![]() ![]() The National Climate Assessment issued in 2014 noted that heavy downpours are increasing across the country, but especially in the Midwest and Northeast, which, in this case, includes Maryland. In a world warmed due to human emissions of greenhouse gases, there is ample evidence that extreme rainfall amounts have increased and are likely to increase further. When these human-factors are put together with an extreme atmospheric “factor,” like these incredibly heavy thunderstorms, the risk of disaster increases. This has turned permeable surfaces like soil into impervious ones like asphalt and concrete. ![]() Plus, in general, land across the United States is much more paved than it was fifty to a hundred years ago. ![]() A wet April in the Mid-Atlantic had led to soggy soils, limiting the amount of additional water that could be absorbed. In this case, a couple of factors exacerbated the flash flooding risk. How the local terrain funnels the resulting unabsorbed water determines which areas might get damaged. When rain falls incredibly fast over any terrain, it’s impossible for the ground to absorb all of it once. For a deeper dive, check out this article. The new ticket doesn’t magically “know” your past history in the lottery. The reason you (very, very likely) won’t win again is because winning the lottery is such an extremely rare thing, not because you have won recently. Now imagine one day you win the lottery (somehow) but decide to keep buying lottery tickets in the future. The chances of winning the lottery are unbelievably low. Provided the climate is not changing, those numbers are true for the start of any year, so even after a thousand-year event occurred in 2016, there still was a 0.1% chance of another event of similar magnitude happening in 20 (and 2019, 2020, 2021… you get it). All a one-in-a-thousand-year event means is that at the start of any given year, there is a 1/1000 or 0.1% chance of a rainfall event of that magnitude occurring. No! But I completely understand the confusion. Don’t we have to wait a thousand years for another thousand year event? Remarkably, less than two years later, the same area observed even more rain in another event. At the time, that event was also called a one-in-a-thousand-year event, since statistics based on historical records estimate it had a 0.1% chance of occurring in any given year. ![]() Two years ago in late July, a similar moisture-rich atmosphere dropped nearly half a foot of rain in just a couple of hours, leading to unbelievably damaging flash floods. The last several days must feel like reliving a horrible nightmare for residents of Ellicott City. graph based on data from NWS Advanced Hydrological Prediction Service. The Patapsco River near Ellicott City, Maryland, went from normal to major flood stage in a little over an hour following a thousand-year extreme rain event. And as the heavy rain poured onto pavement and other hard surfaces north of the city, it drained downhill, turning local streets into raging rivers and causing destruction along the way. Sadly, Ellicott City is located smack dab in that funnel. Thanks to the local topography of the region, all of this water was funneled towards local tributaries and the Patapsco River. Or said another way, these extreme rains were a 1-in-500 to a 1-in-1000-year event. Rain gauges in Catonsville measured more than ten inches of rain, while Ellicott City saw around eight inches of rain.īased on historical records, rainfall amounts this large over this short a period of time have a 0.1-0.2% chance of occurring in any given year. On May 27, an atmosphere laden with moisture was triggered to unleash a barrage of rain to the west of Baltimore, dropping more than six inches of rain in several hours. Mud and flood debris blanketed a section of main street below an overpass in Ellicott City, Maryland, after flash flooding in late May 2018. ![]()
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