Latest Seasonal Assessment -
With the strengthening and expected persistence of La Niña conditions through early 2008, the current Drought Outlook leaned
heavily on precipitation anomalies that typically occur during La Niña episodes. In addition, the current Drought Outlook was
based on the latest short and medium range forecasts, official December and December-February outlooks from the Climate Prediction
Center, climatological considerations, and initial drought conditions. Forecasts favor improvement across the northern Great
Basin, northern Rockies, and northern high Plains. The odds for improvement diminish farther south, with drought persisting
across central and southern California along with the Southwest. A widespread area of drought development is expected from the
southern Rockies into the southern Plains, Gulf Coast, and Florida. Year-to-date rainfall deficits range from 15 to 20 inches in
the area of exceptional drought centered in northern parts of Alabama and Georgia. Drought will likely persist in these areas
along with the Carolinas. Prospects for improvement increase farther north including north Tennessee, eastern Kentucky, and
southern New England. Improvement is also forecast for the Great Lakes and Hawaii.
Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for December - February 2007/08, the four-month drought termination and amelioration probabilities, 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 moisture, the CFS seasonal precipitation forecasts, and La Niña composites of soil moisture.
With the recent intensification of below-normal sea surface temperatures across the central and eastern equatorial Pacific Ocean, and the expectation that these La Niña conditions should persist through early 2008, emphasis was placed on historic La Niña composites (both raw and adjusted toward recent decadal trends). In addition, the outlook was based on the official 5-day, 6- to 10-day, 8- to 14-day, December, and December - February outlooks (the latter of which also relies heavily on La Niña composites), adjusted for considerations related to how typically wet or dry this season is compared to other times of the year, how firmly entrenched drought conditions are, and the seasonal decline in water loss due to declining temperatures, decreased evapotranspirative losses, and reduced public water usage.
Across the southeast, Gulf Coast and Florida, drought persistence or development should be widespread with La Niña conditions typically bringing subnormal precipitation to the region during the forecast time period. Exceptional drought continues for parts of Alabama and Georgia where year-to-date precipitation deficits in excess of 15 inches are common. Louisiana and Texas have experienced dryness recently, and thus may be more sensitive to potential dryness than would normally be the case. Despite near to above normal precipitation during the past 30 days across Florida, high probabilities for below normal precipitation during the outlook period strongly favors drought development excluding the far southeast counties.
Farther inland across eastern parts of Kentucky and Tennessee, near or above normal precipitation typically occurs during La Niña winters. Also, soil moisture composites indicate an increased likelihood for improvement. Therefore, odds are for improvement for northern Tennessee into eastern Kentucky. However, some improvement is forecast for areas suffering from exceptional drought as complete drought elimination is not likely.
Across the Great Lakes, improvement for the small areas of moderate drought is likely due to expected wetness in the medium and long-term forecasts. Along the East Coast from southern New England into the Mid-Atlantic, confidence is not high as conflicting signals exist among the forecast tools. Improvement is most likely for southern New England and the the Virginia piedmont.
Out West, La Niña favors above normal precipitation for the interior Pacific Northwest and northern Rockies. The official CPC seasonal outlook agrees with this scenario. Therefore, improvement was forecast for these regions, with lesser improvement anticipated in areas bordering these to the south, including the lower northern High Plains and parts of the Great Basin. Unlike the previous drought outlook released November 1, a greater area of drought persistence is now forecast for California along with the southern half of Nevada and Utah. This forecast is based on the CFS seasonal precipitation forecast and the official CPC seasonal outlook.
All forecasts on all time scales essentially agree on subnormal precipitation in the Southwest, where drought persistence or deterioration is anticipated. For southern sections of the Rockies and High Plains not already experiencing drought but beginning to feel the effects of short-term precipitation deficits, drought development probabilities are enhanced, as reflected in the forecast. This includes regions in the southern Plains currently in the abnormally dry category as depicted by the Drought Monitor.
Finally, with Hawaii progressing into its wetter time of the year, and with La Niña composites favoring above-normal precipitation, improvement was forecast for drought-affected areas across the island chain.