Increased rainfall raises flood risk this spring

Weather Outlook 1

With a mild but wet winter in Texas’ rearview mirror, forecasters anticipated the spring season would have average amounts of rainfall, warm temperatures and a “near-average” flood potential for most of the Brazos River basin. However, March brought heavy rainfall to the upper and central portions of the basin. This change, along with increased chances of flooding in the upper portion of the basin, also increased the chances for rainfall to lessen drought conditions near the Gulf Coast.

Toward the end of winter, the Brazos River basin’s precipitation levels, soil moisture, water supply storage and streamflow were only slightly above normal. Therefore, the long-range outlook from the Climate Prediction Center for spring and early summer showed equal chances of above, average, and below precipitation along with a continued long-term warming trend. The El Nino-Southern Oscillation pattern was neutral as well, meaning there were no signs of an El Nino or La Nina occurring in the Equatorial Pacific Ocean. The neutral conditions that were expected to continue through the spring lead forecasters to predict a dull and uneventful season.

“Our spring weather will be determined by the vagaries of the weather, which so far have been good for northern and central Texas and bad for southern Texas.,” Texas State Climatologist John Nielsen-Gammon said in mid-March. “Areas around Waco and farther north have received an abundance of precipitation this winter. So, a potent storm system could potentially produce some flooding, since there’s not much extra capacity in the soil to take up more rainfall without producing runoff.”

Since the upper part of the basin enjoyed average rainfall and soil moisture levels through the beginning of March, a normal amount of rainfall would have kept the chance of flooding in the area at its predicted near-average flood potential. However, as the month progressed, most of the upper Brazos River basin received more than twice the normal amount of rainfall so far this year, causing predictions for higher river levels and the possibility of flooding to escalate.

In mid-March, more than five inches of rain fell directly on Possum Kingdom Lake in a matter of hours. The amount of rainfall and very wet soil levels meant there was no place for the rainfall to go, creating runoff that filled creeks and streams. Ultimately, Possum Kingdom Lake rose so quickly that four floodgates were opened in a matter of hours, an event that had not occurred in more than 10 years.

“For larger river basins, the amount of water already in the ground can be just as important for flooding as the amount of rainfall,” Nielsen-Gammon said. “With many soils nearly saturated with water, it does not take much additional rainfall to produce lots of runoff.”

The combination of high amounts of rainfall and saturated soil heightened the potential for additional spring and early summer flooding for the upper basin. But, while the upper and central portions of the Brazos River basin saw an abundant amount of rainfall during the winter and the beginning of spring, the lower portion of the basin still experienced below-normal precipitation and moderate to severe drought.

Weather Outlook 2

Fortunately, the drought conditions in southern Texas have improved as the spring season continues, despite receiving less rain than the northern areas. The southeast portions of the state may potentially improve or even exit drought classification by the end of spring. According to Nielsen-Gammon, the northern margin of the drought areas has decreased significantly and could disappear entirely north of the Bryan-College Station area. Recent rainfall in the region eliminated the chance of drought reemerging in this area.

“Based on the Seasonal Drought Outlook issued by the Climate Prediction Center on February 20th, the CPC anticipates a large portion of the ongoing conditions in southeast Texas to be removed by the end of May 2020,” said Katie Landry-Guyton, a National Weather Service senior service hydrologist, in an online statement.

However, the U.S. Seasonal Drought Outlook, provided by the National Weather Service, shows the coastal region of Texas remaining in a drought state through June. A few good rainstorms could tremendously help the drought conditions in the area.

Even with drought conditions in the southeast portion of the basin, the BRA’s water supply system has remained high, at 99 percent full.