Latest Seasonal Assessment -
The drought outlook for Spring (March–May) 2011, made on February 17, was based largely upon climate anomalies associated with
an ongoing, mature La Niña that has begun to weaken, with ENSO-neutral or La Niña conditions equally likely by May-June. The CPC
monthly and seasonal outlooks indicate enhanced odds for below median precipitation and above median temperatures across the
southern tier of the Nation and in the central Plains which favors drought persistence from southern Arizona eastward into the
southern and central Plains, along the Gulf Coast States, and northward into the Carolina Piedmont. Similarly, drought
development is forecast across much of the rest of the southern U.S., from southwestern Arizona eastward into the southern and
central Plains, northern and southeastern Texas, and along parts of the Gulf and southern and middle Atlantic Coasts. Although
there were some concerns in the Northwest that spring drought development was possible after a mild and very dry January, a good
start to their wet season plus ongoing storms and enhanced odds of above median March precipitation suppresses any notion of
spring drought development. Prospects for improvement are indicated for the lower Mississippi, Tennessee, and Ohio Valleys, with
some improvement predicted for the Arklatex region, the northern Alabama and Georgia border, and in the central Appalachians.
This is based upon enhanced odds for above normal monthly precipitation in this area, plus some hints of wetness in the La Niña
MAM composite and trend, although the frequency of occurrence is low. Drought relief has occurred in Hawaii this winter,
courtesy of heavy rainfall associated with La Niña, and continued improvement is forecast for the islands remaining in drought.
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC precipitation outlook for March 2011 and the long lead forecast for
March - May 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, La Niña composites for the March - May season, the four-month
Palmer drought termination and amelioration probabilities, climatology, and initial conditions.
A very dry and cold December and January, along
with spotty early February precipitation, has resulted in expanding drought across most of the middle and southern Atlantic
States. Many USGS stream flows from western South Carolina northward into central Virginia have fallen below the 10th
percentile (much below normal). An exception to this was in northern Florida and southern Georgia where late January and
early February moderate to heavy rains improved drought conditions and increased stream flows to above-normal values. In
contrast, USGS river flows in north-central Florida, still suffering from long-term drought deficiencies, remained at or
below the 25th percentile. Since La Niña MAM precipitation composites and the CPC monthly and seasonal outlooks strongly
favor enhanced odds for below median precipitation in the southern Atlantic States, drought is expected to persist or develop
across Florida, Georgia, and South Carolina. In the mid-Atlantic (North Carolina to southern New Jersey), the odds for
subnormal monthly and seasonal precipitation gradually decrease in the outlooks as one heads north (e.g. become equal
chances), and actually tilt toward above median monthly precipitation further to the west (e.g. central Appalachians).
However, the La Niña MAM precipitation composites hint at dryness along the coast while lingering effects from last summer's
drought point toward persistence and development.
Forecast confidence for southern Atlantic States are high;
Forecast confidence for middle Atlantic States are moderate.
Across the Southeast, La Niña MAM composites
indicate the highest negative precipitation anomalies along the central and eastern Gulf Coast, from southern Louisiana
eastward to northeast Florida. The CPC monthly and seasonal outlooks indicate the highest odds for below median precipitation
along the eastern two-thirds of the Gulf Coast, and lower odds along the western Gulf. Subnormal precipitation odds in both
the monthly and seasonal outlooks quickly decrease to equal chances as one heads north, and actually transition to above
median precipitation for March in Tennessee and areas northward. Therefore, drought is expected to persist or develop across
most of the Southeast, except in northern sections of Mississippi and Alabama where initial conditions are wetter and the
monthly outlook favors near to above median precipitation. Due to the weak and mixed signals among the precipitation tools
beyond mid-February, some improvement is forecast in western Mississippi and the northern border of Georgia and Alabama.
Forecast confidence for the immediate Gulf Coast States are high;
Forecast confidence for the remainder of the Southeast is moderate.
From mid-November into late January, northwesterly
flow, attributed to a strong and persistent negative Arctic Oscillation/North Atlantic Oscillation (upper-air blocking
pattern), resulted in abnormally cold and dry conditions across the lower Ohio, Tennessee, and middle Mississippi Valleys.
This in turn caused a slight expansion of moderate drought and abnormal dryness in northern Arkansas, southern Missouri, and
southern Illinois. Furthermore, the persistent and strong AO/NOA suppressed the expected winter surplus precipitation that
normally occurs in the Ohio and Tennessee Valleys during a La Niña. Currently, with the AO/NAO a non-factor and less
influential during spring, the CPC monthly outlook favors a tendency for above median precipitation across the Tennessee and
Ohio Valleys. In the lower Mississippi Valley, the seasonal La Niña MAM composites also point toward above normal
precipitation although the frequency is rather low. In addition, the seasonal CPC outlook has equal chances. And as already
mentioned, most forecast tools and the CPC outlooks favor enhanced odds of below normal precipitation as one nears the Gulf
Coast. Accordingly, mprovement is forecast in the Ohio and Tennessee Valleys and into northern Arkansas, while some
improvement is expected for the rest of Arkansas.
Forecast confidence for the Ohio and Tennessee Valleys are moderate to high;
Forecast confidence for Arkansas is moderate.
In most of Texas, near to above normal
precipitation has fallen since late December, resulting in a reduction of drought coverage and severity across southern,
southeastern, and eastern sections of the state. Surplus fall precipitation had carried the Panhandle into the winter without
any drought impacts. However, forecasts favor drier and warmer than normal conditions during March which continue through the
spring, although subnormal seasonal precipitation odds are less than the monthly outlook. Due to these relatively dry and
warm forecasts, drought is expected to continue (or worsen), and return to northern and southern Texas.
Forecast confidence for Texas is moderate to high.
Following a very dry January, two heavy snow storms
blanketed parts of the south-central Plains in early February, but missed most of the main drought areas. Some improvement is
forecast in eastern Oklahoma where heavy snow occurred and the La Niña MAM composites indicate some wetness (but with a low
frequency of occurrence). Elsewhere in the central Plains, precipitation tools at most time ranges indicate an elevated
chance for below median precipitation, especially the La Niña MAM composite which has a high frequency occurrence. Just to
the north, however, forecast tools point toward wetness, plus soil moistures are extremely high (above 90th percentile). This
limits any northward drought expansion due to existing moisture conditions. Accordingly, the persistence area in eastern
Colorado and western Kansas was kept from the previous outlook issued on February 3, while the area of development in the
central Great Plains slightly expanded northward and eastward in line with the monthly and seasonal outlooks, but confined by
the very wet soil moisture conditions in the northern Plains and upper Midwest.
Forecast confidence for the south-central and central Plains are moderate to high.
Since mid-December, drier than normal conditions
have affected much of Arizona and New Mexico. As of February 15, SNOTEL average river basin snow water content values are 30
to 70 percent of normal in southern and central Arizona and New Mexico, with basin average precipitation since October 1
running at about the same values (32 to 75 percent of normal). Precipitation tools at all time ranges indicate enhanced odds
of below median precipitation and above normal temperatures. Due to a lack of adequate precipitation this winter, a tendency
for dryness during La Niña, forecasts of below median precipitation and above normal temperatures, and decreasing
precipitation climatologies for areas already in drought, drought persistence and development can be expected across much of
Arizona and New Mexico, and into southeastern Utah and southwestern Colorado. However, expansion is not forecast for southern
California as many locations have already exceeded their normal winter precipitation and approached their normal ANNUAL
totals from December's excessive precipitation.
Forecast confidence for the Southwest is high.
During the 2010-11 winter, heavy rainfall
alleviated drought and dryness on Kauai and Oahu and diminished drought conditions across the rest of the central and
western Hawaiian Islands which is typical for a La Niña winter. Enhanced rainfall during early-to-mid February and the
ongoing La Niña favor additional improvement in Hawaii.
Forecast confidence for Hawaii is high.