uncertain. Across Hawaii, leeward drought persistence is expected under the summer trade wind regime, while developing drought is possible in Alaska’s Kenai Peninsula region.
A persistent ridge of high pressure maintained dry conditions and much above normal temperatures across the central U.S., exacerbating the widespread drought entrenched across the southern Plains. Daily maximum temperatures in San Angelo, TX have broken 100 degrees Fahrenheit 61 times so far in 2011 (as of July 26th), and there have been 28 consecutive days of highs reaching 100 degrees or more so far (as of July 29th), according to the National Weather Service Office in San Angelo. Both of these events are highly unusual. Mean temperatures for the month of July have averaged as much as 7 degrees F above normal over much of the Red River Valley, including much of Oklahoma and portions of north-central Texas. Isolated thunderstorms across the region have provided little to no relief under the persistent heat and widespread dryness. Drier weather is typical across eastern Texas northeastward through the lower Mississippi Valley during August, with climatological rainfall increasing slightly towards the beginning of autumn. Climatological rainfall increases more substantially in early autumn across southern and southwestern Texas. Short, medium, and extended range forecasts, including the CPC 6-10 and 8-14 day outlooks all indicate enhanced chances for below-median rainfall across the southern Plains states. The CPC August outlook maintains elevated chances of below-median rainfall for most of the southern Great Plains and lower Mississippi Valley. The seasonal precipitation outlook for August-October 2011 calls for equal chances (EC) of below, near, and above-median precipitation for this region. The Constructed Analog on Soil moisture (CAS) outlook for the end of August and the end of October persist the dryness across the southern Great Plains. Based on these outlooks, continued drought persistence is likely. Forecast confidence for the southern Plains is moderate (near the coast) to high (well inland).