Brace Yourself for Fall and Winter Rainfall

19 10 2015

In its Fall 2015 Newsletter, the Brazos River Authority published an article indicating that those of us who reside in the Brazos River watershed, and that includes Fort Bend County, should be aware that we could experience more rainfall than normal this year.  The article is below:

A record-setting El Niño has the potential to deluge Texas with wet weather during the late fall and winter months, which could top off Brazos basin reservoir levels that have been dropping due to returning drought conditions. But the same weather phenomenon could also bring too much of a good thing, with potential flood conditions also a possibility in upcoming months.

The National Weather Service Fort Worth Office’s Climate Prediction Center projects that the El Niño conditions will continue into the spring.

What is El Niño? It is a weather pattern which begins in the Pacific Ocean and results in changing wind patterns and water temperatures. These weather patterns don’t just stay in that area, but move to other places, and affect the weather in the United States.

Climate experts say the current El Niño pattern is one of the largest ever observed, and El Niño is expected to get stronger in the coming months.

What will the impact be for the Brazos Basin?

The Pacific Jet Stream is expected to bring wetter conditions than usual to North and Central Texas, along with cooler winter temperatures. Interestingly, the NWS reports that outbreaks of extreme cold weather are actually less likely during strong El Niño winters, with fewer days of freezing temperatures. So while the weather may be cooler, extremes are less likely.

It is the increased cloud cover and precipitation that should result in lower than usual daytime high temperatures.

Texas experienced a wetter than usual spring this year, but the heat and dry weather of July and August allowed the drought conditions to return to the Brazos basin and other parts of the state.

If El Niño brings significant rainfall, as expected, it will end the ongoing “flash drought.” A flash drought is one that develops quickly but does not last long. Also, because the wetter weather is expected through much of the 2016 spring, that could prevent or delay drought conditions from returning later next year.

Flooding is always a concern, and can occur even during periods of drought, the NWS reports. During a prolonged wet period, however, the likelihood of flooding greatly increases because of saturated soil.

This past spring, the rainfall filled depleted reservoirs. This fall and winter, with most reservoirs remaining close to full, “the runoff from heavy multiple rain events would likely surge lake levels back into their flood pools,” the NWS warns.

Since data has been collected on El Niño patterns, there are El Niño events that can be classified as strong, and only two of these (1991-92 and 1997-98) were extraordinarily wet. However, those two seasons did not have much freezing precipitation. Two years that did have more freezing precipitation than normal were the 1965-66 and 1972-73 seasons, which interestingly, did not have higher precipitation levels overall.

State climatologist’s forecast

State climatologist John Nielsen-Gammon said that we can’t predict with certainly what this El Niño season will bring, but we can be pretty sure of some wet weather.

“Most El Niño events produce above normal rain across Texas, especially from late fall to early spring,” he said. “But the two strongest El Niño events in history were associated with near-normal to slightly below-normal rainfall in Texas. It may be that there’s a sweet spot for El Niño and Texas rainfall, and the strongest El Niño events overshoot the mark. The fourth-strongest El Niño (since 1895) was the 1991-1992 El Niño that helped produce extensive flooding along the Brazos River in December 1991. About the only thing that seems possible to rule out at this stage is the possibility of being substantially drier than normal over the next nine months,” Nielsen-Gammon said.

“Because Texas is near the southern edge of the jet stream on average, storminess increases during El Niño and decreases during La Niña. In general, the chances of wet weather in the winter increase across the southern United States and decrease a bit in the northern United States. Temperatures in the southern United States tend to be a bit cooler than normal during El Niño, and warmer than normal in the northern United States.”

How certain is the impact of an El Niño pattern?

“In some places the effect is more reliable than others,” Nielsen-Gammon said. “In Texas the effect is especially reliable, but even so, it only accounts for about a third of the year-to-year weather variations we experience. Since 1950, Texas as a whole has never had a November through March significantly drier than normal during an El Niño.

“An El Niño winter is typically cold, wet, and cloudy. The best overall adjective would be ‘yucky.’ But think of all the food that’s going to be grown next spring that will have a good solid start thanks to the wet weather over the winter.”

Brazos basin status and forecast

Despite a hot and dry summer that lingered into early fall, the unusually rainy spring resulted in full reservoirs in the Brazos basin. While reservoir levels have declined some as a result of the recent hot, dry conditions, they are still “in pretty good shape,” said Brad Brunett, water services manager for the BRA.

“Reservoirs typically lag behind the onset and ending of drought conditions,” he said. “Following a wet period when lakes fill up, it may take several months of dry weather before the effects are seen in lake levels starting back down. Toward the end of a drought when rainfall starts increasing, it takes a while before enough runoff is generated to start positively impacting lake levels. Because of the rain earlier this year, we’re in good shape heading into fall, and that should be the case heading into next summer, too.”

If El Niño brings heavy rains, what will that mean for the basin?

“If we experience higher than normal rainfalls, there’s a good chance we’ll have to pass more flood water through BRA reservoirs, and the (US Army Corps of Engineers) reservoirs may be up in the flood pools again,” Brunett said.

“The good news is that we should be in great shape as far as water supply heading into the summer months.”