Latest Seasonal Assessment -
Forecasts favor improvement along the Gulf Coast, southern Georgia and the Carolinas. The odds for improvement diminish
further north, with the drought from northern Alabama and northwestern Georgia into the Tennessee and Ohio Valleys
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. Gradual improvement should
continue across eastern Minnesota, northern Wisconsin and the U.P. of Michigan. In the West, the lack of rain and snow
in conjunction with unseasonably warm temperatures during the 2006-07 water year has resulted in the development of
severe drought across the central Rockies, Great Basin and California. The drought will persist through the period.
Drought development is possible over the northern Rockies as well during the period. The summer thunderstorm season
should offer some token relief to Arizona. Drought is forecast to persist in Hawaii.
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.
In the West, a lack of precipitation during the winter and spring has resulted in the development of a severe drought across much of the region. In southwestern California, many locations reported their driest year on record. Los Angeles set a record for the driest water year (July 1 through June 30) recorded in the 130 years since record keeping began. Only 3.21 inches of rain fell during the 2006-07 water year, less than a quarter of average. 2006-07 was the fourth driest water year on record in San Diego. The dearth of precipitation was accompanied by unseasonably warm temperatures throughout the West. This combination of dry and warm resulted in well below normal mountain snowpack across the most of the region. Very warm temperatures in the spring caused an unusually early disappearance of the snowpack Over the next few weeks, a large ridge is expected to keep hot, dry conditions in place over the West. This will continue to keep the risk for wildfires high, especially in the Southwest and Great Basin. Hot, dry conditions may result in drought development over eastern Washington, northern and central Idaho and western Montana. River levels and soil moisture have already decreased to concerning levels over portions of Montana. In Arizona, the Southwest Monsoon may provide some token improvement. In Hawaii, drought is expected to persist over leeward side of the island chain. In Alaska, a drying trend during May and early June raised concerns about drought development across the interior. However, the dry pattern has abated over the state. Increased moisture and no strong indications of dryness in the long range have eased concerns over developing drought in Alaska.
In the central states, relentless heavy rains have resulted in floods over the central and southern Plains. The CPC monthly outlook for July 2007 indicates that the odds favor continued wet, cool conditions across this region. The July outlook also favors above normal rainfall across Arkansas and parts of northwestern Mississippi. As a result, drought conditions are expected to improve in these areas. Gradual improvement is expected across the upper Midwest as an active pattern over the next few weeks will result in increased rainfall and a gradual erosion of long term moisture deficits. Over the Corn Belt, a transition to a wetter pattern and the failure of La Nina to materialize has eased concerns about drought development. However, the waters in the eastern equatorial Pacific remain cooler than normal meaning that La Nina development is still possible. If La Nina develops later on this summer, it would result in an increased chance for the development of dry conditions over the Corn Belt late in the period.
In the East, there are indications that the drought will continue to improve along the central Gulf Coast, as well as Florida, southern Georgia and the Carolinas. An increase in precipitation is expected across the Tennessee and Ohio Valleys as well. Since the deficits accrued over the past several months are quite large, up to 20 inches in some locations, it is unlikely that the drought will dissipate by September. Therefore, drought conditions are expected to persist through the period. It is worth mentioning that the last month of the period coincides with the most active part of the hurricane season. The remnants of a hurricane have the potential to drop a large amount of rainfall over the course of a few days to a week. Rainfall of this intensity and duration can greatly reduce the intensity of a drought, or eliminate it all together. Therefore, the potential exists for rapid improvement late in the period across the South. However, the persistent heavy rains from a tropical storm are by no means a guarantee.