July - September 2007

Latest Seasonal Assessment - Forecasts favor improvement across much of the Deep South into September, with the best prospects for improvement along the Gulf Coast and the South Atlantic Coast. The odds for improvement diminish to the north, with the drought from northern Alabama and Georgia into the Ohio Valley expected to largely persist, although there will be local improvement here as well. Given the increased evaporation and water use expected during the summer, levels in many lakes, reservoirs, and wells will likely continue to drop into September. Soil moisture, small streams, and ponds have a better chance for improvement, but it is extremely unlikely the regional drought affecting the South will end within the next few months. Year-to-date rainfall deficits range from 15 to 20 inches in the area of exceptional drought centered in northern Alabama. As is the case with summer droughts in this part of the country, tropical waves, depressions, storms, or hurricanes could change the picture rapidly, but the paths for such weather systems cannot be foreseen more than a few days into the future. To the north, despite some recent rains, the odds favor drought expansion from Illinois into southwest Pennsylvania, especially during the final week of June. Improvement should continue in the Upper Midwest from Minnesota into Wisconsin and Michigan, but the West should see persisting or worsening drought, with a good chance for expansion northward. The summer thunderstorm season should offer temporary relief to Arizona. Drought is forecast to persist in Hawaii, and may develop in the eastern interior basin of Alaska.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for July-September, 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.

The recent trends favor relief over the Gulf Coast States and persistence to the north, from northern Alabama and Georgia into Kentucky and southern parts of Indiana and Ohio. These trends are especially notable in recent 2-week soil moisture forecasts from the bias-adjusted GFS ensemble model runs. This model shows a large area of improving moisture south of Tennessee during the latter part of June, and a corresponding area of deterioration across the Ohio Valley. As usual, the prospects for change are less certain beyond the first couple of weeks, but the CAS soil moisture tool for the end of September shows considerable improvement across the Southeast. Analogues based on the Palmer Drought Indices for major spring droughts such as occurred in 2000, 1987, 1986, 1985, 1981, and 1942 do not look as optimistic, showing rather mild improvement by September in many areas, but little or no improvement in some other areas, such as northern Mississippi. Even worse prospects for improvement can be found by examining historic reservoir levels following past spring droughts. Archived data for Lake Lanier in northern Georgia, for example, show lowering water levels are likely through early autumn. As if often the case, it is quite a challenge depicting these cross-currents on a single outlook map, which tries to suggest the best areas for improvement and the least likely areas. Hopefully, users recognize that “improvement” depends on the metric used, and soil moisture has better odds for change than the water supply measures.

Short-term rainfall deficits, low streamflows, and below normal soil moisture have developed across Illinois, Indiana, and Ohio. Drought expansion from Illinois to southwest Pennsylvania is largely driven by forecasts of dry and hot weather in the 6-10 and 8-14 Day forecasts. In addition, the CFS precipitation forecast indicates dryness persisting through the end of July but wetter than normal conditions during August. However, dryness returns during September. An enhanced risk for drought to expand north from its current location exists, and forecast confidence is higher than the previous forecast made on June 7. To the west of the Mississippi River, the CPC monthly forecast indicates increased odds for above normal rainfall. Therefore, at this time, drought is not expected to develop across Missouri or Iowa.

Short and medium-range forecasts favor improvement to drought areas across northeast Minnesota, northern Wisconsin, and upper Michigan.

Seasonal forecasts indicate above normal temperatures and below normal rainfall for the Pacific Northwest where drought development is forecast. Elsewhere, across California and the Great Basin, climatology favors persistence or intensification. The some improvement forecast for Arizona reflects the summer monsoon during July and August.

Drought should persist along the leeward areas of the Hawaiian Islands since the dry season has arrived.

Across Alaska, the summer has started with abnormal dryness. Rivers continue to run very low and the wildfire risk remains high over the eastern interior of the state. Therefore, drought development is expected to the north of the Alaskan range.