Latest Seasonal Assessment -
Extreme to exceptional drought persisted and expanded across the Southwest, southern Plains, and lower Mississippi Valley,
worsening particularly across portions of northern and western Texas, southern New Mexico, and the Oklahoma panhandle region.
Drought persistence is expected west of a line from southern Texas through central Kansas, with CPC one-month precipitation
outlooks indicating enhanced odds of below-median precipitation during the climatological peak of spring rains. Further drought
development is forecast across western Arizona and the four corners region due to scant snow water content levels heading into a
climatologically dry period. Across the mid-South and Southeast, in contrast, several rounds of severe weather brought
substantial drought relief to eastern Oklahoma and Arkansas, but also caused widespread flooding and devastating tornadoes.
Drought improvement is forecast across southeastern Oklahoma and northeastern Texas eastward through northern Mississippi as
monthly tools indicate continued wetness. Moderate drought returned to portions of the southern Atlantic coastal Plain
following two weeks of below-average rainfall. Some improvement is expected across much of the Southeast, while longer term
improvement is expected in Florida as the rainy season commences in late May or early June. In Hawaii, drought persistence is
expected in leeward regions as the summer trade winds regime begins.
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC precipitation outlook for May 2011 and the long lead forecast for
May - July 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.
During April and early May, multiple storm systems
traversed the upper South and Ohio and Tennessee Valleys, bringing torrential rainfall and severe weather. April 2011
precipitation values were more than twice average from eastern Oklahoma northeastward into upstate New York. The repeated
heavy rainfall events not only erased drought conditions across Arkansas, they caused widespread flash flooding and river
basin flooding, particularly along the Black/Lower White Rivers, where near-record or record flooding was expected. Across
the remainder of the South, generally below average rainfall was observed, maintaining drought conditions along the Gulf
Coast. Moderate drought development occurred across eastern North Carolina and the Virginia tidewater. Although
short term drought relief is not expected across the southeastern U.S., monthly and seasonal tools do not indicate a
continuation of a dry signal into the summer convective season. Therefore, some improvement of drought is forecast for most
of the Southeast. Improvement is indicated for drought areas on the North Carolina and Virginia coastal Plain in reflection
of the CPC three month outlook, which has increased odds of above-median rainfall across the eastern seaboard from North
Carolina northward. Although areas of extreme drought persist across southern Florida, with several large wildfires
reported, longer term improvement is expected across most of the state, as the sea breeze-driven rainy season typically
begins in late May across southern Florida and lifts northward quickly through June.
Forecast confidence for Florida is moderate to high, and low to moderate across the remainder of the South.
While drought conditions were quickly overturned to
a flooding situation in Arkansas, areas of severe to exceptional drought persisted across Louisiana and eastern Texas. CPC’s
6-10 day, 8-14 day, and May outlooks all indicate enhanced odds for below-median precipitation across the western Gulf Coast.
Therefore, drought persistence is indicated in this outlook.
Forecast confidence for Louisiana and eastern Texas is moderate.
Across the southern Plains and Southwest, widespread
extreme to exceptional drought persisted and expanded during the previous month, except for a small portion of Texas’s
prairies and lakes region. Little to no rain fell across southern and western Texas and eastern New Mexico, while a
combination of persistent above-average temperatures with low humidity and periods of gusty winds exacerbated the drought
impacts. On April 26, Phoenix, AZ tied its record lowest relative humidity at two percent, with an air temperature of 87
degrees Fahrenheit and a dew point of -8 degrees Fahrenheit. The CPC 6-10 day, 8-14 day, and May monthly outlooks all
indicate enhanced chances of below-median precipitation and above-median temperatures persisting across the southern Plains
and Southwestern U.S. Additionally, snow water content levels across the Southwest are generally below 25 percent of
average. Based on these considerations, drought persistence is maintained and further expansion of drought is indicated
across western Arizona and far southern Utah and Colorado.
Forecast confidence for the southern Plains and Southwest is moderate to high.
Heavy La Niña enhanced winter rainfall ended
drought conditions on Kauai and Oahu and eased drought across islands further east. Drought remains, however, across western
portions of Hawaii’s Big Island and central islands. As the summer trade wind regime commences, however, the chances of
substantial precipitation on the leeward sides of these islands diminishes. Further drought reductions, therefore, are not
likely, and persistence is indicated on this outlook.
Forecast confidence for Hawaii is moderate.