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Climate Prediction Center


October - December 2010


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Latest Seasonal Assessment - Widespread above-median rainfall eased drought impacts in South Dakota and Michigan. Heavy rains from the interaction between a closed upper-air low and the remnants of Tropical Storm Nicole brought significant drought relief across many areas from the Carolinas to the Northeast. In contrast, scant precipitation across the southeastern quadrant of the United States promoted drought expansion and intensification. In the Pacific Northwest, although widespread rainfall overspread Washington and western Oregon, it fell primarily outside of the Great Basin drought areas. The updated drought outlook for the October - December period indicates drought improvement for the Northeast, Upper Midwest, northern Wyoming and southern Oregon, with some improvement to drought conditions over northern Nevada, parts of Utah, western Wyoming, northwest Arizona, the central Appalachians and the mid-Atlantic. Drought conditions are likely to persist and/or intensify in the southeast, the lower Ohio Valley, the southern Plains, central Nevada, eastern Colorado, with drought expected to develop across areas in the southeast not currently in moderate drought conditions.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC precipitation outlook for October 2010 and the long lead forecast for October - December 2010, 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.

Interaction between an anomalous upper-level closed low and the remnants of Tropical Storm Nicole brought widespread heavy rain and substantial drought relief across an area from the Carolinas to New England. A few areas of at least moderate drought remain in parts of the central Appalachians, southern New Jersey, northern Connecticut and eastern Maine. Additional improvement is likely in Connecticut and Maine as short-term forecasts indicate continued rains, and CPC's extended range and monthly forecasts also favor above-average rainfall for these areas. In the mid-Atlantic and central Appalachians where drought impacts are slightly greater, short and medium-range forecasts also suggest continued rain but monthly and seasonal forecasts offer equal chances for above- or below-median precipitation. Therefore, drought improvement is forecast for the Northeast, but with some improvement indicated for the central Appalachians and Mid-Atlantic due to the less optimistic monthly and seasonal outlooks.
Forecast confidence for the central Appalachians, mid-Atlantic and Northeast is moderate.

Marked dryness during the previous two weeks intensified drought conditions across the remainder of the Southeast and the lower Mississippi River and Ohio Valleys. Climate anomalies observed during La Nina events favor below-median precipitation with a high frequency of occurrence. Short and medium range forecasts and both the 6-10 and 8-14 day outlooks indicate enhanced chances of below-median rainfall across the Mississippi and Ohio River Valleys, and the seasonal outlook for October-December shows odds tilted in favor of below-median precipitation continuing across the lower Mississippi Valley. Abnormally drier conditions associated with La Nina are expected to extend eastward and southward during the upcoming three months as the probability of tropical cyclone activity decreases towards the end of the 2010 Hurricane Season. Therefore, drought persistence is maintained across the Southeast and Ohio Valley in areas that have slipped into at least 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 Tennessee Valley, and most of the immediate Gulf Coast and the Florida panhandle. The potential for tropical cyclone activity early in the seasonal period adds some uncertainty to the forecast, and if a late-season tropical cyclone were to affect parts of the southeast more significant improvement would be likely.
Forecast confidence for the Southeast, Delta, and Lower Ohio Valley is moderate.

Continued above-median precipitation across the northern Plains and upper Midwest during the previous two weeks continued to ameliorate drought conditions, with no areas of moderate drought remaining in South Dakota or Michigan. Following some additional light to moderate precipitation in the short term, CPC's 6-10 and 8-14 day outlooks favor near-average precipitation chances. The monthly and seasonal outlooks maintain equal chances for above, near, or below median precipitation. Climate anomalies observed during La Nina events show a trend towards above-median precipitation in northern Minnesota, although the frequency of occurrence is not high (near 50 percent). Drought improvement is maintained for the upper Midwest.
Forecast confidence for the northern Plains and upper Midwest is low to moderate.

Soil moisture remains plentiful across much of the southern Plains following the passage of Tropical Storm Hermine in early September, and continued above-median precipitation in far southern Texas during the previous two weeks. Forecasts from the short range through the seasonal outlook, however, all indicate enhanced chances of below-median precipitation for the southern Plains during the drought outlook period. The probability of tropical cyclone landfalls in the western Gulf Coast diminish during the autumn as mid-latitude troughs dig further south and tend to recurve tropical cyclones eastward away from the region. Therefore, drought persistence is maintained for the small areas remaining in moderate drought in Oklahoma, and a small development area is indicated across southern Texas, where La Nina conditions are strongly associated with decreased precipitation.
Forecast confidence for the southern Plains is moderate.

Abnormal dryness has increased throughout much of northern and eastern Colorado, with an area of moderate drought across parts of north-central Colorado where below-average precipitation anomalies are prevalent. Short-term forecasts show preciptiation is expected in this region but is forecast to remain fairly light. CPC's 6-10, Week-2 and monthly outlooks indicate a slight tilt in the odds for below-median precipitation, and GFS soil moisture anomaly forecasts also show a slight decrease in soil moisture during the Week 1 and Week 2 periods. La Nina composites for the seasonal period also show a slight tendency for below-median precipitation in northern Colorado. Thus, drought persistence is indicated for northern Colorado.
Forecast confidence for northern Colorado is moderate.

Rainfall has increased recently over parts of southern Oregon and the northern Great Basin, largely due to an upper-level closed low stationed over southern California. The upper-low is forecast to move northeastward over the next few days. As winter approaches the atmospheric response to La Nina shifts the favored Pacific storm track into the Northwestern U.S., bringing increased precipitation to the Northwest. Accordingly, La Nina composites for OND show above-median precipitation across the Pacific Northwest, with a sharp transition to below-median precipitation occuring near the northern borders of California and Nevada. The CPC monthly and seasonal outlooks indicate enhanced chances of above-median precipitation extending from the Pacific Northwest eastward as far as Wyoming. Therefore, drought improvement is forecast for southern Oregon and northern Wyoming; some improvement is indicated across northern California, far northwestern Nevada, and west-central Wyoming; and drought persistence is forecast for central Nevada.
Forecast Confidence for the Northwest and Great Basin is moderate.

While monsoon rainfall remained well east of the northern Arizona drought area during September, a recent late-season surge in monsoon moisture ahead of the upper-level closed low over southern California has resulted in increased shower and thunderstorm activity across western parts of the northern Arizona drought region. Additional rain is possible over the next couple of days over northwestern Arizona as the closed upper low moves northeastward. Beyond that time, CPC's 6-10 day, Week-2 and seasonal outlooks favor enhanced chances for dry conditions, and La Nina composites also favor dryness in the southwest. Some improvement to conditions is possible in northwestern Arizona, but more significant improvement is not likely given the increasingly dry long-range forecasts. Drought persistence is indicated for the rest of the northern Arizona drought area with short-term rains expected to fall west of this region.
Forecast confidence for the Southwest is moderate.

Drought developed in Hawaii due to El Nino conditions that persisted through early 2010, and drought conditions have continued to persist or intensify. Hawaii typically receives enhanced rainfall during a La Nina northern hemisphere winter season, but this effect primarily occurs beyond the forecast range of this outlook, in January - March. Therefore, drought persistence is maintained across Hawaii for the remainder of the autumn months, though rainfall should begin to increase as the winter rainy season approaches.
Forecast confidence for Hawaii is moderate.

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Page Author: Climate Prediction Center Internet Team
Page last modified: October 7, 2010
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