August - October 2008

Latest Seasonal Assessment - Tropical Storm Edouard brought drought relief to the north coast of Texas on August 5. The storm’s remnants benefited parts of central Texas the following day, and were expected to bring additional relief to parts of central and west Texas at the start of the forecast period on August 7. Improvement is also on tap for northern and eastern Texas over the 3-month forecast period, with more limited relief for central Texas, and drought persistence for south-central Texas. Over the West, the official forecasts for all time ranges show little rainfall across California except in the north, and it is unlikely that the state will experience significant improvement during the ongoing dry season. In fact, it is likely that the area of drought over the West will expand in Oregon and Washington. Drought development is also likely in northern Montana and is expected to persist over eastern Montana and western North Dakota, with some relief in central North Dakota. The Colorado plains and adjacent areas of Oklahoma, Kansas, and Nebraska should see increased moisture. Wet weather in the short term, near to above normal precipitation predicted during the extended range, and a forecast of above-normal precipitation for August-October support the optimistic forecast. In the Southeast, an area of improvement is indicated across coastal areas and into the Piedmont region of the Carolinas, associated with the forecasts of above-median rainfall in the short and medium ranges, and the increased possibility of tropical systems into autumn. The Hawaiian Islands are likely to experience expanding drought in some leeward areas, but improvement is expected over windward locations on the Big Island and Kauai.

Tools used in the Drought Outlook included the official CPC precipitation outlook for August 2008 and the long lead forecast for August-October 2008, 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 (CAS) moisture, the CFS seasonal precipitation forecasts, and climatology.

The improvement for drought across the interior Southeast is little changed from the previous forecast, with the area of some improvement expanded into southwestern Virginia, where in-house Palmer Drought Index probabilities based on historical data show relatively less improvement by the end of October. Also, medium range forecasts continue to show less chance for above-normal rains toward the northern fringes of the drought region. Again, it should be noted that hydrological drought indicators such as reservoirs and groundwater are unlikely to see significant improvement during the summer in the absence of a tropical system, so the improvement denoted in the forecast applies mostly to soil moisture, pasture conditions, and small streams. In Georgia, for example, Lake Lanier levels have been declining and levels on average do not increase until late November, so significant increases by October will be difficult. In contrast, Lake Okeechobee levels in Florida continue to rise, although they remain about 3 feet below average. Levels should continue to improve.
Confidence for the Southeast: Moderate

In Texas, Tropical Storm Edouard dumped 2 to 6 inches of rain along the northern coast, relieving drought there. However, rainfall amounts were lower in other areas, and the storm appeared to be unlikely to make a big difference in the extreme drought affecting the San Antonio-Austin area. Rainfall forecasts for the first 2 weeks of the period south of the remnant’s path look quite modest, while the Palmer drought probabilities for October show a 65 to 75 percent chance for persistence or intensification in Climate Division 7 in south-central Texas. For these reasons, an area of persisting drought was introduced in southern Texas. Northern and western areas of Texas, as well as Louisiana, should benefit from plentiful rains along the periphery of the dry area. Farther west, drought relief has occurred as forecast in New Mexico and southwest Texas.
Confidence for the Texas and Louisiana area: Moderate

In the Colorado and western Oklahoma area, where drought had been intensifying into early August due to high temperatures and low rainfall, the pattern changed to a wetter regime by August 5, and short and medium-range forecasts indicated continued wetness during the first 10 days of the forecast period. Long-range forecasts for 30 and 90 days also indicate above-normal rainfall for parts of this region. Flash flood watches were posted on August 6 for large portions of Colorado’s drought area.
Confidence for the central Plains: High

There was little change to the forecast for the North Dakota-eastern Montana area, with persisting drought in the western part of the drought area and some improvement toward the east. Some rain is forecast over the first 5 days of the period in this area, but seasonal Palmer and soil moisture forecasts based on climatology suggest the odds for improvement are relatively slim in eastern Montana and adjacent North Dakota.
Confidence: Low

A small area of drought developed in early August in south-central Minnesota. Forecast tools provide a mixed picture for relief prospects. With the latest runs of the Coupled Forecast System leaning wet in this area for the 3 months and the Palmer probability statistics showing low odds for drought development, this Outlook went with improvement. But the medium range forecasts are somewhat dry, so this area could see problems persisting well into August if not longer.
Confidence for Minnesota: Low

For the West, the ongoing dry season results in expected persisting drought for the California area, while the CPC 30 day outlook for August and the 90 day Outlook for August-October indicate a tendency for below-median rainfall from northeastern California into at least southern Idaho. For these reasons, an area of drought development continues to be denoted for parts of northeastern California, eastern Oregon, and southeastern Washington State. However, an upper level trough was expected to deliver some rain to the Northwest in the medium range, while showers spread across the region on August 6. Also, October often sees winter storm systems penetrating the region, so conditions can improve at that time. The odds for persisting drought are greater over the Great Basin and southern California based on climate factors.
Confidence for the West: Moderate

A new area of development has been introduced in northern Montana, where July rainfall was low and short term forecasts show little rain accompanied by above-normal temperatures. Palmer index probabilities show a substantial risk for drought development in this area by late October.
Confidence: High

In Hawaii, recent rains have resulted in the forecast showing improvement for the windward sides of Kauai and the Big Island. Prospects for increased shower activity also resulted in dropping development from northeastern Oahu, eastern Molokai, and Lanai. Drought development continues to be forecast over scattered leeward areas.
Confidence: Moderate

In Puerto Rico, climatology and prospects for abundant tropical rains in coming weeks continue to suggest drought improvement.
Confidence: High