May - July 2004

Latest Seasonal Assessment - Below-normal rain and snow accompanied by record warmth worsened prospects for spring and summer streamflows and water supplies in the drought-plagued West. The Southwest recorded its warmest March in 110 years of record-keeping, according to preliminary data. The first half of April saw increased rain and snow, with record precipitation in parts of New Mexico on April 3-4 and significant mountain snows in Wyoming and Colorado on April 8-9. Temperatures turned cooler. Although more precipitation is forecast during April in the West, the winter-spring snow season is drawing to a close, resulting in diminishing prospects for significant drought improvement. The latest outlook indicates that drought is expected to persist at least through July. Prospects improve farther east, with some improvement expected over the High Plains and into New Mexico, and more significant improvement "at least a one category change in drought intensity" expected in a swath extending from Minnesota into Kansas and eastern New Mexico. The improvement in the Midwest is based on expectations of climatological normal rainfall occurring during the forecast period, as well as the stormy pattern shaping up for the last half of April. In the Southeast, heavy rains eased severe short-term dryness during the second week of April, but some areas remained in need of moisture. The year-to-date precipitation through April 14 in Athens, Georgia, for example, totaled 8.36 inches, which was 7.26 inches below normal. Because long-range forecasts show a weak tendency toward warmth and dryness during May-July, there is a risk of drought development in Georgia, Alabama, and northern Florida. The risk for drought development is lower over the Florida peninsula due to the seasonal wetter conditions that typically develop by late May or early June. Given the typical rainfall patterns in the region and the indications from our latest soil moisture models, the overall drought risk in the Southeast may peak in the May-June period.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for May-July, the Palmer Drought Index Probability Projections for July, 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 Constucted Analogues for the season.

The drought forecast is more pessimistic in the West this month because of the lower water supply prospects resulting from the abnormally warm and dry March weather. The drought outlook considers the official spring-summer streamflow forecasts made in early April as well as the climatological probabilities for drought relief during this time of the year. Even though medium-range forecasts through late April suggest some beneficial rain and snow in the West, it is believed that it is too late in the season to have much of an impact on overall drought conditions. For much of the interior West, it looks like significant relief from hydrological drought will not arrive before next year. In southern California, although water supply conditions are not a big problem, precipitation deficits have been mounting. In San Diego, for example, January 1 to April 14 precipitation of 3.67 inches was 3.40 inches below normal. Going into the seasonal dry season, short-term dryness could increase the wildfire danger to above-normal levels, especially if temperatures are unusually high, as is suggested by the official CPC May-July forecast.

In contrast, drought relief appears headed for the Plains states. The forecast is driven largely by the favorable climatological probabilities for drought improvement for April to July, as well as the wet pattern suggested by the circulation pattern shaping up for the last half of April. Forecast signals from the models were not strong enough to warrant CPC placing any areas of above or below-normal rainfall outside of the Southeast on the May-July precipitation forecast, but the Constructed Analogue soil moisture (CAS) forecasts for May and July suggests a bias toward wetness over the southern half of the Plains and improvement in the Minnesota area as well. The same model also shows a tendency for drought to persist in the northern Plains. The resulting hatched area on the outlook map in the northern Plains was a compromise between the improvement suggested by climatology and the dryness suggested by the CAS.

In the Southeast, which was seeing near-drought conditions until the rains moved through the region around April 11-13, the risk area highlighted is based on the areas that missed the heaviest rains from the April event as well as the areas shown by the CPC May-July forecast as more likely to be warm and dry. Medium-range forecasts were also considered. Although the May-July precipitation forecast shows below normal for the Florida peninsula, the drought risk area did not extend into this part of the state because of climatological considerations. After the thunderstorm season gets underway in late May and early June, it usually becomes more difficult to sustain or develop drought. In addition, irrigation supplies offset the impacts from below-normal rainfall in peninsular Florida during the summer. The CAS indicates that the soil moisture anomalies in the Southeast should start to improve after May, and may be nearly gone by the end of July. The forecast for drought development has relatively low confidence, given the mixed signals from the various forecast tools.