August - October 2007

Latest Seasonal Assessment - Short and long range forecasts favor improvement across much of the South and the East. Showers and thunderstorms are expected in the short range while the seasonal forecast favors above normal rainfall during the August-October period. Drought conditions are expected to gradually improve across the Tennessee and southern Ohio valleys. However, substantial moisture deficits will likely persist. Year-to-date rainfall deficits range from 10 to 20+ inches in this region. Although these deficits are expected to slowly decrease, the chances of these deficits and their impacts being eliminated are small. The forecast period covers the height of the hurricane season. The remnants of a tropical system can reduce drought conditions very quickly. However, the occurrence of such events can not be accurately forecast more than a few days in advance. Over the Upper Midwest, moderate to locally severe drought has developed. In the short range, little if any improvement is expected. Drought conditions will likely worsen during the rest of July into early August. This may have a negative impact on agriculture. As September and October progress, opportunities for significant rainfall and drought improvement will increase. Over the western states, record low precipitation totals for the 2006-07 water year in some areas has resulted in the development of severe drought across the Southwest. In the northern Rockies, recent heat and dry weather have resulted in drought development. Drought conditions will persist through the period, with possible expansion into northwestern California, Washington, Oregon and central Montana. In Hawaii, drought is expected to persist across the leeward side of the island chain through October. On the Big Island, Tropical Storm Cosme may generate some significant rainfall during the first few days of the period. The best rains will likely be on the eastern side of the island.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for August-October, the four-month drought termination and amelioration probabilities, various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil moisture, and the CFS monthly precipitation forecasts.

Over the South, a lack of significant rainfall and above normal temperatures has resulted in the development of severe drought. The worst conditions are being observed in northern Alabama and adjacent parts of Georgia and Tennessee. So far in 2007, precipitation totals are only 40 to 70 percent of normal across the Tennessee Valley and parts of the Southeast. This has resulted in moisture deficits of 10 to 20+ inches. Dry weather during the spring and early summer has resulted in drought development across the Ohio Valley as well. Rainfall totals over the past 90 days are 6 to 12 inches below normal. Recent hot and dry weather has resulted in drought expansion into parts of the Mid Atlantic. In the short term, showers and thunderstorms are expected to result in some improvement across the Southeast and Mid Atlantic. Showers and thunderstorms will also result in some improvement across the Ohio and Tennessee Valleys. The long range outlooks support a gradual erosion of the rainfall deficits across the East and South. It is worth noting the forecast period encompasses the hurricane season. The remnants of a tropical system have the potential to drop a large amount of rainfall over the course of a few days to a week. Rainfall of this intensity and duration can greatly reduce the intensity of a drought, or eliminate it all together. Therefore, the potential exists for rapid improvement late in the period across the South and along the Eastern Seaboard. However, the persistent heavy rains from a tropical storm are by no means a guarantee. Drought conditions are forecast to improve across the Ohio and Tennessee Valleys as well. However, effects of the drought are expected to persist through the period due to the magnitude of the rainfall deficits accrued so far this year.

Long term hydrologic drought was gradually easing across the upper Midwest during the spring. However, recent dry weather has resulted in drought redevelopment across eastern Minnesota, northern Wisconsin and the U.P. of Michigan. Recent dry weather and heat has also resulted in drought development over southwestern Minnesota and western Iowa. Showers and thunderstorms are expected to result in some improvement in the short term across northeastern Minnesota, northern Wisconsin and the U.P. of Michigan. In the long range, the odds greatly favor improvement by the end of the period. However, a period of hot, dry weather over the next few weeks will result in a worsening of the drought across southwestern Minnesota and western Iowa. The heat and dry weather may also result in drought development in adjacent areas. This could have a negative impact on local agriculture. While there is the potential for the drought to persist into October and beyond, the odds favor drought improvement in these areas as summer transitions to fall. It is worth noting that the waters in the eastern equatorial Pacific remain cooler than normal meaning that La Niña development is still possible. If La Niña develops during the summer, it would result in an increased chance for the development of dry conditions over the Corn Belt during the middle of the period.

In the West, a dearth of precipitation during the winter and spring has resulted in the development of a severe drought across much of the region. In southwestern California, many locations reported the driest year on record. Los Angeles set a record for the driest water year (July 1 through June 30) recorded in the 130 years since record keeping began. Only 3.21 inches of rain fell during the 2006-07 water year, less than a quarter of average. 2006-07 was the fourth driest water year on record in San Diego. Many stations in smaller cities and towns across southwestern California set records for the driest year on record. The lack of precipitation during the winter was accompanied by unseasonably warm temperatures throughout the West. This combination of dry conditions and warm temperatures resulted in well below normal mountain snowpack across the most of the region. Very warm temperatures in the spring caused an unusually early disappearance of the snowpack More recently, extreme heat and a continued lack of moisture has resulted in conditions favorable for the development of wildfires. Already this year, large wildfires have consumed over 3,300,000 acres nationwide compared with a 10 year average of 2,700,000 acres. In the short term, late day thunderstorms are expected across the Rockies and the Great Basin. While these thunderstorms may boost moisture in some locations, drought conditions will persist across the northern Rockies and Great Basin. Furthermore, many of the thunderstorms will likely have small rain cores, which will result in a continued high risk for dry thunderstorms and wildfires. In the Southwest, the monsoon may provide some improvement. However, long term drought conditions are expected to persist. In the Northwest, the outlook for August through October favors drier than normal conditions. This may result in drought development across northwestern California, Oregon and eastern Washington where conditions are already being classified as unusually dry. There are also indications that drought conditions could spread eastward into central sections of Montana and Wyoming. In Hawaii, drought is expected to persist over leeward side of the island chain. On the Big Island, however, tropical storm Cosme may produce significant beneficial rainfall on or about July 21. Most of this rain, however, will likely fall on the east side of the island where the drought is less intense.