Latest Seasonal Assessment -
Climate anomalies attributed to the Summer 2010 - Spring 2011 La Niña event promoted widespread drought development and
intensification across the southern tier of the U.S. In many locations, significant deterioration occurred, with 65 percent of
the current drought areas categorized as extreme or exceptional, and nearly 42 percent of drought areas categorized as
exceptional (D4). During the previous two weeks, a persistent ridge of high pressure maintained hot, dry conditions across the
core drought areas of the southern Plains states, while a series of stalled fronts sparked scattered to numerous thunderstorms
across the Southeast and mid-Atlantic, bringing localized drought relief. Copious rainfall fell across the central and eastern
Gulf Coast, bringing more substantial relief, while daily wet-season thunderstorms throughout the Florida Peninsula began
eroding entrenched drought conditions. While monsoon rainfall was generous across much of Arizona, Utah, and Colorado, many
core drought areas of New Mexico and western Texas received subnormal rainfall. Continued drought improvement is expected
under the monsoon regime in the Southwest, although the prospects for significant relief across the eastern monsoon region are
less certain due to the lackluster start to the summer rainfall. Continued drought improvement is expected across Florida and
the Southeastern coastal plain as the rainy season continues and enhanced rainfall is more likely during the upcoming three
months. Below-average August rainfall is more likely across the southern Mississippi Valley region, however, and prospects for
substantial drought relief across the southern Plains remains limited through October. Across Hawaii, leeward drought
persistence is expected under the summer trade wind regime, while developing drought is possible in Alaska's Kenai Peninsula
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC temperature and precipitation outlooks for August 2011
and the long lead forecast for August - October 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, the four-month Palmer drought termination and amelioration
probabilities, climatology, and initial conditions.
A persistent ridge of high pressure maintained dry
conditions and much above normal temperatures across the central U.S., exacerbating the widespread drought entrenched across
the southern Plains. Daily maximum temperatures in San Angelo, TX have broken 100 degrees Fahrenheit 54 times in 2011, more
than any other year except 1969. Isolated thunderstorms across the region have provided little to no relief under the
persistent heat and widespread dryness. Drier weather is typical across eastern Texas northeastward through the lower
Mississippi Valley during August, with climatological rainfall increasing slightly towards the beginning of Autumn.
Climatological rainfall increases more substantially in early Autumn across southern and southwestern Texas. Short, medium,
and extended range forecasts, including the CPC 6-10 and 8-14 day outlooks all indicate enhanced chances for below-average
precipitation across the southern Plains states. The CPC August outlook maintains enhanced chances of below-median rainfall
for the southeastern Plains and lower Mississippi Valley. Based on these outlooks, as well as extemely low NCDC 3-month
drought amelioration probabilities for the southern Plains, continued drought persistence is likely. Further drought
development is possible across northeastern Texas, the only region of the state currently not in drought, as well as
portions of southwestern Arkansas, due to recent extreme heat and dryness and the dry outlooks through August.
Forecast confidence for the southern Plains is moderate.
During the previous two weeks, widespread heavy
rainfall overspread the central and northeastern Gulf Coast, with accumulation departures from normal of 1 to 3 inches in
most locations, but up to 7 inches in portions of southern Mississippi and the western Florida Panhandle. The heavy
rainfall substantially improved streamflows and ameliorated the widespread exceptional drought conditions. The CPC 6-10 day
outlook indicates a continuation of above-median precipitation across the central and northeastern Gulf Coast, while the
August-October seasonal outlook maintains enhanced chances of above-median precipitation for the eastern Florida Panhandle
only. Given the potential for continued short term relief combined with the peak of the Atlantic hurricane season falling
in the middle of the outlook period, continued drought improvement is expected across the Gulf Coast.
Forecast confidence for the central and northeastern Gulf Coast is moderate to high.
Daily thunderstorms associated with the
seabreeze-driven rainy season, as well as a brief influx of moisture associated with a tropical wave maintained generally
above-average rainfall across the Florida Peninsula, gradually easing the areas of entrenched drought. Keetch Byram Drought
Indices across the state remained generally below 400, indicating a diminished risk for wildfire spread. Additionally, no
areas of exceptional drought (D4) remain in the state. As the rainy season continues across Florida, further drought
reduction is likely. Both the CPC monthly outlook for August and the August-October seasonal outlook maintain enhanced
chances for above-median rainfall across the Florida Peninsula. The peak of the Atlantic hurricane season also falls in
early September, the middle of this outlook period. Based on these factors, drought improvement is maintained for this
Forecast confidence for the Florida Peninsula is high.
Across the remainder of the Southeast,
below-average rainfall was observed during the previous two weeks across northern Mississippi, Alabama, and Georgia, as well
as much of the Tennessee Valley, although scattered thunderstorms provided some local drought relief. The August-October
climatology across the interior South trends slightly drier in the late summer, with many locations receiving only 15-20
percent of their annual rainfall during the period. The CPC August-October seasonal outlooks do not tilt the odds in favor
of above or below median precipitation across the southern piedmont, though the August monthly outlook maintains enhanced
chances of below-median rainfall across Mississippi and far western Alabama. Moisture from tropical systems is also a
possibility during the late summer and early Autumn months. Based on these factors, improvement is maintained for the
coastal Plain regions, but a less certain some improvement is forecast for the Piedmont drought areas.
Forecast confidence for the Southeast is low to moderate.
Monsoon rainfall commenced during July across
portions of the Southwest, primarily in Arizona, Utah, and Colorado. In New Mexico, however, where drought was present
across the entire state and nearly half of the state was experiencing exceptional (D4) drought conditions through July 12,
thunderstorm activity was more sparse. The CPC 8-14 day outlook indicates a greater chance for enhanced monsoon rainfall
across the New Mexico and Arizona border, but suppressed thunderstorm activity is more likely to continue across the eastern
monsoon region. Equal chances are maintained in the CPC monthly and seasonal outlooks. Based on these forecasts, continued
drought improvement is forecast for Arizona, western New Mexico, and southern Colorado, but improvement is less certain
across eastern New Mexico and western Texas due to the late start to the monsoon rains. Monsoon moisture riding north of
the summertime ridge over the southern Plains may bring some improvement as far east as west central Kansas.
Forecast confidence for the Southwest is moderate.
In Hawaii, the summer trade wind regime, which
favors dryness on the lee sides of the islands, favors persistence of the existing drought areas in the state.
Forecast confidence for Hawaii is moderate.
Abnormal dryness has persisted across southern
Alaska for several months. Streamflows across the Kenai Peninsula region in particular have continued to decline.
Therefore, drought development across Alaska's Kenai Peninsula is possible during the upcoming three month period.
Forecast confidence for Alaska is low to moderate.