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 Ni? that has begun to weaken, with ENSO-neutral or La Nina 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 AR-LA-TX 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 Nina 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 Nina, and continued improvement is forecast for the islands remaining in drought.
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, improvement 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.
SOURCE: National Weather Service
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