May - July 2007

Latest Seasonal Assessment - Prospects for significant drought relief across California, the Southwest, and the Great Basin are dim as the snow season comes to a close and snow pack remains well below normal. As of mid-April, California statewide snow water content stood at just 32 percent of normal. Although some precipitation is expected near the start of the forecast period, overall drought conditions will not improve significantly across most of the region. Over Arizona, however, the onset of the thunderstorm season in July should bring some short-term relief. Varying degrees of improvement are on tap for the western Dakotas, western Nebraska, and parts of Wyoming, although complete eradication of the long-running drought is unlikely. Drought relief is also expected in northern Minnesota, but drought-ending rains in the near future are unlikely there as well. The drought in the Tennessee Valley saw some relief in mid-April, and more help is on the way. With cumulative rainfall since the start of the year a foot below normal in parts of the region, it should take a while for the drought to end, but improvement is forecast to continue. Parts of Florida and southern Georgia also saw beneficial rains in April, and some additional improvement is expected by the end of July, thanks in part to the onset of the summer thunderstorm season. Soil moisture will initially benefit, but lakes will take more time to recover. The lingering drought in southeast Puerto Rico is expected to end. Dry conditions prevail over Hawaii, and the leeward areas could see drought development by the end of July.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for May-July, 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.

Consistent with last month's Outlook, a large area of drought is expected to persist into July across California, the Southwest, and the Great Basin. The area of development that was forecast in the March 15 release has transitioned into drought, according to the latest U.S. Drought Monitor depictions. With the snow season coming to an end, significant relief is unlikely in coming months, although sporadic precipitation can be expected. Over an inch of precipitation is forecast across northern California and the Sierra during the first 5 days of the Outlook period. Any precipitation is helpful, but significant relief is doubtful this late in the water year. Due to below-normal spring snow pack, the April 1 forecast issued by USDA’s Natural Resources Conservation Service indicated spring-summer streamflows less than 50 percent of normal for much of California, Utah, Nevada, Arizona, and eastern Oregon, as well as portions of the adjacent states. Good reservoir levels will provide some cushion from the expected poor streamflows and runoff. April 1 reservoir storage was above average in California, Colorado, Idaho, Nevada, and Washington, and only slightly below-normal in Oregon and Utah, providing some cushion from the expected poor stream flow and runoff. In contrast, Arizona, Montana, New Mexico, and Wyoming reservoirs were at below-normal storage. The onset of the summer monsoon thunderstorm season in the Southwest should offer some dryness relief by the end of July, with the best bet for beneficial moisture in Arizona.

In the Rockies and High Plains, the Outlook indicates increasing confidence for improvement toward the north and east. Long and short range precipitation forecasts, as well as climatology, are considered in this forecast. The latest CPC May-July precipitation outlook centers dryness over southwestern Wyoming, with decreasing odds for dryness to the north and east. Soil moisture outlooks for the last half of April indicated improving conditions across the Rockies, with the greatest improvement over Montana and the Dakotas southward. Palmer drought index change probabilities based on climatology were also a factor in the improvement indicated for the Plains states, including the improvement over northern Minnesota. Improvement depicted over Wyoming and Montana was limited due to a drier pattern forecast for later in April, as shown in the week-2 forecast for April 24-28. There are several statistical and numerical seasonal models suggesting a trend toward dryness during May-July in the Plains states, including the constructed analogue soil model (CAS), the Ensemble CCA, and the UKMET, the latter centering the dryness over the Midwest in the latest available release (March). These models do not form a consensus, and the near-term environment is too wet to consider drought expansion at this time, but the situation will be monitored to determine if more forecasts move in this direction.

Recent rains have eased the drought in the Tennessee Valley, and more beneficial moisture is forecast during the last week of April. Climatology also favors drought improvement this time of the year, so improvement is forecast, with a relatively high confidence. The odds for improvement are less from southern Georgia into northern Florida, where medium-range forecasts suggest little improvement, and CAS indicates low soil moisture for the end of July. CAS points to more substantial improvement over southern Florida, consistent with the boost to soil moisture expected from the summer thunderstorm season, but with significant increases to lake levels unlikely before late summer, this area is forecast to show more limited improvement as well. As of April 17, Lake Okeechobee elevation stood at 10.02 feet, 4 feet below average.

Seasonal forecast models continue to show wetness for the Puerto Rican area, so drought improvement is indicated there. In Hawaii, recent conditions have been abnormally dry, and various models point to normal to below-normal rainfall going into the upcoming dry season. As a result, there are enhanced odds for drought development in the leeward areas. In Alaska, low snow pack and winter precipitation have resulted in abnormally dry conditions in the interior. With the seasonal forecast indicating equal chances for wet or dry, no drought development is indicated at this time.