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Climate Prediction Center


August - October 2011


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Latest Seasonal Assessment - Climate anomalies attributed to the Summer 2010 - Spring 2011 La Niña event promoted widespread drought development and intensification across the southern tier of the U.S. In many locations, significant deterioration occurred, with 65 percent of the current drought areas categorized as extreme or exceptional, and nearly 42 percent of drought areas categorized as exceptional (D4). During the previous two weeks, a persistent ridge of high pressure maintained hot, dry conditions across the core drought areas of the southern Plains states, while a series of stalled fronts sparked scattered to numerous thunderstorms across the Southeast and mid-Atlantic, bringing localized drought relief. Copious rainfall fell across the central and eastern Gulf Coast, bringing more substantial relief, while daily wet-season thunderstorms throughout the Florida Peninsula began eroding entrenched drought conditions. While monsoon rainfall was generous across much of Arizona, Utah, and Colorado, many core drought areas of New Mexico and western Texas received subnormal rainfall. Continued drought improvement is expected under the monsoon regime in the Southwest, although the prospects for significant relief across the eastern monsoon region are less certain due to the lackluster start to the summer rainfall. Continued drought improvement is expected across Florida and the Southeastern coastal plain as the rainy season continues and enhanced rainfall is more likely during the upcoming three months. Below-average August rainfall is more likely across the southern Mississippi Valley region, however, and prospects for substantial drought relief across the southern Plains remains limited through October. Across Hawaii, leeward drought persistence is expected under the summer trade wind regime, while developing drought is possible in Alaska's Kenai Peninsula region.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC temperature and precipitation outlooks for August 2011 and the long lead forecast for August - October 2011, 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 (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

A persistent ridge of high pressure maintained dry conditions and much above normal temperatures across the central U.S., exacerbating the widespread drought entrenched across the southern Plains. Daily maximum temperatures in San Angelo, TX have broken 100 degrees Fahrenheit 54 times in 2011, more than any other year except 1969. Isolated thunderstorms across the region have provided little to no relief under the persistent heat and widespread dryness. Drier weather is typical across eastern Texas northeastward through the lower Mississippi Valley during August, with climatological rainfall increasing slightly towards the beginning of Autumn. Climatological rainfall increases more substantially in early Autumn across southern and southwestern Texas. Short, medium, and extended range forecasts, including the CPC 6-10 and 8-14 day outlooks all indicate enhanced chances for below-average precipitation across the southern Plains states. The CPC August outlook maintains enhanced chances of below-median rainfall for the southeastern Plains and lower Mississippi Valley. Based on these outlooks, as well as extemely low NCDC 3-month drought amelioration probabilities for the southern Plains, continued drought persistence is likely. Further drought development is possible across northeastern Texas, the only region of the state currently not in drought, as well as portions of southwestern Arkansas, due to recent extreme heat and dryness and the dry outlooks through August.
Forecast confidence for the southern Plains is moderate.

During the previous two weeks, widespread heavy rainfall overspread the central and northeastern Gulf Coast, with accumulation departures from normal of 1 to 3 inches in most locations, but up to 7 inches in portions of southern Mississippi and the western Florida Panhandle. The heavy rainfall substantially improved streamflows and ameliorated the widespread exceptional drought conditions. The CPC 6-10 day outlook indicates a continuation of above-median precipitation across the central and northeastern Gulf Coast, while the August-October seasonal outlook maintains enhanced chances of above-median precipitation for the eastern Florida Panhandle only. Given the potential for continued short term relief combined with the peak of the Atlantic hurricane season falling in the middle of the outlook period, continued drought improvement is expected across the Gulf Coast.
Forecast confidence for the central and northeastern Gulf Coast is moderate to high.

Daily thunderstorms associated with the seabreeze-driven rainy season, as well as a brief influx of moisture associated with a tropical wave maintained generally above-average rainfall across the Florida Peninsula, gradually easing the areas of entrenched drought. Keetch Byram Drought Indices across the state remained generally below 400, indicating a diminished risk for wildfire spread. Additionally, no areas of exceptional drought (D4) remain in the state. As the rainy season continues across Florida, further drought reduction is likely. Both the CPC monthly outlook for August and the August-October seasonal outlook maintain enhanced chances for above-median rainfall across the Florida Peninsula. The peak of the Atlantic hurricane season also falls in early September, the middle of this outlook period. Based on these factors, drought improvement is maintained for this outlook.
Forecast confidence for the Florida Peninsula is high.

Across the remainder of the Southeast, below-average rainfall was observed during the previous two weeks across northern Mississippi, Alabama, and Georgia, as well as much of the Tennessee Valley, although scattered thunderstorms provided some local drought relief. The August-October climatology across the interior South trends slightly drier in the late summer, with many locations receiving only 15-20 percent of their annual rainfall during the period. The CPC August-October seasonal outlooks do not tilt the odds in favor of above or below median precipitation across the southern piedmont, though the August monthly outlook maintains enhanced chances of below-median rainfall across Mississippi and far western Alabama. Moisture from tropical systems is also a possibility during the late summer and early Autumn months. Based on these factors, improvement is maintained for the coastal Plain regions, but a less certain some improvement is forecast for the Piedmont drought areas.
Forecast confidence for the Southeast is low to moderate.

Monsoon rainfall commenced during July across portions of the Southwest, primarily in Arizona, Utah, and Colorado. In New Mexico, however, where drought was present across the entire state and nearly half of the state was experiencing exceptional (D4) drought conditions through July 12, thunderstorm activity was more sparse. The CPC 8-14 day outlook indicates a greater chance for enhanced monsoon rainfall across the New Mexico and Arizona border, but suppressed thunderstorm activity is more likely to continue across the eastern monsoon region. Equal chances are maintained in the CPC monthly and seasonal outlooks. Based on these forecasts, continued drought improvement is forecast for Arizona, western New Mexico, and southern Colorado, but improvement is less certain across eastern New Mexico and western Texas due to the late start to the monsoon rains. Monsoon moisture riding north of the summertime ridge over the southern Plains may bring some improvement as far east as west central Kansas.
Forecast confidence for the Southwest is moderate.

In Hawaii, the summer trade wind regime, which favors dryness on the lee sides of the islands, favors persistence of the existing drought areas in the state.
Forecast confidence for Hawaii is moderate.

Abnormal dryness has persisted across southern Alaska for several months. Streamflows across the Kenai Peninsula region in particular have continued to decline. Therefore, drought development across Alaska's Kenai Peninsula is possible during the upcoming three month period.
Forecast confidence for Alaska is low to moderate.

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Page Author: Climate Prediction Center Internet Team
Page last modified: July 21, 2011
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