It’s been a crazy month for severe weather on the southern Plains, with Texas and Oklahoma making a spectacular recovery from drought by drowning under more rain than they’ve seen in years. Leaving behind hundreds of victims and millions (if not billions) of dollars in damage, the rain will finally start to subside this week.
The disastrous rains culminated in a catastrophic flash flood event in Houston on Monday night, when a slow-moving squall line stalled out over the city near the Gulf Coast, dumping up to a foot of rain on Houston’s western suburbs. The above graphic is an infrared satellite image of the complex of storms as they raged over Houston on Monday. Cloud tops above the updrafts reached temperatures as low as -81°C (-113°F), or a few degrees colder than dry ice. Extremely cold cloud tops are a great indication of vigorous thunderstorms that can lead to copious amounts of precipitation, as Houston saw.
Almost all of the dangerous flooding we’ve seen in the region over the past couple of weeks came from intense thunderstorms training over the same location. Training occurs when thunderstorms move over the same city one after the other, like train cars on railroad tracks. Training is implicated in some of the worst flash floods we’ve seen in the United States (and in other parts of the world), and this was the case last night in Houston.
The rainfall totals were immense. Rain fell at several inches per hour for most of the night, overwhelming drainage systems and natural waterways and causing them to spill over their banks and inundate whatever was in the way. Some communities just west of downtown Houston wound up with around a foot of rain by the time the skies cleared out. Interstates, homes, parks, television stations, and even a major mall (The Galleria) were all affected by rising waters or the crush of extreme runoff. The map above shows the rain that fell between Monday morning and Tuesday morning.
The local National Weather Service office took the rare step of issuing a flash flood emergency for Houston and its suburbs during the catastrophe, urging residents to stay off the roads and seek higher ground if their home or business is threatened by rapidly rising waters. The situation was so bad that the Harris County emergency management office raised their alert status to Level 1, which they haven’t done since Hurricane Ike made landfall in 2008.
Earlier in the year, we talked about the stubborn pattern in the jet stream that sent a huge ridge of high pressure over the West Coast—keeping them hot and dry—while a persistent trough existed over the eastern half of North America, spinning off one blizzard after another in New England and allowing cold temperatures to filter south for extended periods of time.
We’ve had a similar situation leading to the recent streak of foul weather across the south-central Plains, where the jet stream had trough stuck just east of the Rocky Mountains, allowing one surface low after the next to form in the central Plains, allowing instability from the south to flow into a region of wind shear and allow severe weather outbreak after outbreak to take shape across Texas and Oklahoma.
Thankfully, the jet stream is going to pull back to the north over the next couple of days, allowing the southern United States to return to its typical regime of pop-up daytime thunderstorms rather than the daily end of the world we’ve seen since the first of the month.
Between May 1 and this afternoon (May 26), Texas and Oklahoma alone have seen 1,188 reports of severe weather, most of which have been centered around the Red River, which is a notorious hot spot for severe weather activity thanks to its location.
The flash flooding has been the biggest story from these storms, but the severe weather is a major concern as well. Some areas have seen hail up to the size of softballs, and strong tornadoes have destroyed hundreds of homes and businesses in these intense storms. One person died when a tornado struck his home in Milam County, Texas, on Monday, becoming the ninth person to die as a result of a tornado so far in 2015.
Severe weather will remain a threat to the region over the next couple of days—a tornado watch is in effect for central Texas as of the publication of this post—but the threat for flash flooding will subside. However, given the already-high water levels and ground saturated to capacity, it won’t take much heavy rain to trigger further flooding. Around northern and eastern Texas, it will only take rainfall rates of one to two inches per hour to allow flash flooding to begin, but it would take a brief heavy rain shower near Houston to do the same.