Spring Drought/Flood Outlook
for the Lower Rio Grande Valley


Dry, Warm Weather Expected to Continue

Existing Conditions
Rainfall: Rainfall totals have been below normal during the winter (December - February) over Deep South Texas and the Lower Rio Grande Valley. During this time period, rainfall totals have averaged 3 inches or less over the region. Brownsville had its 14th driest winter on record (since 1878), and the driest since 1999. Harlingen had its 12th driest winter on record (since 1911), and McAllen recorded its 19th driest winter since 1941. Rainfall over the region has been below normal since August 2007.

Soil Moisture: Below normal rainfall and above normal temperatures have allowed for soil moisture to be below normal over Deep South Texas and the Lower Rio Grande Valley. Daily evaporation rates during the past month have been a quarter of an inch to near half an inch.

Reservoir Conditions: Storage at Amistad is at 87 percent of normal conservation. Storage at Falcon is currently at 51 percent of normal conservation. Falcon storage levels are expected to decrease during the next few weeks in response to an increase in water releases. These releases are expected as peak irrigation season begins over the Lower Rio Grande Valley.

Drought Conditions: Moderate drought conditions persist over much of Deep South Texas. Portions of Zapata County are experiencing severe drought conditions. Moderate to severe drought conditions are expected to persist through the spring.

Meteorological Outlook
Guidance from the Climate Prediction Center indicates that the strong La Niña pattern in the tropical Pacific Ocean will persist throughout the rest of March 2008. This will likely maintain a very warm and dry weather pattern over Deep South Texas and the Lower Rio Grande Valley. This pattern will continue a fairly steady progression of storm systems moving across the United States, generally from west to east throughout the remainder of March. This progression of storm systems will usher weak to moderate cold fronts across the area over the next few weeks. However, overall atmospheric moisture values with these systems will be fairly limited for Deep South Texas and the Lower Rio Grande Valley. Accordingly, isolated to scattered shower and thunderstorm activity is expected across the area between March 10th through March 14th, but not enough to make a notable dent in the dry conditions, considering the elevated winds. Significant and widespread soaking rainfall is not expected throughout the remainder of March.

Long Term Outlook
Longer range guidance from the climate prediction center indicates that the moderate La Niña pattern will likely continue throughout the rest of spring 2008. This persistent pattern will in turn maintain above normal temperatures and below normal rainfall for the region through May. As a result, drought conditions are expected to persist and even intensify across Deep South Texas and the Lower Rio Grande Valley. However, the Center expects that the current La Niña pattern will likely weaken a bit as we move into summer. This weakening La Niña pattern may allow for somewhat increased rain chances during the summer, which may provide some improvement of the current drought conditions affecting the region. Near to above normal temperatures may continue into summer for Deep South Texas and the Lower Rio Grande Valley.

Spring Flood Outlook
Historically, March is in the driest month of the year for Deep South Texas and the Lower Rio Grande Valley. With aggregate rainfall forecasts to remain below average for the next several months, the potential for spring floodins is considered low. That said, one must remain alert to localized slow moving or stationary thunderstorms as we move toward summer. Such storms can produce very heavy amounts of rainfall in a short period of time, causing flash flooding of poor drainage areas, arroyos, and other prone locations.

Below are graphical data showing, from left: The March 6th Texas Drought Monitor; The latest March through May forecast temperature departures from average, and the latest March through May forecast precipitation departures from average. The Rio Grande Valley is inside the box. Click each for a larger image.

Texas Drought Monitor, March 6th, 2008 (click to enlarge) Climate Prediction Center probability of departure from normal, temperature, March through May, 2008 (click to enlarge) Climate Prediction Center probability of departure from normal, rainfall, March through May, 2008 (click to enlarge)
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