June -August 2007

Latest Seasonal Assessment - The Outlook calls for some improvement for the severe drought covering much of the Southeast, although drought should persist well into summer. The areas with the best odds for improvement include the Gulf Coast, the southern and central Florida Peninsula, and the Carolinas. Despite scattered showers forecast for the region, no significant improvement is expected during the last half of May, and some areas, including the scene of the serious wildfire outbreaks in southern Georgia and northern Florida, may see deterioration. In contrast, normal rainfall should benefit southern Florida over at least the first 2 weeks of the forecast period. Despite the forecast for at least some improvement in most of the Southeastern drought areas by the end of August, complete eradication is unlikely as deficits accumulated since the first of the year exceed one foot in many locations. Tropical weather systems can potentially break droughts quickly, but future tracks of such storms are not known beyond a few days, adding to the uncertainty of seasonal forecasts. Elsewhere, the drought area that extends from California into the Great Basin is going into the dry season, so little lasting relief is expected in this region. The summer thunderstorm season running from July into September should bring some relief to Arizona. Farther north, forecasts for unseasonable warmth and dryness this summer result in a potential expansion of drought in Oregon and Idaho. Drought is expected to persist in Montana, Wyoming, the western Dakotas and western Nebraska, but improve in the Upper Midwest. Dryness may transition to drought in Hawaii, mainly in the leeward areas. Although drought is not forecast in Alaska, large areas have been unseasonably dry, and any future warm, dry weather could contribute to a serious wildfire season over the interior.

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 main forecast challenge this month is the drought in the Southeast, which continued to persist or even expand into mid-May, and was having serious impacts on agriculture, water supplies, and wildfire danger in various locations. The updated June-August rainfall outlook indicates equal chances wet or dry, so medium-range forecasts and historical analogues played a large role in the current drought outlook. Widespread severe spring dryness across the South also took hold in 2000, 1987, 1986, 1985, and 1981, as seen in soil moisture archives (the VIC model, for example). Of these years, only in 2000 did abnormally dry soil conditions persist throughout the summer for most of the region. In 1987, there were intra-seasonal fluctuations, conditions improving and then worsening. The other years saw widespread improvement. Changes in the Palmer drought index were more modest, although the composite for these years did show overall increases in the index (toward more wetness). The Palmers for a longer time period, including droughts in the 1940s and 30s, showed a tendency to persist during May to August for many areas of the Southeast. Given large rainfall deficits going into summer, it is understandable that a longer-term drought index would take longer to show improvement. The general forecast of some improvement is consistent with the odds favoring short-term improvement benefiting soil moisture but less substantial longer term improvement, a factor in lake and groundwater capacity. The constructed analogue soil moisture forecast (CAS) for July and August shows general improvement also, with a tendency for dryness to linger in northern Florida and southern Georgia, an area with perhaps the least odds for improvement. Of nine years of Palmers showing drought (-2.0 or worse) in May over south-central Georgia, six years showed little change into August. For southern Florida, short, medium, and long-range model forecasts show normal to above-normal rainfall, so confidence is relatively high for improvement there, although it may take a long time for Lake Okeechobee levels to rise substantially from current near-record lows. Areas of improvement along the Gulf Coast and the Carolinas are mainly from climatology, including the NCDC Palmer drought amelioration probabilities. This drought outlook makes no attempt to speculate where any tropical weather systems may strike this summer, although such systems can have a major impact on drought. Historic storm landfalls may, however, have contributed to the climatology used in this forecast.

To the north, short-term rainfall deficits have led to low streamflows and below-normal soil moisture in parts of the Ohio Valley. Medium range and long-range rainfall forecasts do not support developing drought at this time, but the area bears watching. Looking at a larger area, if the eastern tropical Pacific Ocean temperatures rapidly transition to moderate La Niña conditions (cooler than normal), this could increase the chance of heat in the central Corn Belt late in the forecast period. Although high temperatures can dry out soils more quickly, La Niña composites that adjust for climate trends show no strong tendency for dryness in the Midwest during the summer, and the historical analogues of El Niño to La Niña transition years accompanied by ample spring soil moisture in the Great Plains do not show a consistent dry signal.

In the Upper Midwest, there has been some recent expansion of the dryness or drought in Minnesota and Wisconsin, but medium and seasonal rainfall forecasts are consistent with improvement, as is the CAS for August. Farther west, the bulk of the improvement may have already taken place in the High Plains, and the odds for improvement in this region begin to drop as the summer heat ramps up. Persistence is now favored for this region.

Medium range and seasonal forecasts indicate warmth and dryness for the northern Intermountain region, resulting in the odds favoring drought expansion in Oregon and Idaho. Spring-summer streamflow forecasts and recent soil moisture estimates are consistent with the area of expansion shown. Climatology favors persistence this time of the year in California and the Great Basin, and the area of some improvement in AZ is expanded northward to reflect more of a monsoon impact going into August and the forecast for above-normal rainfall shown in the SWcast statistical model coming from the ESRL research lab in Boulder.

The development forecast for Hawaii continues, as abnormal dryness has persisted, the dry season lies ahead, and models show normal to below-normal rainfall for June-August.