Graphical outlook for increased rain chances, May 15th to May 18th, 2008 (click to enlarge)
Drought Conditions Now Severe to Extreme
May 14th Update: Additional Rainfall through May 18th Should Help

What's New
Rainfall and Drought Conditions: Drought conditions have improved a little since the end of April, particularly in the Rio Grande Plains of central Jim Hogg and northern Zapata County, where a complex of thunderstorms during the early morning dropped between 1 and 3 inches on May 5th. While locally oppressive heat returned after the rains, it was generally accompanied by much more humidity than had been seen earlier this spring. On the evening of May 11th, a few showers edged through Zapata County, dropping a little more rainfall. Still, the local nature of the rains, leaving many areas of the Lower Rio Grande Valley at a significant rainfall deficit for the past several months, is maintaining solid Severe to Extreme Drought conditions, for now.

These conditions could edge down another notch after the weekend of May 15th through 17th. A definitive pattern change is underway across the western half of the United States in the short term. A high amplitude ridge of high pressure, extending along the Pacific coast and creating an unusually early heat wave in California, will allow energy to dive south and southwest ahead of it. This combination will further strengthen the subtropical jet stream across northern Mexico and South Texas, and disturbances riding along this jet stream will combine with warm, humid air at the surface, to increase the likelihood of widespread, perhaps locally heavy, rainfall across the Lower Rio Grande Valley.

The Heat Is Off, for Now: After a protracted period of rather hot and unusually humid conditions for early May, the expected pattern change and increased cloud cover will allow daytime temperatures to drop back into the more reasonable 80s, or perhaps a bit lower should more widespread rains develop. The lower temperatures, combined with still high dew points, will maintain high relative humidity across the Valley, which will further aid moisture retention and help alleviate drought conditions, at least a little bit. Even with the cooler period, temperatures are expected to end up above average for the March through May period.

The relief may not last long, particularly as the region continues heading into the hottest time of the year. Much will depend on how the atmospheric pattern evolves over the next month or so. April was yet another month with above normal average temperatures, ranging from 1 to 3°F. While the long lead outlook is for "equal chances" of above or below normal temperatures through July (below, center), that is little comfort considering average daytime readings soar well into the 90s to around 100, area-wide, by mid July.

Agricultural Impacts: Needless to say, the worsening drought conditions continue to rapidly dry out near surface soils, and soil moisture across most of Deep South Texas and the Lower Rio Grande Valley is abnormally to excessively dry. According to the United States Department of Agriculture and the Texas A&M Agrilife Extension Service, the worsening drought is beginning to take a toll on crops and cattle alike. Dryland crops continue to be increasingly stressed by the persistent winds and warm, dry weather. The potential exists for heavy crop losses in non-irrigated soils. Corn, cotton, spring vegetables, sorghum, onions, cantaloupes, and watermelons were faring well under irrigation. Producers continue to provide supplemental feed to local cattle ranches due to the lack of spring forage. Daily evaporation rates have slowed a little, but are still notable, averaging one quarter to one half an inch.

Wildfire Danger: The continuation of above normal temperatures, abundant dry fuels, occasional surges of very low humidity, and persistent winds, continued to maintain a moderate to high threat for wildfire development and growth through most of April. The recent rainfall did little to reduce the Keetch-Byram Drought Index (KBDI) across areas in excessive drought, where values continue to exceed 700, or high to extreme. Where heavier rains fell, the KBDI dropped below 500 (eastern Cameron through coastal Kenedy County), but this development may be temporary, at least for the first half of May. Burn bans remain in effect for all but Kenedy County, and residents are strongly urged to use extreme caution when disposing of lighted materials, and refrain from any outdoor burning activities.

Hydrologic Impacts: Water storage levels, while remaining fairly high, continue to decrease during this warm, dry spring. At Amistad Reservoir, storage has dropped to 74 percent of normal conservation; storage at Falcon Reservoir is now down to 41 percent. Water storage levels are expected to continue decreasing across the Lower Rio Grande Valley reservoirs in response to increasing demands from municipalities, and the need for peak irrigation during the heart of the spring growing season. According to the State of Texas Commission on Environmental Quality, there are currently three public water systems in Hidalgo County under mild rationing.

Meteorological Outlooks
Through May 14th: Guidance from the Climate Prediction Center (CPC), as well as medium to long range global models, indicates that the pattern of fast moving systems across the temperate regions of the U.S. will continue through at least the first half of May. For Deep South Texas and the Lower Rio Grande Valley, this could well mean that the welcome rain event on April 27th and 28th was only an aberration, and be replaced by the persistent south to southeast flow of warm to hot air across the entire area, with frequent periods of drying, compressed heat out toward the Rio Grande Plains. Hot and dry weather will be the rule through part of the weekend in these areas, with perhaps a brief respite in the heat with a chance for some rain before the hot and dry conditions return between May 6th and May9th. Readings above 100 are expected on a number of days across the hardest hit areas of Starr, Jim Hogg, western Hidalgo, western Brooks, and Zapata Counties.

Long Term Outlook: Longer range guidance from CPC indicates that the moderate La Niña pattern will likely continue into early summer. This persistent pattern is likely to maintain near to above normal temperatures and below normal rainfall for the region through July. As a result, extreme to excessive drought conditions are expected to continue to spread further across Deep South Texas and the Lower Rio Grande Valley. After July, CPC expects that the current La Niña pattern could weaken toward a more neutral condition as we head into late summer and toward autumn. This weakening La Niña pattern, combined with favorable flow patterns that increase rain chances in August and beyond, may provide some improvement of the current drought conditions affecting the region. At this point, the best bet for widespread relief would be copious rains generated by tropical cyclones.

Spring Flood Outlook: With aggregate rainfall forecasts to remain below average for the next several months, the potential for widespread spring flooding remains virtually nil. That said, one must remain alert to localized slow moving or stationary thunderstorms as we move toward summer, and low level atmospheric moisture continues to increase. 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 April 29th Texas Drought Monitor; The latest May through July forecast temperature departures from average, and the latest Mayl through July forecast precipitation departures from average. The Rio Grande Valley is inside the small box. Click each for a larger image.

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