Right now, Houston is in the midst of a catastrophic flood disaster as tropical storm Harvey, which made landfall as a Category 4 hurricane on the central Texas coastline Friday night, continues to unleash torrents over the Houston/Galveston metropolitan area. Harvey is expected to drop an additional 15 to 25 inches of rainfall over the next few days, which, combined with 1-2 feet of rain that fell over the weekend, has created a “worse than worse case scenario for Houston,” and could lead to some of the highest rainfall totals the nation has ever seen.
Tropical storms get their fuel from warm, wet air evaporating off the ocean. After making landfall, they tend to dissipate quickly, losing energy and organization as they blow across cooler, drier, inland air masses. What makes Harvey both incredibly dangerous and highly unusual is that it has barely budged over the last few days, channelling nonstop belts of rainfall from sea to land. “There is virtually no precedent for such a slow-moving system maintaining at least tropical storm strength along the Texas coast for five days,” meteorologists Jeff Masters and Bob Henson wrote yesterday on their weather and climate science blog, Category 6.
So, why won’t Harvey leave Texas alone?
“The problem with Harvey is that it is trapped,” Phil Klotzbach, atmospheric scientist and tropical storm expert at Colorado State University, told Gizmodo in an email.
Harvey’s unusual trajectory (or lack thereof) stems from the fact that it’s stuck between two areas of strong upper-level high pressure, one in the western US, and another centered around the southeast. “Hurricanes effectively move as pebbles in a stream, that is, they are steered by large-scale weather patterns,” Klotzbach said. “The combination of these high pressure areas means that the storm is currently stationary.”
A tropical storm stalling out due to weak steering currents isn’t so unusual in itself, but the additional circumstances surrounding Harvey have created a nightmare scenario for coastal residents. “What makes this exceptional is that (a) it’s a former Category 4 hurricane, (b) it’s located very near a U.S. coastline, and (c) the upper-level pattern is going to be very persistent, which means the stall could last for days,” Henson explained via email.
Harvey is trapped close enough to the Gulf that it’s able to siphon an endless supply of energy and moisture from exceptionally warm waters, which are also likely to have contributed to the storm’s rapid intensification last week. Unfortunately, some weather models are now showing that Harvey could drift back out to sea over the next few days, re-intensifying a bit before hammering the coastline again.
According to Klotzbach, the closest analog may be tropical storm Allison (2001), which tracked slowly across Texas and Louisiana for several days, dropping over 40 inches of rainfall on some parts of Houston. That storm, which flooded more than 70,000 homes, was much weaker than Harvey.
Events like this always cause folks to ask about the connection between hurricanes and climate change. The science is still evolving, and the answer is not simple—while theory suggests that higher sea surface temperatures will yield more intense storms, whether we’re actually seeing an uptick in cyclone intensity worldwide is unclear. “The trend signal has not yet had time to rise above the background variability of natural processes,” according to a recent draft of the National Climate Assessment.
Still, climate scientists generally agree that elevated sea surface and air temperatures play a role in intensifying storms, so it wouldn’t be a shock if future attribution studies linked features of Harvey to rising temperatures. And storm surges—one of the most dangerous aspects of tropical cyclones in terms of life and property—are being made worse by sea level rise.