May - July 2003

Latest Seasonal Assessment - A change in the storm track brought needed moisture to the upper Midwest in March, with a deep low pressure system dumping abundant rain and snow on the Great Lakes region during March 27-28. The wet pattern continued into April for parts of the Midwest, and a large storm system affected much of the nation during April 14-17. Forecasts point to continued widespread precipitation during the last half of April. With the prospects for increasing soil moisture during April, the Midwest, Great Plains, and the Rockies will be seeing additional relief from the drought that has affected many areas since last fall or longer. The Drought Outlook depicts improvement across the Midwest and into the High Plains, and limited improvement across the West outside of the Great Basin and Southwest. The forecast improvement does not necessarily mean the end of drought problems, as it should take a long time to eradicate drought in areas where moisture deficits are especially large, such as western Nebraska and western South Dakota. Some locations in northwestern Missouri, where low water supplies may persist, have gone 10 consecutive months with below- normal precipitation. Farther north, with relatively less rainfall expected in northern South Dakota and North Dakota, drought may largely persist. Long-term moisture deficits still prevail across much of the West, so even the normal mountain snow pack observed in some states this spring is not enough to bring significant relief to water supplies except in scattered locations. Additional rain or snow could result in an improvement of pasture conditions, an increase in soil moisture, and reduced wildfire risk. However, due to the large accumulated moisture deficits, there will be less improvement of streamflow and water supplies. In many western areas, water shortages are expected to continue throughout the period. The odds for improvement in the hydrological drought are lowest in the Great Basin and the Southwest due to low snow pack or the limited precipitation expected during this time of the year. In Maine, where drought lingers in the north, improvement is expected in coming months. In Hawaii, the outlook calls for limited relief.

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 and various medium and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts. The latest western water supply forecast is also considered.

The forecast shows much more improvement this month because of the change to a wetter circulation pattern that is expected to continue at least through month's end. Medium range forecast models have been consistent in showing a broad southerly jet originating in the Pacific racing across the southern U.S. Forecast surface maps show a series of low pressure systems crossing the West and central states through the end of April, spreading rain or mountain snows over much of the country. A broad area of low pressure at the surface and aloft is likely through the end of April over southern and central portions of the U.S., with ridging in western Canada leading to above-normal pressures in the northern Plains. The latter is one area where drought may persist for the forecast period. The other large area of persisting drought extends across the Great Basin and into the Southwest. Low snowpack will result in continuing water supply problems for the Great Basin even with some upcoming short-term relief, and the Southwest is unlikely to see very much precipitation until the summer monsoon starts in July. The seasonal drought outlook places considerable weight on the short and medium-range forecasts, as the long- range precipitation outlook for May-July shows equal chances of wet or dry for most of the country.

Although the outlook does not depict any areas of developing drought, the southern plains has trended dry in recent weeks and months, and will need to be monitored in case the dryness intensifies. Rainfall was less than 40 percent of normal for the past 60 days across northern Texas and a large portion of Oklahoma as of April 15.

Confidence in the outlook is relatively high for the early part of the forecast period, given the consistency of forecast models in showing the stormy pattern. The outlook beyond April is much less certain given both the time of the year and changing sea surface temperature patterns. La Niña-like conditions in the Pacific would tend to bring drier weather to many parts of the country, and there has been some recent cooling of equatorial waters in the eastern Pacific and warming west of the dateline suggestive of this kind of pattern. The current drought outlook is based on SST conditions not changing very much in the next few months.