June -August 2007

Latest Seasonal Assessment - The Outlook calls for some improvement to the severe to extreme drought affecting the Gulf Coast and parts of the Southeast. With rainfall deficits exceeding 12 inches in many locations of the Southeast, drought elimination is highly dependent on tropical storm activity. The greatest likelihood of improvement across the Southeast exists over the Carolinas and southeast Georgia. Improvement is also expected for Florida’s peninsula, but Lake Okeechobee water levels will remain low. Although periods of beneficial showers are expected, the ongoing drought will likely persist across north Alabama, northwest Georgia, Tennessee, and Kentucky. Drought has recently spread north into southern Ohio. Drought is forecast to expand into southeast Missouri, Indiana, and Ohio. Although drought development is not forecast for the western and central Corn Belt, a trend towards dry, hot weather may occur by late summer. Elsewhere, improvement will continue across the upper Mississippi Valley. Across the interior West and California, drought will persist or intensify, but some improvement is expected for Arizona due to the onset of the monsoon. Seasonal forecasts of above normal temperatures and below normal rainfall result in drought development across central Idaho. The wet short-term, including mountain snow, favors some improvement in southwest Montana and northwest Wyoming. The leeward areas of the Hawaiian Islands will remain in drought. Seasonal forecasts call for above normal temperatures across much of Alaska. Drought development is now forecast for parts of Alaska where an above normal wildfire season is expected.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for June-August, 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 Constructed Analogue on Soil moisture.

The forecast challenge remains the drought in the Southeast and potential expansion into the eastern Corn Belt. The June-August rainfall outlook indicates equal chances for wet or dry, but the June-August temperature outlook indicates odds favoring above normal. Above normal temperatures would increase evaporation rates and reduce the effects of favorable summer rainfall. The constructed analogue soil moisture forecast (CAS) indicates dry conditions persisting for the Southeast through the end of July. Across the Southeast, NCEP’s coupled forecast system model (CFS) indicates below normal rainfall during June and July but shows more wetness during August. Tallahassee recorded its driest March-May. Due to the large rainfall deficits over the Southeast, major improvement will likely not occur until a tropical storm affects the region. Based on historical data, the hydrological impacts of the drought will likely persist throughout the summer, but soil moisture anomalies should slowly diminish. The forecast of some improvement for the Gulf Coast and southern Appalachians is consistent with short-term soil moisture improvement associated with climatology but less long-term hydrological improvement. The forecast of drought persisting across Alabama, northwest Georgia, Tennessee, and Kentucky is based on the CAS forecast of dryness through the end of July and the monthly precipitation forecasts from the CFS. The greatest likelihood of improvement is over the Carolinas, southeast Georgia, and northeast Florida where medium-range forecasts indicate opportunity for relief. Meanwhile, over south Florida, short, medium, and long-range forecasts favor above-normal rainfall. Therefore, confidence is relatively high for improvement. However, the water levels of Lake Okeechobee that are currently near a record low will only slowly respond.

Short-term rainfall deficits, low streamflows, and below normal soil moisture have recently developed across southeast Missouri and southern portions of Illinois, Indiana, and Ohio. Short-range forecasts, through Day 5, show little or no rainfall for these areas. Medium-range forecasts indicate near to above normal rainfall but above normal temperatures. The CAS indicates dryness continuing through the end of July, while the CFS shows below normal rainfall during June and July. The CFS does show more wetness for August in Ohio, but persists the dryness across southern Illinois. For the remainder of the Corn Belt, a rapid cooling of the eastern tropical Pacific waters (La Nina) would increase the odds for heat. La Nina composites do show increased probability for heat in the Ohio Valley. However, La Nina composites that adjust for climate trends show no strong signal for dryness in the Midwest during summer. Furthermore, the historical analogues of El Nino to La Nina transition years along with abundant spring soil moisture on the Great Plain do not show a consistent tendency towards dryness. Bottom line is an enhanced risk for drought to spread into southeast Missouri and southern portions of Illinois and Indiana, and central Ohio, but forecast confidence is moderate.

Short and medium-range forecasts favor improvement or some improvement to drought areas of the upper Mississippi Valley, central high Plains, and parts of the northern Rockies.

Seasonal forecasts indicate above normal temperatures and below normal rainfall for the Pacific Northwest, where drought development is forecast. However, a wet short-term forecast will limit the extent of the expansion. 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. The seasonal forecast calls for above normal temperatures with equal chances of wet or dry. The drought development is based on the ongoing dryness and the CFS forecast for below normal precipitation during July and August.