Storm was on weather steroids

On Monday night the heavens opened over Houston, dousing the city with some of its heaviest rains since the near biblical floods of Tropical Storm Allison.

While forecasters predicted a wet Memorial Day weekend, no one expected as much as 1 foot of rain to fall during a single night, nor the ensuing mayhem caused by widespread flooding.

So what caused these intense storms to regenerate and dump 162 billion gallons of water on Harris County? There’s only one answer: the Gulf of Mexico. It provided the additional fuel for storms that, had water been collected across the county, could have filled the Astrodome more than 500 times.

Forecasters had good reason to be confident about rain this weekend.

Heading into Memorial Day weekend, the greater Houston region lay more or less beneath the jet stream, a large river of swiftly moving air about 30,000 feet above the Earth’s surface. This location beneath the jet stream provided the impetus to lift air at the surface into

the upper levels of the atmosphere.

Moreover, it was clear the air at the surface was going to be moist this weekend. The best measure of this moisture is so-called precipitable water, or the total amount of water in a column of air from the ground to space. For Monday these precipitable water values were in excess of 2 inches, which is very high. That’s part of the reason why Memorial Day felt so muggy outside, almost pregnant with rain.

So it was no surprise when a large line of thunderstorms approached Houston from the west around sunset on Monday. This system was forcing moist air at the surface to rise, which generated thunderstorms.

Reflecting the very moist, tropical nature of this air, these thunderstorms produced in excess of 4 inches of rainfall per hour in some locations across west and southwest Houston.

Storm train

Typically as these systems move through a region they work over the atmosphere, as if wringing the moisture out of a wet towel. Such lines of storms normally produce 2 to 4 inches of rain, perhaps more where the most intense storms develop. But then the system moves on.

Only this didn’t happen Monday night. As the initial line swept into west Houston it slowed, and another line of storms formed behind it. And then another. The storms were lining up one after the other like freight trains.

Something was fueling these newly regenerating storms so that they didn’t burn out, moving eastward in search of more moist air to wring out.

Instead of being depleted of moisture, the atmosphere over west Houston, from Katy to Sugar Land to Missouri City, was being fed by a feature known as the “low-level jet” — essentially a fast flowing of current of air about one mile above the surface of the earth. In this case, the low-level jet acted as a conveyor belt to bring more moisture northward from the Gulf of Mexico, essentially up Texas Highway 288, allowing the storms to redevelop.

The result was lots of rain, which occasionally is to be expected living next to a large, warm, tropical body of water such as the Gulf of Mexico.

Rains this spring — as much as 40 inches have now fallen across parts of Houston during the past three months, nearly the city averages in a given year — have definitively ended a drought that began in 2010.

Now the question becomes whether this wet pattern will extend into the warm summer months, when there will be still more ample fuel from a warming Gulf for additional flooding events.

In the immediate future, it appears the region will see a typical summertime pattern on Wednesday and Thursday, with scattered thunderstorms developing during the afternoon, sparked by daytime heating. This break from widespread storms should allow swollen bayous to ease back into their banks, and for flooding on parts of the San Jacinto and Trinity rivers to crest and recede.

However, another upper-level low-pressure system could very well approach the region this weekend, brought on in part by the jet stream, and again providing the lift needed for additional storms. Given the sodden ground, the possibility for these kinds of storms will have to be watched closely.

Hurricane season

Looking further ahead, other threats loom. The Atlantic hurricane season starts Monday. While a strong El Niño like that now developing in the Pacific Ocean often dampens activity during the peak months of the Atlantic season — August and September — Texas could easily be threatened by a rainmaking tropical storm in June or July.

Seasonal forecast models, which attempt to forecast overall weather patterns a few months ahead, aren’t overly accurate. But for what it’s worth, they’re generally predicting a wetter than normal June followed by drier conditions in July and August.

One upside to the region’s wet spring has been lower daytime temperatures — because of clouds and rain. Houston has yet to record a 90-degree day this year, and is now nearly four weeks late for its first one. The city hasn’t gone this late into spring without hitting 90 degrees since 1997.

With more sunshine expected on Wednesday and Thursday of this week that could soon change.