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Climate Prediction Center


November - January 2010


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Latest Seasonal Assessment - Since the last U.S. Drought Outlook (Oct. 7), a drier weather pattern returned to most of the lower 48 States and Alaska while mostly dry weather persisted in Hawaii. Exceptions to this included the Northeast, where above-normal rainfall has eased or eliminated drought in much of New England, and in the Southwest where a vigorous upper-air low produced unseasonably heavy rains and even severe weather (tornadoes) in Arizona. This rain was enough to diminish drought by a category in northwestern Arizona, western Nevada, and the Sierra Nevada. Meanwhile, abundant precipitation has soaked the southeastern Alaskan Panhandle and Puerto Rico. The drought outlook for November 2010 - January 2011 periods indicates drought improvement for the Northwest, upper Midwest, and Ohio Valley, with some improvement in the mid-Atlantic, Tennessee Valley, northern California, and Hawaii. Drought conditions are likely to continue in the Southeast and Southwest, with drought expected to develop and expand into much of the Southeast not currently in moderate drought, along with parts of the southern Plains and portions of the Southwest.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC precipitation outlook for November 2010 and the long lead forecast for November 2010 - January 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, La Niña composites for the September - November season, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

After widespread heavy late September and early October rains produced substantial drought relief from the eastern Carolinas into New England, additional light to moderate (and locally heavy) rains in the Northeast have erased drought in New England and diminished dryness in the mid-Atlantic. Some dry pockets, however, remained in the central Appalachians and southern New Jersey. In these areas where drought impacts are slightly greater, short and medium-range forecasts suggest continued surplus rain, but the monthly and seasonal forecasts offer equal chances for above- or below-median precipitation in the central Appalachians while the seasonal outlook tilts slightly toward dryness along the coast. Therefore, some improvement is forecast for the mid-Atlantic due to the less optimistic seasonal outlook and the moderate to strong La Niņa years composites.
Forecast confidence for the mid-Atlantic is moderate.

Continued dryness during the past two weeks maintained or intensified drought conditions across the remainder of the Southeast and the lower Mississippi and Ohio Valleys. Climate anomalies observed during La Niña events favor below-median precipitation with a high frequency of occurrence, except in the Ohio Valley where above-normal precipitation is common during the late winter and spring season. Short and medium range forecasts and both the 6-10 and 8-14 day outlooks, however, indicate near to above-median precipitation in the Mississippi and Ohio River Valleys and parts of the Southeast, although the November and November-January outlooks have odds tilted in favor of below-median precipitation across the lower half of the Mississippi Valley and the Southeast. Therefore, with ample precipitation expected in the short and medium term and in the seasonal outlook, improvement was shown in the Ohio Valley. Elsewhere, abnormally drier conditions associated with La Niña are expected to extend eastward, southward, and westward during the upcoming three months as tropical cyclone activity markedly decreases during November. Therefore, drought persistence is maintained across the Southeast in areas that have slipped into moderate drought. A large area of drought development is forecast for most of the Southeast not currently in moderate drought, including parts of western Arkansas, eastern Texas, the southern Appalachians, and along the Gulf Coast. The normal seasonal decrease in tropical cyclone activity adds confidence to the forecast, along with the ongoing moderate to strong La Niņa event expected to persist into the spring.
Forecast confidence for the Southeast, Delta, and Ohio and Tennessee Valleys is moderate to high.

Much drier weather returned to the northern Plains and upper Midwest during the previous two weeks, ending (for now) a wet pattern that commenced this summer which nearly ameliorated all drought conditions in the region. Only a small area in northeastern Minnesota remained in drought. The short and medium term forecasts expect light to moderate precipitation in the short term, with CPC's 6-10 and 8-14 day outlooks favoring above-average precipitation chances. The monthly and seasonal outlooks maintain equal chances for above, near, or below median precipitation. Climate anomalies observed during La Niña events show a trend towards above-median precipitation in northern Minnesota, although the frequency of occurrence is not high (near 50 percent). With expected short and medium term surplus precipitation, an ongoing wet trend, and ideal time for moisture replenishment (cool weather, no evapotranspiration, growing season ended), drought improvement is maintained for the upper Midwest.
Forecast confidence for the upper Midwest is moderate.

Soil moisture that was plentiful across much of the southern Great Plains following the early September passage of Tropical Storm Hermine has dwindled as much drier weather has occurred the past 45 days. An exception to this has been extreme southern Texas where ample mid- to late September rains have maintained wetness. With the exception of heavy rains during the next 5-days (from HPC) in northern Texas and Oklahoma, all forecasts from the short to medium range through the seasonal outlook indicate enhanced chances of below-median precipitation and above-normal temperatures. And with the likelihood of little or no "bonus" tropical cyclone rains (such as from Hermine) in the western Gulf Coast due to decreased tropical cyclone formation and southward-displaced mid-latitude troughs that tend to recurve any tropical cyclones eastward away from the region, drought persistence is maintained in Oklahoma and northern and southwestern Texas. In addition, development was extended across most of south-central Texas, northern Texas and Oklahoma, and parts of eastern New Mexico where La Niña conditions are strongly associated with decreased precipitation and increased temperatures.
Forecast confidence for the southern Plains is moderate to high.

Abnormal dryness has increased throughout much of eastern Colorado, western Nebraska, and eastern Wyoming as moderate drought has expanded from northeastern Colorado into western Nebraska. Short-term forecasts show light to moderate precipitiation in this region, while CPC's 6-10 and Week-2 outlooks portend above-normal precipitation. The monthly and seasonal outlooks, however, show no odds toward either above- or below-median precipitation, although both outlooks favor above-normal temperatures. The La Niņa composites for NDJ period show a slight tendency for below-normal precipitation in the central Great Plains and above-normal precipitation in the central Rockies. Thus, without any strong indicators for improvement or deterioration in the central High Plains, drought is maintained.
Forecast confidence for the central High Plains is low to moderate.

The 2010-11 Water Year got off to a wet start in the Sierra Nevadas and Great Basin as unseasonably heavy early October precipitation (and severe weather) was produced by a strong upper-air low that tracked northwestward from the Southwest. There was enough precipitation to diminished drought and abnormal dryness in this region. As winter approaches, the atmospheric response to La Niņa shifts the favored Pacific storm track northwestward, bringing increased precipitation to the Northwest. Accordingly, La Niņa NDJ composites show above-median precipitation across the Pacific Northwest and into the northern Rockies, with an abrupt transition to subnormal precipitation in central California (albeit a weak signal). The CPC monthly and seasonal outlooks indicate enhanced chances of above-median precipitation extending from the Pacific Northwest eastward into Wyoming. Therefore, drought improvement is forecast for southern Oregon, extreme northern California, western Wyoming, and northern Utah; some improvement is indicated across northeastern California (transition area); and drought persistence is forecast for the northern Sierra Nevadas and south-central and northern Nevada.
Forecast Confidence for the Northwest and Great Basin is moderate to high.

The same upper-air low that soaked the Sierra Nevadas and Great Basin in early October first impacted the Southwest with severe weather (tornadoes) and unseasonably heavy rains. Hardest hit was northern Arizona where several tornadoes produced damaged west of Flagstaff. Two to as much as six inches of rain fell on northern Arizona, southern Utah, and California's Sierra Nevada, with 1 to 2 inches of rain over most of Nevada. The early season bonus rains were enough to diminish drought conditions in parts of Arizona, Nevada, Utah, and California. The short-term forecast calls for scattered showers and thunderstorms, some locally heavy, while CPC's 6-10 day and Week-2 outlooks have equal-chances (no odds) for precipitation. Further out, the monthly and seasonal outlooks depict enhanced chances for dry conditions, and La Niņa NDJ composites also favor dryness and warmth in the Southwest, especially in eastern areas. Some slight improvement to conditions is possible in Arizona in the short-term, but more significant improvement is not likely given the increasingly dry long-range forecasts. Drought persistence is indicated for the rest of the Southwest drought area, with development in northeastern Arizona and northern New Mexico where the early October rains missed and soil moisture (NLDAS) is much lower.
Forecast confidence for the Southwest is moderate.

Drought expanded across much of Hawaii due to the El Niņo event from mid-2009 until early 2010, and has persisted or intensified since. Since rainfall normally increases during the winter, and Hawaii typically receives enhanced precipitation during a La Niņa northern hemisphere winter season, one would expect favorable odds for above-normal rainfall. However, this La Niņa effect primarily occurs after the forecast range of this seasonal outlook (DJF through MAM). Additionally, not all La Niņa events equate to wetter than normal conditions. During the 1999 and 2000 La Niņas, the leeward sides recorded subnormal rainfall, resulting in a 4-year drought (1998-2001). Therefore, with this uncertainty and being just outside the La Niņa "wet" time frame, some improvement is possible, especially along the windward sides of the islands.
Forecast confidence for Hawaii is moderate.

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Page Author: Climate Prediction Center Internet Team
Page last modified October 21, 2010
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