Latest Seasonal Assessment - A classic and very persistent La Nina precipitation pattern has dominated the country since Autumn 2010, resulting in broad areas of drought across the southern tier of states, in the southern half of the Plains, and along parts of the lower and middle Atlantic seaboard as of mid- May 2011. Extreme to exceptional drought covers most areas across the southern Rockies, the central and southern Plains, the immediate Gulf Coast, and the southernmost Atlantic seaboard. Drought has been most acute through Texas and the southern High Plains where many locations experienced the driest October through mid-May period on record, in some cases by wide margins. Frequently high wildfire danger, dramatic agricultural impacts, and increasing hydrologic concerns have been felt throughout these areas. Farther east, drought dates back more than a year in much of eastern Texas and Louisiana. However, an historic influx of water from the north wiped out any drought impacts in the large sections of southeastern Louisiana; specifically, in areas along and near rivers, across the region’s extensive marshland and natural levees, and through a large area east of the Atchafalaya River intentionally inundated to divert floodwaters away from the Baton Rouge to New Orleans corridor of the Mississippi River.
The La Nina precipitation pattern should gradually loosen its grip through late spring and the meteorological summer of 2011, leaving a lot of uncertainty about how drought conditions will evolve by the end of August 2011. The mid-May 2011 Drought Outlook map was synthesized from a combination of initial drought conditions, current drought duration, forecasts for the last two weeks of May, and Climatology (especially whether June - August is a relatively wet or dry season) to varying degrees. Drought is expected to persist in the interior Carolinas and through much of Louisiana and adjacent areas, where drought dates back farther than in most other areas. In other existing areas of drought, the odds at least nominally favor some degree of improvement, though there is nothing pointing toward anything broad-scale and substantial. To wit, the forecasts of “some improvement” and “improvement” for the parched areas of Texas and the southern High Plains were driven by the approach of a neutral to climatologically wet season (summer) which should provide at least some surface moistening, even if amounts aren’t unusually heavy. There is nothing to indicate that widespread, significant drought relief should be expected during the forecast period, though that of course is a possibility.
The first Drought Outlook valid through the 2011 meteorological summer (August 2011) finally anticipates the dissolution of the classic La Niña precipitation pattern across the contiguous United States dating back approximately to October 2010. As of mid-May 2011, this pattern has covered the southern tier of the country from Arizona eastward through Florida, parts of the southern and middle Atlantic Coast, and the southern half of the Plains in drought. Extreme to exceptional drought covers many areas from southeast Arizona and central New Mexico eastward through Florida and southern Georgia, and much of the central and southern Plains. Precipitation has been most critically lacking in much of Texas and the southern High Plains, where October through mid-May totals were unprecedentedly low in many areas.
The unwavering La Niña pattern is expected to wane as spring and summer 2011 progress, and this, along with the typical decline in long-lead precipitation predictability during these seasons, has led to a sharp drop in Drought Outlook confidence nationwide compared to the forecasts issued for more than the last half- year. In fact, the June - August 2011 long-lead forecast indicates no tilt of the odds toward either above- or below- normal precipitation in any of the areas currently experiencing drought (except in Hawaii). As a result, the mid-May 2011 Drought Outlook valid through August 2011 leans primarily on initial drought conditions, the day 1 through 14 forecasts extending through the end of May, and climatology.
From southern Mississippi northeastward into the lower mid- Atlantic coast, drought recently expanded northwestward into interior Alabama and Georgia, and intensified in a few areas near the immediate Gulf and Atlantic coasts. Only light precipitation is expected through May 23, but thereafter through the end of May the odds favor below-normal rainfall only in southeastern Georgia and adjacent Florida. June - August is climatologically a little wetter than other times of the year along the immediate coasts and the southeastern half of Georgia. Given these factors, most of the region should experience limited improvement by the end of August except through interior portions of the Carolinas, where drought is expected to persist. Drought dates back farther in the central Carolinas than in other parts of this region, and therefore will not respond as readily to any rain that falls. Confidence in the Southeast and along the eastern Gulf and southern Atlantic coasts is low.
Drought has also gripped the Florida Panhandle to varying degrees for the past several months, which coincides with its driest time of the year. The odds favor continued below-normal precipitation through the end of May, but the advent of the rainy season in June should at least alleviate any surface moisture shortages existing at that time, even if June - August 2011 totals are unremarkable. Confidence in the Florida Panhandle is high.
In the lower Mississippi Valley, and in particular southeastern Louisiana, a dramatic contrast between drought and flooding exists. Seriously deficient precipitation has fallen since Autumn 2010, and consistently below-normal amounts date back well over a year through much of Louisiana. At the same time, upstream rains led to historic flooding along rivers and across the swamps, marshes, and natural levees that cover most of southeastern Louisiana, with a large area along and east of the Atchafalaya River intentionally inundated in an effort to divert floodwaters from the Baton Rouge to New Orleans corridor of the Mississippi River. As of mid-May, drought is the least of this region’s concerns; however, with serious, protracted rainfall deficits in place and unlikely to change substantially over the summer, drought is expected to redevelop. Meanwhile, in other sections of the state, including some places within a few miles of swampland and inundated areas, impacts from the protracted drought are more readily apparent. Odds tilt only nominally toward drier than normal conditions through the end of May, and June - August is climatologically neither a wet nor dry time of year. However, the entrenched and long-term nature of precipitation deficits in this area will not respond readily even to increased precipitation. Confidence in the lower Mississippi Valley is moderate.
The southern half of the Plains states, and in particular the southern High Plains, are experiencing the most acute drought conditions in the country. Even though dryness dates back only to last October, the period since then has brought record dryness to many locations, with some parts of western Texas and eastern New Mexico recording only a few tenths of an inch during the period. Serious and intensifying agricultural impacts and frequently high wildfire danger has been a mainstay for several months now, and longer-term hydrologic impacts have been on the rise. Fortunately, between one and three inches of rain are expected by May 23 from eastern and northern Kansas southward through part of northeastern Texas, but elsewhere, little if any rain is forecast during the period, and the odds favor below- normal precipitation through the end of the month across Texas and New Mexico. Through the remainder of the summer, the only indicators of which direction the drought will take are initial conditions and climatology. The northern and eastern parts of the region, where moderate rains are expected in the short-term with no tilt of the odds toward dryness thereafter, improvement is forecast. In contrast, drought should persist relatively intact in southern Texas, which is pulling out of a relatively wet time of year, and in part of eastern Texas, where dryness dates back considerably farther than in other parts of the state. The outlook for the swath of land from the western halves of Texas, Oklahoma, and Kansas westward through the southern High Plains was driven exclusively by initial conditions and climatology. Western and northern parts of this region are headed into their wet season as summer progresses, making it likely that at least some surface moisture improvement will be felt by the end of August, though this forecast should not be interpreted as calling for widespread, significant relief. In other areas, where June - August is not particularly wet and/or precipitation since the start of the water year has been particularly sparse, only limited improvement is expected at best, even if only a marginal boost in surface moisture. Confidence in the southern half of the Plains is low.
Drought also covers the southwest from western New Mexico through central Arizona. Little or no rain is expected through the end of May 2011, although this is a dry time of year for the region anyway. Thereafter, June - August tend to be wetter, though not to the extent observed a little farther to the east. The result of these considerations is a forecast for some improvement. Confidence in the Southwest is low.
In Hawaii, there are enhanced chances for drier than normal June - August conditions, overcoming any relief felt through the end of May, and resulting in drought conditions similar to or slightly worse than at the start of this period. Confidence in Hawaii is moderate.
SOURCE: National Weather Service
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