Latest Seasonal Assessment -
La Niña conditions developed during the northern hemisphere fall for a second year in a row, with below average equatorial Pacific
sea surface temperatures expected to persist through the winter season. Therefore, climate anomalies associated with La Niña winters
were closely considered both for the U.S. Drought Outlook and the CPC monthly and seasonal guidance. Ongoing drought conditions
across the southern tier of U.S. states are expected to persist or worsen, with drought development likely across the Florida
Peninsula and portions of the desert Southwest. Drought development is also possible across eastern North Carolina, where the CPC
seasonal outlook indicates enhanced odds of below median precipitation. Although the seasonal outlook has no tilt in the odds
towards below or above median precipitation across Colorado, a dry climatology yields little opportunity for significant drought
reduction. In contrast, La Niña winters favor a wetter pattern across the northern U.S., increasing the chances for drought
improvement in the Great Lakes region. Recent excessive dryness across portions of the Corn Belt, however, particularly in
southern Minnesota and northern Iowa, coupled with a drier winter climatology, reduce the prospects for significant improvement of
drought impacts. La Niña conditions also favor a winter storm track centered along the Ohio Valley region, which may ameliorate
drought across northwestern Tennessee, but improvements along the fringes of this region, including eastern Arkansas and the
southern Appalachians, are less certain.
Discussion for the Seasonal Drought Outlook
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC temperature and precipitation outlooks for December 2011
and the long lead forecast for December 2011 - February 2012, 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.
Following the La Niña event of 2010-2011, widespread drought remains entrenched across the southern tier of U.S. states,
especially across the southern Plains, where large areas of exceptional (D4) drought remain. The summer rainy season
across the Florida peninsula eliminated drought conditions, while Tropical Storm Lee brought a swath of soaking rains
across the east-central Gulf Coast region. Additionally, recent episodic rainfall events have brought some relief to
parts of northern Texas and eastern Oklahoma. During the fall 2011 season, however, La Niña conditions re-emerged, and
below average equatorial Pacific sea surface temperatures are expected to persist through the outlook period. Therefore,
the CPC 1-month December and 3-month December-February outlooks both favor below median precipitation across the southern
tier of the U.S., particularly in Florida, Texas, and southern New Mexico. Accordingly, persistence or intensification
of existing drought areas is likely across the southern U.S. states. Although a surge of moisture during the late summer
brought an influx of water into Lake Okeechobee in southern Florida, lake levels heading into the dry season remain below
average, and drought redevelopment in the Florida Peninsula is likely. Drought expansion across parts of western New
Mexico, Arizona, and southwestern California is also possible.
Forecast confidence for the southern tier of U.S. states is high.
Following significant rainfall from Hurricane Irene earlier in the summer, rainfall has been near to slightly below
average across the southern and mid-Atlantic coastal plain. Although the CPC December outlook does not tilt the odds in
favor of above or below median precipitation, the December-February outlook does maintain enhanced chances of abnormal
dryness across eastern North Carolina and far southeastern Virginia. Additionally, precipitation anomalies associated
with a smaller subset of second year La Niña events also reveal a dry pattern across the southern Atlantic states. Based
on these factors, drought development is forecasted for eastern North Carolina and far southeastern Virginia.
Forecast confidence for the southern-Atlantic and mid-Atlantic coasts is moderate.
During a La Niña winter, the central Plains lie in between the typically drier southern U.S. and typically wetter
northern tier. The CPC December outlook maintains equal chances for near, above, or below median precipitation across
the southern Plains, while the winter seasonal outlook brings enhanced odds of below median precipitation as far north as
central and eastern Kansas. In these areas, drought persistence is forecasted, but some improvement is indicated for
eastern Nebraska, north central Kansas, and northern Missouri. Drought persistence is forecasted for southeastern
Colorado, however, due to the very low precipitation received climatologically during the winter season.
Forecast confidence for the central Plains is moderate.
Drought has expanded during recent weeks across the western Corn Belt region and the western Great Lakes, with record
rainfall deficits observed in southern Minnesota and northern Iowa. Short term forecasts bring meager precipitation to
Minnesota, while the CPC 6-10 and 8-14 day outlooks both favor abnormal dryness continuing across the western Corn Belt.
The December monthly outlook maintains equal chances of near, below, and above median precipitation, while the winter
seasonal outlook favors wetter conditions. Although the seasonal outlook and climate anomalies associated with La Niña
both favor enhanced wetness across the western Corn Belt, the region typically receives less than 10 percent of its
annual precipitation during the winter months. Therefore, it is less certain that enhanced precipitation during this
period can overcome the extreme deficits observed during the wetter late summer and early fall months. Based on these
considerations, a low confidence some improvement forecast is maintained for the drought areas of the western Corn Belt,
while improvement is forecasted for the western Great Lakes region.
Forecast confidence is moderate for the western Great Lakes, but low for the western Corn Belt.
During a La Niña event, winter storm tracks frequently favor a path across the Ohio Valley region, bringing enhanced
wetness to the central Mississippi Valley, Ohio Valley, central Appalachians, and New England. This anomaly factors into
the CPC winter seasonal outlook, which tilts the odds in favor of above-median precipitation across the Tennessee and
Ohio Valleys. Accordingly, drought improvement is likely across northwestern Tennessee during the upcoming three and a
half months. Both the CPC seasonal outlook and climate anomalies associated with a smaller sample of second year La Niña
events indicate a spatially reduced wet signal when compared to a typical La Niña, however, with increased dryness
forecasted along the fringes of the Ohio Valley region, including the lower and middle Mississippi Valley and southern
Appalachians. Therefore, some improvement rather than improvement is forecasted for the drought areas in eastern
Arkansas and along the western border of North and South Carolina.
Forecast confidence for the middle Mississippi and lower Ohio River Valleys and the southern Appalachians is moderate.
Drought redeveloped during the summer months across the western portions of the Hawaii. During a La Niña winter, wetter
than average conditions are typically observed, particularly after January. Based on these climate anomalies, drought
improvement is forecasted for the leeward sides of the Hawaiian Islands.
Forecast confidence for the Hawaii is low to moderate.