March - May 2006

Latest Seasonal Assessment - A dearth of rain and snow into February promoted intensifying drought across the Southwest from Arizona into west Texas and Oklahoma, and the odds favor continued drought across the region into May, if not beyond. Although shorter-range forecasts show at least a temporary change to a wetter pattern during the last half of February in Arizona and New Mexico, significant drought relief is not expected. The seasonal temperature and precipitation outlooks for March-May point to the likelihood for above-normal temperatures and below-normal precipitation, supporting drought continuation well into spring. Reservoir storage across much of the Southwest remains in good condition thanks to last year’s storms, but the ongoing drought poses a threat to upcoming spring and summer streamflows, as well as agriculture. Wildfire danger remains a major concern across the region. In addition, there is a risk that the drought currently affecting the southern Plains could spread northward across Kansas and eastern Colorado and merge with the drought in Nebraska and Iowa. The odds for improvement continue to look better to the east, with more moisture on tap for eastern portions of Texas and Oklahoma, as well as Arkansas. Nevertheless, with 12-month rainfall deficits exceeding 20 inches in the northeast Texas-southeast Oklahoma region, drought-ending rains are unlikely anytime soon. Ongoing drought, but with some improvement, is seen for Illinois and eastern Iowa. A large storm system near the start of the forecast period on February 15-16 was bringing widespread moisture to the region, but twelve-month precipitation deficits of 12 inches and greater from northern Illinois into eastern Iowa mean that this region is also a long ways from seeing an end to their drought. In the Southeast, heavy rains in early February ended short-term dryness concerns, but the seasonal and monthly forecasts of below-normal rains suggest increased risk for drought from Florida into southern Georgia.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for March-May, the drought termination and amelioration probabilities for May, 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.

This Outlook is consistent with the previous month’s depiction in focusing on drought in the Southwest and Plains. The U.S. Drought Monitor has expanded drought across much of the Southwest in the past month, and even increased drought to D3 levels over a large part of Arizona. Given the drought extent and severity already evident, the latest Outlook map shows persisting drought, but no longer indicates additional expansion or intensification across the Southwest. The seasonal precipitation outlook for March-May implies increasingly dry conditions across southern California and southern Nevada. Factors that discouraged expansion of drought into these states included adequate water storage in many reservoirs due to last year’s storms, and short-term weather forecasts suggesting a turn toward a wetter pattern, at least for the last half of February.

In the plains, abnormal dryness has been increasing north of the Texas-Oklahoma drought area, and the latest official monthly and seasonal precipitation outlooks tilt the odds toward dryness in Kansas, eastern Colorado, and into Nebraska. As a result, the Drought Outlook continues to indicate a risk of the southern Plains drought merging with the ongoing drought in Nebraska.

The seasonal precipitation outlooks show equal chances for wet, dry, or normal in the northern Rockies. The forecasts for the last half of February suggest normal to above-normal precipitation for the lingering drought areas in Montana and Wyoming, and trend-adjusted La Niña composites show a slight tendency for wetness over parts of the region for March-May, so some improvement is again indicated for this region.

Forecast models have been quite accurate these past few months in showing increasing odds for drought improvement farther east over eastern portions of the Mississippi Valley, and this theme is continued with the current drought outlook. Drought has ended in nearly all the areas shown as improving in last month’s Outlook, including areas in Kentucky and southeast Louisiana. From this point on, it appears that the heaviest rains, at least in the first 2 weeks, will fall mainly in areas that have already seen drought elimination, with mainly the eastern edges of the ongoing drought region in Arkansas, Texas, and Oklahoma expecting significant precipitation. Some improvement is indicated for areas generally expecting normal rains in the first 10 days of the period and equal chances for wet or dry during March-May.

Precipitation forecasts ranging up through seasonal time frames tend to show wetness over the Ohio Valley, and a storm near the start of this Outlook period in mid-February brought beneficial moisture to the drought area in Iowa and Illinois. The consensus of the forecasts is consistent with some improvement for the ongoing drought, with the better odds for improvement to the east, and lesser odds to the west. With 12-month precipitation deficits still exceeding 12 inches across northern Illinois, drought elimination is very unlikely over the next few months, and low subsoil moisture levels could become a concern if conditions turn hot and dry during the growing season.

Consistent with the current La Niña, the majority of statistical and numerical forecast models point to dryness over parts of the Southeast during March-May. Although unusually heavy rains in early February removed immediate concerns for developing dryness over the Florida peninsula, a return to dry conditions or even drought development over much of the state by the end of May remains a risk. The 6-10 day, 8-14 day, monthly, and seasonal precipitation forecasts all call for below-normal rainfall across most of the Florida peninsula, so the Drought Outlook continues to indicate an enhanced risk for drought. Given the seasonal precipitation outlook’s indications for dryness northward to North Carolina, there is some risk for drought expansion through the Carolinas, but the current map limits the drought to Florida and southern Georgia due to forecasts for abundant rains farther north during the last half of February.