With Rains like These, Who Needs Hurricanes?
August 13, 2016
A devastating flood event was unfolding over southeast Louisiana on Friday, and conditions may get worse yet, as an extremely slow-moving center of low pressure is dumping colossal amounts of rain on the region. This sprawling, “stacked” low is carrying more water vapor than many tropical cyclones, and its slow motion is leading to persistent rains that could add up to all-time record totals in some places.
Multi-sensor analyses indicate that several areas in southeast Louisiana and southermost Mississippi racked up more than 6” of rain from 7:00 am CDT Thursday, August 11, to 7:00 am Friday (see Figure 1). More than 10” of rain was analyzed just northeast of Baton Rouge, the hardest-hit area thus far. In the 24 hours from 2:00 pm CDT Thursday to 2:00 pm Friday, Baton Rouge Metropolitan Airport recorded a preliminary total of 8.49” of rain. Since records began in 1892, the city’s largest calendar day total is 11.99” (set on April 14, 1967), and the largest two-day calendar total is 14.03” (June 6-7, 2001). Given the very slow motion of the stacked low, these all-time records are conceivably within reach. A cooperative observer in Livingston, LA, reported 17.09” of rain from midnight to 3:00 pm CDT Friday. The state’s official 24-hour record is 22 inches, reported near Hackberry on August 28-29, 1962.
By mid-morning on Friday, more than a foot of rain had fallen near Kentwood, Louisiana, in just a 12-hour stretch — a downpour with an estimated likelihood of just once every 500 years, and roughly three months’ worth of rainfall during a typical hurricane season. It’s the latest in a string of exceptionally rare rainstorms that are stretching the definition of “extreme” weather. It’s exactly the sort of rainstorm that’s occurring more frequently as the planet warms.
In response to the ongoing heavy rains, Louisiana Governor John Bel Edwards declared a statewide state of emergency on Friday, and local governments are distributing sandbags, conducting water rescues, and facilitating evacuations. The New Orleans Times-Picayune is maintaining a live blog of the latest developments. The Tickfaw River north of New Orleans soared 18 feet in about 12 hours to a new record crest on Friday morning, beating the water level of April 1983, and five feet higher than the high-water mark during Hurricane Isaac in 2012, the last hurricane to make landfall in Louisiana.
Meanwhile, a lot more rain is still on the way. High-resolution weather models predict an additional one or two feet of rain by Saturday evening, a total the local National Weather Service referred to as “scarily high.” The NWS has issued its highest alert for excessive rain and warned of “significant to catastrophic flash flooding.” A “flash flood emergency” is in effect for the hardest-hit regions, a warning reserved only for the direst and most life-threatening events.
Obviously, this is no ordinary storm. Though the overall structure of this meteorological event does not meet the technical requirements for a tropical storm or hurricane (it’s attached to a stalled weather front, for example), the NWS is treating it roughly the same way, and the physics of the rain clouds themselves are similar. (Tropical rain clouds are generally more efficient at converting cloud moisture into raindrops.)
This storm’s tropical nature, in combination with record-warm water temperatures just offshore in the Gulf of Mexico, are creating a nearly perfect environment for extremely heavy rain and record flooding in one of the wettest places in the country. As the atmosphere warms thanks to greenhouse gas emissions, it can hold more water vapor — and this effect makes it exponentially more likely that extreme rainfall events will occur. The weather balloon released on Friday morning from the New Orleans office of the NWS measured near all-time record levels of atmospheric moisture, higher than some measurements taken during past hurricanes. The NWS meteorologist who reported this morning’s reading remarked simply, “obviously we are in record territory.”
Although 2015 saw a Northern Hemisphere record for major hurricanes, Cat 3 and above. Still we hear climate deniers talking about a “hurricane drought”. Only for those who look beyond the Dogpatch County line.
In fact, the major “hurricane drought” in these stories is so arbitrary and frankly irrelevant to the lives of most Americans living along the coast as to render it beyond misleading. A journal article published over two months ago — aptly titled “The Arbitrary Definition of the Current Atlantic Major Hurricane Landfall Drought” — pointed out:
From a societal context, human and financial losses matter most, and Irene [2011; $8 billion (U.S. dollars)] and Sandy (2012; $88 billion) occurred during the current drought.
D’oh! We could get hit by a Sandy every year — heck, every month — for the entire century and still be in this so-called drought!
How is that possible? Well, the primary “drought” people are talking about (yes, there is more than one) is, as the Post put it “A major hurricane hasn’t hit the U.S. Gulf or East Coast in more than a decade…. The streak has reached 3,937 days, longer than any previous drought by nearly two years.”
But wait, you say, Sandy and Irene were both Category 3 hurricanes — and Ike (2008) was a Category 4 — and the definition of a “major hurricane” is a Category 3, 4, or 5 hurricane. True, say the drought-ists, but the semantic drought we’re talking about is a drought in hurricanes that were major when they made land-fall!
But wait, you say, Sandy was the second costliest hurricane in U.S. history (after Katrina), and “truly astounding in its size and power,” as hurricane Hunter and meteorologist Dr. Jeff Masters has explained. It had a “larger area of tropical storm-force winds” than any hurricane on record, and “At landfall, Sandy’s tropical storm-force winds spanned 943 miles of the the U.S. coast”:
At its peak size, twenty hours before landfall, Sandy had tropical storm-force winds that covered an area nearly one-fifth the area of the contiguous United States…. Sandy’s area of ocean with twelve-foot seas peaked at 1.4 million square miles — nearly one-half the area of the contiguous United States, or 1% of Earth’s total ocean area. Most incredibly, ten hours before landfall, the total energy of Sandy’s winds of tropical storm-force [was] the highest value for any Atlantic hurricane since at least 1969. This is 2.7 times higher than Katrina’s peak energy, and is equivalent to five Hiroshima-sized atomic bombs.