Discussion for the Seasonal Drought Outlook
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC temperature and precipitation outlooks for October 2011
and the long lead forecast for October - December 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 hot and dry conditions across the south-central U.S. into early September, maintaining a large area
of extreme to exceptional drought. Texas had its driest summer on record, with a statewide average of 2.44
inches of rain. 2011 was the hottest summer on record for Oklahoma and Texas with average temperatures of
86.5 degrees F and 86.6 degrees F, respectively. According to the National Climatic, Texas recorded the
hottest summer for any state. On September 13, Dallas-Fort Worth broke the previous record (69 in 1980) for
most 100 degree days in a year. At the beginning of the period, a cold frontal passage is expected to bring
1-2.5 inches of rainfall to southeast Kansas and northeast Oklahoma where some improvement is forecast. Across
the remainder of the southern Plains, tools on all time scales and consecutive La Niña composites indicate
enhanced odds for below median precipitation. Therefore, persistence is forecast for much of Oklahoma and
Texas. However, forecast confidence is tempered by the potential for tropical cyclone activity through the end
of November that could provide relief to the western Gulf region.
Forecast confidence for the southern Plains is moderate.
During the summer, monsoon rainfall
provided some drought relief to eastern Arizona and western New Mexico. Locally heavy rainfall and flash
flooding occurred in the desert Southwest during early-to-mid September. Therefore, development that was
forecast in the previous outlook across the desert Southwest was removed. The waning of the monsoon coupled
with forecasts of enhanced odds for below median rainfall during the upcoming three months favors persistence
or development across Arizona, southeast Utah, southwest Colorado, and much of New Mexico. Some improvement is
forecast in southeast Colorado and northeast New Mexico due to moderate to heavy rainfall expected at the
beginning of the period.
Forecast confidence for the Southwest is moderate.
Across the Southeast, heavy rainfall
associated with Tropical Storm Lee eliminated drought and abnormal dryness across southeast Louisiana along
with parts of Alabama and Mississippi. Outside of Lee’s track, drought continued in Arkansas, western
Louisiana, southeast Alabama, central and southern Georgia, the Carolinas, and parts of Florida. During the
remainder of September, an upper-level trough is expected to increase the odds for near to above median
rainfall in North Carolina where improvement is forecast. The October-December climatology across south
Florida trends much drier, with this region receiving only 10-20 percent of their annual rainfall (versus a
normal distribution of 25 percent) during the period. Without heavy rainfall from a tropical system,
persistence can be expected for the small drought area at Lake Okeechobee in south Florida. Based upon the
seasonal CPC precipitation outlook and consecutive La Niña composites, persistence or development is forecast
for the Southeast where heavy rainfall did not occur with Tropical Storm Lee.
Forecast confidence for the Southeast is moderate.
Across the upper/middle Mississippi
Valley, lower Ohio Valley, and western Corn Belt, a lack of late summer rainfall resulted in drought expansion.
Based upon consecutive La Niña composites, persistence and development is forecast for the middle Mississippi
Valley and lower Ohio Valley. To the west and north of this region, signals among tools for October-December
are rather weak and the seasonal outlook for OND indicates equal chances for below, near, and above median
precipitation. Since the climatology tends slightly drier during this period, odds are tilted towards
persistence for the upper Mississippi Valley and western Corn Belt.
Forecast confidence is moderate for the middle Mississippi Valley and lower Ohio Valley and low for the
upper Mississippi Valley and western Corn Belt.
In Hawaii, drought continues on the
western Big Island, Maui, and Molokai. Although the return of La Niña favors above median precipitation across
the Hawaiian Islands, the wet signal typically occurs later during the winter season. Therefore, persistence
is forecast at this time for the ongoing drought areas.
Forecast confidence for Hawaii is low.