February - April 2008

Latest Seasonal Assessment - Widespread rains during the last half of January will contribute to at least some degree of drought improvement for the entire Southeastern drought area outside of Florida. The improvement should be more limited over the longer term from southern Alabama into central and southern Georgia and the Carolinas due to below-normal rainfall forecast during February-April. The drier weather expected later in the season means that conditions could deteriorate following initial improvement, especially in areas near the Gulf and Atlantic coasts. Also, even with several inches of rain during the first 2 weeks of the forecast period, many reservoirs and wells will remain low due to lingering impacts from the extreme rainfall deficits incurred last year. Peninsular Florida will see relatively less rain over both the short and the seasonal time frames, so persisting drought is still expected there, with the chance for expansion toward the east coast due to the dry 3-month forecast. Elsewhere, the odds favor drought expansion by the end of April in central Texas toward Oklahoma, with additional expansion from western Kansas into eastern New Mexico. To the north, some improvement is likely for most of the northern Plains from the Dakotas into central Montana, with the exception of north-central North Dakota, while more significant improvement is expected across the remaining drought areas in the interior Northwest and Great Basin. The Pacific storm in early January substantially boosted snow pack in California and other parts of the West, but below-normal precipitation is expected during February to April for the Southwest, so the odds favor limited improvement at best for this region.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for February – April 2008, the monthly precipitation forecast for February, 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, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil moisture, the CFS seasonal precipitation and soil moisture forecasts, and La Niña composites of soil moisture.

Widespread rains were falling across the Southeast drought area as the Drought Outlook was being completed on Wednesday, January 16, and weather forecasts indicated that a series of Gulf lows would continue to bring beneficial rains to the drought region for the next 2 weeks. The 5-day rainfall forecast (QPF) for January 16-21 showed over 2 inches of rain near the Gulf Coast extending to the southern tip of South Carolina. The raw GFS model output on Wednesday indicated over 6 inches of rain during the 384-hour period for much of Alabama and Georgia. The challenge in making the seasonal drought forecast was reconciling the drought relief expected during the first 2 weeks of the outlook period with the likely return of drier weather during February-April. The final map placed the straight improvement area in the parts of the Southeast with the relatively lower probabilities for below-normal rainfall during Feb-Apr, and this was over central and northern Alabama, northern Georgia, and points north. It should be noted that 10 to 12 inches of rain are needed to bring the Palmer Drought Index to near zero (normal) over northeastern Alabama, northern Georgia, and northwestern South Carolina, so drought impacts are likely to linger for a long time in these areas despite short-term relief. It also will be very difficult to bring the lowest reservoirs up to full storage in coming months. Lake Lanier levels in northern Georgia remain over 14 feet below normal as of January 15. Nevertheless, the rains expected during the first 2 weeks will boost soil moisture, raise river levels and benefit groundwater and surface supplies.

Florida is expected to see less relief from both the short-term rains and the Feb-Apr rains, although substantial rains will hit northern Florida during the first 2 weeks. The odds still favor drought persistence and even expansion across central and southern Florida into April based on expected La Nina impacts over the longer term and the medium range forecasts for the shorter term. Both the 6-10 day and 8-14 day forecasts issued on January 16 indicate below-normal rains for central and southern Florida.

The development area across Texas has been expanded from the last outlook because of recent dryness. Little to no rain fell across north-central Texas during the first half of January. The development area in central Texas and also southwestern Kansas into northeastern New Mexico is consistent with the drop in soil moisture forecast by the CFS model by April for both areas. The University of Washington’s 3-month runoff and soil moisture forecasts based on historical La Nina events were used to tweak some of the areas on the outlook map, including expansion of the development area in eastern New Mexico as well as the persisting area in North Dakota. Their Surface Water Monitor forecasts were also considered in pulling back some of the improvement from central Montana.

In the Southwest, with the big snow-producing Pacific storm of early January now history, and the seasonal forecasts still indicating below-normal precipitation across the region, the Outlook is somewhat less optimistic, although the improved mountain snow pack should benefit water supplies.