August - October 2007

Latest Seasonal Assessment - Hot, dry weather during the first half of August will contribute to persisting drought from southern Michigan into western parts of Kentucky and Tennessee, with a good chance drought may expand into southeastern Missouri and southern Illinois. From the northern Gulf States into eastern Kentucky and southern Ohio, a return to drier weather in early August should be followed by more seasonal rains, resulting in some overall improvement. Hot, dry weather in early August will cause drought to worsen from Maryland and Virginia into North Carolina, but the odds favor increased rainfall later in the season, resulting in overall improvement by October. The outlook remains favorable for improvement over most of the Southeast, especially along the Gulf Coast during the first days of August. Across this region, improvement will tend to be gradual unless tropical storms make landfall. Recent thunderstorms have boosted soil moisture for crops, but many reservoirs remain low. Forecasters expect an active Atlantic storm season, but the impacts of any future storms on the U.S. are unknown at this time. In contrast, the Upper Midwest will experience beneficial rains in early August from Wisconsin into Minnesota, but overall improvement may be limited due to high temperatures from August to October and unreliable rainfall later in the season. In the West, the summer thunderstorm season will continue to provide spotty relief for many areas, especially from Arizona northward into Utah and Colorado, with the best odds for relief in Arizona. Drought will persist over leeward areas of the Hawaiian Islands.

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, the best odds for drought improvement continue over the Gulf region based on climatology, medium-range soil moisture forecasts, and the CPC August-October precipitation outlook. The confidence for improvement remains relatively high for this region, especially along the coast, which is most susceptible to tropical weather systems, and where heavy rainfall is forecast during the first 5 days of the period. The drought outlook continues to show less conviction for improvement from northern Alabama into Kentucky, a region that has been depicted by the "some improvement" category and now includes an area of persistence from northwestern Tennessee northward. This region is forecast to see declining soil moisture levels during the first half of August, and long-range seasonal forecasts are more equivocal on the odds for abundant rains away from the coast. Hot, dry weather during the first 2 weeks in August should lead to worsening drought in much of the Ohio Valley and mid-Atlantic region, but the odds for improvement increase later in the season, as shown by the drought amelioration probability maps and the analogue soil moisture forecasts (CAS). The latter maps show significant improvement in soil moisture between August and October in the mid-Atlantic region. Most of the areas designated as "some improvement" in the South and mid-Atlantic should see persisting or worsening drought early in the forecast period followed by improving conditions by the end of October. The reason for the more pessimistic assessment for the Midwest, Tennessee Valley, and mid-Atlantic since the July 19 release of the Drought Outlook is the hot, dry outlook for the first half of August, leading to dropping soil moisture levels and significant impacts to vulnerable crops.

The drought outlook remains mixed for the Upper Midwest. Rains expected early in August will bring some improvement, but there are a number of long-range forecast tools that suggest improvement may be tough to accomplish, mainly from western Wisconsin into Minnesota. These include the October CAS and the Climate Forecast System (CFS) soil moisture forecasts. In addition, La Niña rainfall probability charts show a slight tilt toward dryness for August-October from Iowa into the Great Lakes region, as well as above-normal temperatures. It appears that near-La Niña conditions are developing, so seasonal La Nina composites were looked at, although they were not major factors behind the drought outlook.

The main change in the West from the July 19 Outlook was the removal of the drought expansion areas, as some of the expansion has already taken place, and medium-range 6-10 day forecasts of cooler and wetter weather make it less likely additional expansion will take place. The first 2 weeks of August look quite wet in the Southwest, indicating a robust monsoon pattern. This led to slight expansion of the area of some improvement, and the forecast shows an additional area of straight improvement in Arizona based on the location of expected above-normal rains during the first half of August.

There was no change to the drought outlook for Hawaii, where drought is expected to persist in the leeward areas.