Spring flood risk only slightly higher than normal
April kicked off with the central portion of the Brazos River basin receiving some of the highest three-day rainfall accumulations in the state.
The rainfall is just the beginning as the Brazos River basin moves into spring.
Texas State Climatologist John Nielsen-Gammon said the two concerns for river flooding are antecedent soil moisture and expected precipitation.
Soil moisture, which is a factor in determining the level of both drought conditions and how quickly water runs off during rain events, is looking near normal and precipitation outlooks from the Climate Prediction Center are tilted toward above-normal precipitation, but not dramatically so, Nielson-Gammon said.
“Taken together, the risk of flooding this spring is only slightly higher than normal,” he said.
He added that there’s no way to accurately predict the weather during a particular week several weeks in advance.
A spring flood outlook takes into account recent rainfall, soil moisture, streamflow and water supply conditions, said Maren Stoflet, National Weather Service senior service hydrologist, in a statement. Those figures are then partnered with 90-day temperature and rainfall outlooks to assess flood potential across the river basins, she said.
“It is important to note that the primary driving factor for river flooding in the spring is excessive rainfall in relatively short periods of time,” Stoflet said. “It is typical for this region to experience springtime thunderstorms that can produce excessive rainfall with rapid runoff to result in significant river flooding as well as flash flooding.”
The rain arrives as BRA reservoirs are at or near 100 percent capacity, except for the outlier Lake Whitney, which hovered at 88 percent at the beginning of April. Releases from the hydropower storage at Lake Whitney, which is controlled by Southwestern Power Administration, over the last several months has caused lower storages there than the remainder of the reservoirs within the basin.
While the reservoirs are near capacity, they looked a lot different a year ago.
In August 2018, 99 percent of the Brazos River basin was experiencing drought conditions. The drought was followed by what would be one of the longest releases from the uppermost reservoir in the Brazos River Authority system. Gates were opened at Possum Kingdom Lake’s Morris Sheppard Dam, and within six weeks, the basin was entirely drought free.
Though the spring months are usually some of the wettest for the Brazos River basin, the area is also approaching the Atlantic hurricane season, which begins June 1 and ends each year on Nov. 30.
The Atlantic basin hurricane season is expected to have slightly below normal activity, according to the Colorado State University Tropical Meteorology Project. The current weak El Niño event appears likely to persist and perhaps even strengthen this summer or fall, according to the project.
“We anticipate a slightly below-average probability for major hurricanes making landfall along the continental United States coastline and in the Caribbean,” according to the April 4 report. “As is the case with all hurricane seasons, coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them. They should prepare the same for every season, regardless of how much activity is predicted.”
The school predicts 13 named storms. Five of those are expected to become hurricanes while two are expected to become major hurricanes, according to the forecast, which is the earliest seasonal forecast issued by Colorado State University.