May-July 2005

Latest Seasonal Assessment - Although valley rains and mountain snows beginning in late March benefited areas of the Pacific Northwest, the northern Rockies, and High Plains, drought remains a major concern from the Cascades into the western Dakotas and western Nebraska, as mountain snow pack remains near record lows in some areas. In Washington, Oregon, and western parts of Idaho and Montana, drought conditions are expected to persist or intensify over the next several months. In contrast, prospects brighten farther eastward, where the wetter outlook, especially for the first weeks of the forecast period, holds out hope for short-term improvement from eastern and central Idaho through Montana into North Dakota. The best odds for improvement extend from Wyoming into southwestern South Dakota, Nebraska, and parts of Colorado, where storms during the early part of the period could significantly boost soil moisture, resulting in more favorable prospects for farmers and ranchers while reducing wildfire danger. The hydrological drought situation, however, will likely see little improvement across this region, as the mountain snow packs, which are the source for the area's lakes and rivers, remain low and the spring storms will likely come too late to boost water supply prospects in such areas as the North Platte basin in Wyoming and Nebraska. In the Southwest, where drought has been significantly reduced following near-record snowfalls this past season, snowmelt will boost water supplies, and even threaten major flooding, while benefiting those reservoirs that remain low. Elsewhere, short-term dryness has developed in the Great Lakes region, South Texas, and in south-central Oklahoma and adjacent parts of north Texas. Short-term and seasonal weather forecasts do not suggest drought development in these areas, although some drought is likely to persist in the southern Oklahoma-north Texas area, and the other dry areas bear watching. In the Southeast, a trend toward abnormally dry, warm weather is expected during May-July, as indicated in the latest CPC long-lead outlook. Current ample moisture conditions in this region, including Alabama, Georgia, and Florida, will make it difficult for drought to gain a foothold, but this area should also be monitored.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for May-July, the drought termination and amelioration probabilities for July, 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 Analogues for the season, and the latest western water supply forecasts.

Pacific storms since March have eliminated drought in western Oregon and Washington and boosted mountain snow pack and soil moisture into Idaho, but the latest medium-range forecasts point to a trend toward drier and warmer weather for the first 2 weeks of the Outlook period in Washington and Oregon, as well as parts of Idaho. Although the official CPC May-July precipitation outlook shows equal chances of above or below-normal precipitation for the West, this is too late in the season for significant drought alleviation. Spring-summer streamflow forecasts from east of the Cascades into Idaho and parts of western Montana depict flows of just 25 to 49 percent of normal, and mid-April snow water content remains below 50 percent of normal in Washington and northern Oregon. The Palmer drought probability data for a climate division in northern Oregon, CD 6, shows a 100 percent chance that severe drought on the Palmer scale will persist into July. Because the medium-range soil moisture forecasts show a trend to lower values over the first 2 weeks, the map label has been changed to indicate the possibility of intensifying drought.

To the east, a storm was bringing rain and snow to the northern Rockies at the start of the Outlook period, with significant precipitation amounts expected in parts of Wyoming, Montana, and South Dakota on April 21-22. Forecast models suggest that several more storms should hit the region in the following 2 weeks. The near-term wet pattern, in addition to the May-June period being a seasonally wet period, led to the depiction of drought improvement from Wyoming into the Dakotas and Nebraska. It should be noted that this is too late in the year to appreciably affect the hydrologic drought in this region, so concerns for water supplies will continue in Wyoming and the Upper Missouri Basin in general, but spring moisture can be very beneficial for farmers and ranchers, and can reduce the danger from wildfires.

Most of the lingering drought area in the Southwest changed from the straight improvement category to the limited improvement category this month because of the region entering a drier time of the year. Melting snow will result in above-normal streamflows across the region, boosting water storage.

There are several parts of the country where drought development is a concern. The U.S. Drought Monitor already shows a small area of D1 drought in southern Oklahoma, and 2-week soil moisture forecasts based on the GFS model show increasing dryness extending from OK into Texas. Most seasonal forecast models do not point to heat and dryness in this area, so drought expansion was not shown, but there appears to be a good chance that the existing drought area will continue well into the Outlook period. In the southern Plains, it can be difficult to alleviate drought that persists into the hotter time of the year.

Drought has recently developed in South Texas. This is not shown on the Drought Outlook because it is not expected to expand given the lack of dryness shown in the seasonal forecast tools. But the seasonal outlook does call for above-normal temperatures, so this is an area of concern.

In the Great Lakes region, a dry area centered in Michigan is being monitored. A storm expected during the first few days of the Outlook period should reduce the short-term dryness in Michigan, but may do little to benefit areas to the west. Parts of the Northeast have also experienced short-term dryness and reduced streamflows, but the April 22-24 storm should benefit this area also. The forecast area of above-normal rainfall centered over Wisconsin in the CPC May-July outlook precludes drought development in this region, although the short-term outlook for continued dryness in the western Great Lakes region means that this area bears watching.

In the Southeast, the May-July outlook for above-normal temperatures and below-normal rainfall increases the odds for drought development from Florida northward into Georgia, Alabama, and South Carolina. An area for drought development was considered for this Drought Outlook, but current high soil moisture values coupled with a cool, wet pattern in the medium range resulted in holding off development for now.

Puerto Rico has some lingering areas of dryness and local drought, but long-range models show normal to above-normal rainfall during May-July, so expanded drought is not expected on the island.