May - July 2006

Latest Seasonal Assessment - Increased rainfall over the central and southern Plains states during the last half of April should lead to short-term improvement in the moderate to extreme drought conditions, with the best chances for improvement extending from central and eastern Oklahoma into northern and eastern Texas. More significant improvement is expected over Arkansas and adjacent parts of Texas and Oklahoma. Nevertheless, drought is expected to continue across a large portion of the Plains states into July, as the outlook for May to July shows the odds tilting toward above-normal temperatures and below-normal rainfall over the central and southern Plains. Varying degrees of improvement are anticipated from eastern Wyoming into South Dakota thanks in part to ample precipitation early in the period. In the Southwest, with the dry season starting, little relief from the severe to extreme drought affecting Arizona and New Mexico can be expected into June, but the summer monsoon rains that typically begin in July may start to provide some relief. Due to unusually low mountain snowpack this spring, USDA/NOAA hydrologists have forecast spring and summer streamflows to be less than 25% of normal for several river basins in both states. Farther east, some improvement is expected in the middle Mississippi Valley, as well as the drought area that has recently expanded across the Gulf of Mexico coast. For the drought that has affected parts of the Mid-Atlantic region, significant improvement is on tap early in the period from New Jersey into North Carolina from a frontal system that should bring up to 2 inches of rain.

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, and the soil moisture tools based on the GFS model and the Constructed Analogues for the season.

There is much uncertainty about the drought outlook in the Plains, with precipitation forecasts for the first 2 weeks of the valid period nearly the complete opposite of the seasonal precipitation outlook for May-July, the short-term showing above-normal rainfall over much of the central and southern Plains where the May-July outlook shows below-normal rainfall. The fading La Niña also makes the seasonal rainfall outlook a bit more tenuous. The resulting seasonal drought outlook, which attempts to meld the forecasts for all time periods, expands the area of limited improvement slightly farther westward in Texas and Oklahoma, covering most of the area where the latest 2-week CPC soil moisture outlook has the greatest positive change. The best chance for improvement lies where favorable short-term rains are expected near or outside the boundary of the dry area in the seasonal period, and this extends from Arkansas into extreme northeast Texas and southeast Oklahoma. Some of the area shown as persisting drought, including eastern Colorado and western Kansas, will likely see some good precipitation from one or more low pressure systems crossing the region in late April, but this may be only a temporary benefit, given the seasonal outlook for warmth and dryness. The Constructed Analogue Soil moisture guidance shows severely low soil moisture persisting across much of Texas through at least the end of July.

The Outlook for the Southwest is based on the theme of little change or even worsening conditions as the dry season becomes established, but climatological odds for rainfall during July, the start of the monsoon rainy season. The area of limited improvement depicted on the map for parts of Arizona and New Mexico is largely driven by the NCDC drought amelioration probabilities for July and the western boundary of the dry area in the May-July precipitation outlook. The spring and summer streamflow forecasts show less than 50 percent of normal for most of the river basins in Arizona and New Mexico due to the poor spring mountain snow pack.

The outlook for the recently expanded drought along the Gulf of Mexico coast from Louisiana into the Florida Panhandle is mixed, with Alabama and Florida expected to see some relief in the first 5 days of the period, and the seasonal rainfall outlook showing equal chances of wet or dry. The CAS shows some improvement, and the drought amelioration map indicates around a 50-60 percent chance of improvement. The area is hatched to indicate that the drought may continue, but some relief by July is likely. Of course, a tropical disturbance coming out of the Gulf could turn the situation around rapidly.

In the East, the area of drought development shown in last month’s Drought Outlook is dropped, as the La Niña dry signal is much less apparent for this region by the end of July. This does not rule out drought development over parts of the Southeast before July, and the risk continues to be elevated for drought over the Florida peninsula and southern Georgia. Farther north, however, a frontal system crossing the eastern U.S. on April 21-22 promises to provide moderate to heavy rains for the mid-Atlantic drought regions, with as much as 2 inches of rain in some areas. This should easily reduce the extent of drought, and improvement is shown on the map for the northern and western parts of the drought region. More limited improvement is shown where rainfall totals should be less or the drought is more firmly established.

The outlook for the area from Missouri into Iowa and parts of Illinois is mixed. Iowa and Illinois have seen recent improvement from severe thunderstorms, but the weather could turn drier during the first 2 weeks of the valid Outlook period as a high pressure ridge extends from the northern Plains into central Canada. The seasonal CFS model shows dry weather for much of the western Corn Belt for May-July, but with limited skill. The CAS shows improvement by July, but this forecast tool also has limited skill in this area. Climatology as shown in the drought amelioration probability data implies improvement for parts of the region, so the result is a depiction of some improvement. The small area of lingering drought in northeastern Illinois should see improvement.

The short-term outlooks suggest improvement of varying extent from eastern Wyoming into western South Dakota.