September - November 2007

Latest Seasonal Assessment - Hot, dry weather during the first half of August worsened drought in the Southeast, Tennessee Valley, and middle Mississippi Valley. While more heat was on tap for the third week of August, the tropics were becoming more active, and by August 15 Tropical Storm Erin was aiming for Texas and Tropical Storm Dean was headed for the Caribbean. Although it was unknown if Dean would eventually affect the U.S. mainland at the time this drought forecast was produced, an active tropical storm season during late summer and fall increases the odds for drought improvement, especially near coastal areas along the Gulf and the Atlantic. As a result, drought improvement is forecast by November along the Gulf and mid and south Atlantic coasts, with decreasing odds for improvement toward the interior. Despite some improvement likely later in the season, drought should largely persist from Kentucky into western Tennessee, and could even expand westward into Missouri and Arkansas. Varying degrees of improvement are anticipated in the Great Lakes region, with the best odds for relief extending from southern Minnesota eastward into southern Michigan and parts of Indiana and Ohio. In the West, drought should persist for most of the region, although the summer thunderstorm season will bring scattered short-term relief to the Southwest, and above-normal rains forecast during September-November should offer improvement to the interior Northwest. In Hawaii, despite some rains from Tropical Storm Flossie on August 14-15, drought is forecast to persist into November over the leeward areas of the islands.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for September-November, 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, and the soil moisture tools based on the GFS model and the (CAS) Constructed Analogue on Soil moisture.

Over the South, the forecast improvement areas are little changed since the August 2 forecast, with the best odds for improvement continuing over the Gulf and Atlantic coastal regions based on climatology, medium-range rainfall forecasts, and the CPC September-November precipitation outlook. The confidence for improvement is not quite as high as in the earlier forecasts, given the recent intensification of drought caused by the August heat wave, which was forecast to continue into the first days of the forecast period. The drought outlook continues to show relatively less conviction for improvement from northern Alabama into Kentucky, a region that has been depicted by the "some improvement" category and also includes an area of persistence in large parts of Kentucky and Tennessee, as well as over the new drought area in southern Illinois and southeastern Missouri. This region is forecast to see declining soil moisture levels during the first 2 weeks of the forecast period, and long-range seasonal forecasts are more equivocal on the odds for abundant rains away from the coast. Of course, the tracks of future tropical weather systems are unknown, and the drought situation could improve quickly if soaking tropical rains move northward from the Gulf. With near-record rainfall deficits this year in the core D4 drought area in Tennessee and Alabama, even a direct hit by a hurricane may not be enough to end the drought. The Palmer drought index probability maps produced by the National Climatic Data Center show the odds for the index to increase to marginal drought levels at -2.0 by November range from 0 to 7 percent over northeastern Alabama and parts of Georgia and Florida. The odds for drought ending rains by that time (index of -0.5) are given as 0 to 2 percent for much of the Southeast.

To the north, this forecast does show more improvement than in the previous forecast, based on a continuing wet pattern from the Upper Midwest across the northern parts of the Ohio Valley for the first 10 days of the period. The Palmer probabilities for improvement to November for northern Michigan, Wisconsin, and Minnesota are not as high as in the areas farther south, so this area has been changed to show more limited improvement. The forecast for less improvement from northern Minnesota into northern Michigan is also consistent with the CAS forecast of soil moisture for November, which shows dry soils tending to linger in this region. CAS also shows marked improvement farther south from northern Indiana into southern Michigan, as well as over the mid-Atlantic region.

The main change in the West from the August 2 Outlook was the introduction of the improvement area from eastern Oregon into northwestern Montana and south into northern California. The main reason for the change is that this forecast period goes into November, when cooler weather and increased rain and snow typically reduce drought in the Northwest. The fact that the official September-November precipitation forecast shows an area of above-normal from Washington into western Montana increases confidence that some degree of improvement will occur. Increased wetness would also be consistent with La Niña composites for this time period, the odds favoring La Niña developing shortly and continuing into winter.

In Hawaii, rainfall amounts from Tropical Storm Flossie in mid August generally totaled less than 2 inches, and amounts tended to be under 1 inch in the drought areas, so the storm brought little relief. Long-range models continue to shift the odds slightly toward below-normal rainfall into November for the islands, so continued drought is forecast.