The latest assault by severe storms on Friday in Southeast Texas has folks scratching their rain-soaked heads and wondering: Why has it been so rainy in Houston lately? Is this normal or is something going on? Here are five things we point to for some answers:

Time of the year

The National Weather Service establishes what it considers “normal” climate by finding the averages from 30 years of weather data, and forecasters are currently using the data set from 1991 to 2020. According to those averages, May is the third-wettest month for Houston, behind June and October, and it logs an average monthly rainfall total of 5 inches.

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However, when you take all of Houston’s weather records, even going back to the 1880s, May actually is the city’s wettest month, averaging 4.8 inches of rainfall each year.

Still, this year, rain gauges at Bush Intercontinental Airport, Houston’s official climate observation site, collected about 8.7 inches of rain in May, or close to 4 inches above normal. Storms in May are not news. May is typically the most active month for severe weather in Texas. But let’s look at some particular factors.

Instability in the atmosphere

Houston Chronicle newsroom meteorologist Anthony Franze said the weather patterns over the past several weeks can be attributed to a steady stream of moisture moving in from the Gulf of Mexico. Couple the moisture with temperatures statewide that have been hotter than normal, and you get an atmosphere has been especially unstable for weeks, he said.

During April and May in Texas, these severe weather ingredients come together most often. But by the time we get closer to summer, the upper-level jet stream starts to shift farther north, and Texas is left with far fewer atmospheric disturbances that can help to spark thunderstorm formation, Franze said.

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Series of shortwaves

With an unstable atmosphere, we only need minor atmospheric disturbances to trigger strong to severe thunderstorms, Franze said. Those atmospheric disturbances have included a steady stream of low pressure systems, weak cold fronts and stationary fronts lingering across the state almost every day this month.

Newsroom meteorologist Justin Ballard pointed specifically to how moisture-laden air flowing inland off the Texas Gulf Coast has interacted with upper-level atmospheric disturbances called shortwaves, which have pushed into the Lone Star State recently.

Shortwaves act as localized areas of low atmospheric pressure that tend to push warm air upwards in front of them, according to the weather service.

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“The shortwaves are really the primary driving force of the uncertainty in these (weather) patterns,” said Cameron Self, a meteorologist at the National Weather Service office overseeing the Houston and Galveston metro areas.

Heat dome in Mexico

Texas also sits on the periphery of a sprawling ridge of high atmospheric pressure aloft, often called a “heat dome,” centered over Mexico, Ballard said. A similar heat dome sat over Texas last summer, helping to produce scorching heat and exceptional drought for months. 

“Along the periphery of this high pressure, there have been numerous shortwave disturbances embedded in the west to northwest flow aloft,” Self added.

Remnants of El Niño

El Niño, the warming of tropical waters in the eastern Pacific that has brought wetter and cooler weather to Texas this winter, is already surrendering to La Niña, which is the cooling of those same waters. Although El Niño appears to be losing its influence in the atmosphere, broad large-scale atmospheric changes don’t happen overnight.

The effects of an unusually warm eastern Pacific, El Niño, can include a southward shift in the jet stream, a river of air flowing from west to east around the northern hemisphere. As the jet stream dips closer to Texas, it allows cooler air to encounter tropical air from the Gulf of Mexico. It’s that confrontation of cold and warm air that produces rain, which can lead to so many storms for Texas.

Forecasters at the Climate Prediction Center have said “the ocean part is weakening,” and that La Niña “could emerge as soon as June to August.”