Latest Seasonal Assessment -
La Niña conditions developed and strengthened during the latter part of the summer, and the latest CPC ENSO Diagnostic
Discussion indicates that La Niña is expected to last at least through the Northern Hemisphere winter months. Therefore,
analyses of climate anomalies typically observed during La Niña events contributed to this outlook. The remnants of Tropical
Storm Hermine brought significant rainfall and flooding to eastern Texas and Oklahoma, reducing drought impacts in these
states, while drought conditions persisted or developed across the lower Mississippi Valley, Ohio Valley, Southeast, and
Mid-Atlantic states. Enhanced chances of above-median precipitation extend across the northwestern U.S. during the upcoming
three months, consistent with La Niña and favoring some drought amelioration across southern Oregon and possibly into
northeastern California. In contrast, drier than average conditions are favored across the southern tier of states, with the
forecast complicated along the Atlantic and Gulf coasts by the possibility of tropical cyclone activity. Drought persistence
or expansion is forecast in Arizona and much of the interior southeastern quadrant of the U.S., while some improvement is
indicated closer to the coasts in respect of the ongoing hurricane season. Further drought improvement is indicated for South
Dakota, the Great Lakes, and northern New England due to short-term forecasts of significant rainfall. In Hawaii, drought
persistence is indicated, as the rainy season primarily begins beyond the OND forecast period.
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.
Recent rainfall has continued to alleviate moderate drought conditions in Maine, while below average precipitation persisted across the remainder of New England and the Mid-Atlantic states. Short and medium range (0-5/6-10/8-14 day) forecasts call for a likelihood of continued above-median rainfall in Maine, while below-median rainfall is expected to continue across the mid-Atlantic. Monthly and seasonal forecasts indicate equal chances of below, near, or above median precipitation in the region. Based on these factors, further drought improvement is forecast for Maine, while some improvement is indicated for the remainder of New England. Across the mid-Atlantic, drought persistence is forecast across the Piedmont and Appalachian regions, while some improvement is maintained closer to the coast to account for the possibility of tropical cyclone activity.
Forecast confidence for the Northeast is moderate.
Below-average rainfall continued across the Southeast during the past two weeks, with moderate drought conditions developing across the South Carolina Piedmont. Climate anomalies during La Niña events favor below-average precipitation, with a high frequency of occurrance, particularly in the southern Atlantic states. Both the 6-10 and 8-14 day forecasts indicate enhanced chances of below-median precipitation for the Southeast excluding the Florida peninsula, with a time range that extends towards the end of September, when tropical cyclone activity typically begins to decrease. Monthly and seasonal forecasts indicate equal chances of below, near, or above-median rainfall, with the possibility of tropical cyclone activity balancing the climatologically favored dryness. Since the short and medium range guidance indicates a lack of rainfall across the Southeast, along with a climatological decrease of tropical cyclone frequency beyond September, this outlook indicates drought persistence throughout the interior Southeast. Drought development is most likely in the already abnormally dry areas of the Piedmont and Appalachian regions from central Alabama through western North Carolina and extreme eastern Tennessee. Some improvement is maintained for drought areas on the Coastal Plain, as tropical cyclone activity is still possible during the autumn months.
Forecast confidence for the Southeast is moderate.
The remnants of Tropical Storm Hermine generated a swath of heavy rainfall across eastern Texas, Oklahoma, Missouri, and northern Arkansas, easing drought impacts in the affected areas. Elsewhere, hot weather and below-average precipitation was observed across the Ohio River Valley, lower Mississippi Valley, and far southern Texas. Moderate drought pushed northward along the Ohio River as far east as western Ohio. Short and medium range forecasts indicate increased odds of above-median precipitation for southern Texas due to tropical moisture, while drier conditions are favored for the lower Mississippi Valley. Precipitation is more likely across the mid-Mississippi and upper Ohio Valleys due to thunderstorm activity generated by eastward propagating shortwave troughs. OND Seasonal forecasts indicate a tilt towards drier conditions across the mid-Mississippi valley, with equal chances prevailing elsewhere. Climate anomalies observed during previous La Niña events indicate enhanced chances for abnormally dry weather across the entire south-Central U.S. Based on these factors, drought persistence is maintained for the lower Midwest and much of the Ohio Valley, with development likely across western Arkansas and far western Tennessee. North of the Ohio River, some improvement is indicated due to the potential for shortwave trough activity, while some improvement is maintained for this outlook across central Louisiana and eastern Texas as we pass the peak of hurricane season.
Forecast confidence for the lower and central Midwest is moderate to high.
Widespread heavy rainfall continued from the Northern Plains through the Great Lakes region during the previous two weeks, further easing long-term drought conditions. Short and medium range forecasts as well as the October monthly outlook all indicate a continuation of the rainy pattern, while the seasonal outlook indicates equal chances. Climate anomalies during La Niña events also lean towards above-average precipitation in the north-central U.S., though the frequency of occurrance is near 50 percent. Based on the persistently wet forecasts through the October monthly time frame, continued drought improvement is forecast for the nothern Plains and Great Lakes regions.
Forecast confidence for the north-central U.S. is high.
Generally below-average precipitation has continued across the southwestern U.S. during the previous two weeks, with drought conditions persisting in northern Arizona. During La Niña events, the southwestern U.S. has been anomalously dry with a high frequency, particularly in Arizona, where 60-80 percent of the OND periods during La Niña events since 1950 resulted in below-average rainfall. CPC 6-10, 8-14, October monthly, and OND seasonal forecasts all favor drier conditions across much of the Southwest. Therefore, drought persistence is forecast for northern Arizona, with drought development likely in areas already experiencing abnormal dryness.
Forecast confidence for the Southwest is high.
Drought conditions have persisted across the Great Basin and parts of the northern Rockies region throughout the summer dry season. As winter approaches, the atmospheric response to La Niña shifts the favored storm track into the Pacific Northwest, bringing increased precipitation. Accordingly, OND La Niña precipitation anomalies show above-average precipitation across the northwestern quadrant of the U.S., with the frequency of occurrence increasing towards the west. The region of increased precipitation can extend as far south as northern California, but a sharp gradient exists between the enhanced precipitation in the Northwest and the typically drier Southwest during a La Niña. Widespread rainfall is expected to begin along the Pacific Northwest during the upcoming 5 days, with the 6-10, 8-14, monthly, and seasonal forecasts all predicting enhanced precipitation during the OND period. For northern Wyoming, the CPC October monthly outlook yields a 40 percent chance of above-median rainfall. Due to the high likelihood of enhanced precipitation across the Pacific Northwest, drought improvement is forecast for southern Oregon, while some improvement is indicated for northern California and far northwestern Nevada. Persistence is forecast for the remainder of the Great Basin, as the La Niña climate anomalies trend drier towards the south. Drought improvement is also foreast for northern Wyoming, with some improvement possible in western Wyoming.
Forecast confidence is high for the northern Great Basin, and moderate for Wyoming.
Drought developed in Hawaii due to El Nino conditions in early 2010, and has persisted through the summer months. Hawaii typically receives enhanced rainfall during a La Niña winter, 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 autumn, though rainfall should begin to increase as the winter months approach.
Forecast confidence for Hawaii is moderate.