The images of the devastation from Houston after Hurricane Harvey are incredible, in the absolute worst sense possible (until, perhaps, images of Hurricane Irma's devastation come along). The official death toll from the floods stood at about seventy as of Thursday, September 7, and it may wind up as the costliest natural disaster in U.S. history (again, pending Irma), with estimated damages approaching $200 billion. America's fourth-largest city is completely devastated, with the cleanup and recovery process barely under way.
While Harvey made landfall in south Texas as a Category 4 hurricane with winds measured as high as 132 miles per hour, the devastating amount of rainfall is what took this storm from a really strong hurricane to one that'll forever be linked with a city, just as any mention of New Orleans immediately brings up memories of Katrina.
Comparing Harvey's rainfall numbers to Denver's might help explain the staggering scope of devastation. And it might help illustrate how astonishing those numbers truly were. Let's start on the conservative side of things:
Houston's official five-day total rainfall for the storm came out to 31.26 inches, as measured at the city's George Bush Intercontinental Airport on the north side (the highest overall totals in metro Houston were generally on the east and south side).
The 31.26 inches of rain that Houston saw is equivalent to every raindrop and snowflake that landed on the City of Denver between May 20, 2015, through today. In other words, in five days Houston saw what Denver saw in the last two-plus years, or about 27 months.
Unimpressed? Consider this: Houston's highest official one-day total during the height of the storm, 16.07", on August 27, was more than Denver's average rainfall in a full year (14.30"). That's right: Houston saw more rain in a day during Harvey's peak than Denver gets in a year.
It gets even weirder when you compare the highest observed total for the entire storm, from a rain gauge in Cedar Bayou, about thirty miles east of Houston: a ridiculous 51.88" in the same five-day period, a record for the most rainfall ever seen from a tropical cyclone in the lower 48. (The rain gauge actually went out just before the end of the storm, so the actual total is likely a bit higher.)
That 51.88" total is equivalent to every drop of rain and snowflake that Denver's seen since...May 21, 2014. That's a nearly forty-month stretch of Denver rainfall to measure up to what some parts of Houston saw in less than a week.
Could that kind of rain happen here?
The quick answer: no. But that doesn't mean Colorado doesn't get serious floods, of course.
Before you start building an ark here in Denver, consider this: It's very hard for significant tropical moisture, such as the kind that led to the devastation in Houston, to impact our area. The most likely scenario for tropical moisture to push into Denver would come from a tropical storm or hurricane in a completely different ocean basin: the eastern Pacific. Weakening tropical cyclones that move through northwest Mexico often impact Arizona, New Mexico and southern California, and every so often, some of that moisture can lift north into Colorado.
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But for any moisture to arrive in Denver or just along the Front Range, it would have to move through the Rockies, which would wring out most, if not all, of the moisture from a tropical system. The forced uplift from the 14,000-foot high mountains essentially serves as a block for big rainfall in the Mile High City, and that's the main reason that Denver only averages a little over a foot of precipitation a year.
Still, flooding — such as the kind that the Front Range saw back in September 2013 — can and does occur. Two main meteorological scenarios cause big flooding in Denver: slow-moving thunderstorms, and a slow-moving area of low pressure that can tap into moisture from the Gulf of Mexico and even the Pacific.
The latter scenario is what caused the 2013 floods in Colorado. An area of low pressure sat over the Four Corners and drew up Gulf and Pacific moisture for a week, creating a localized 1,000-year flood that caused at least a billion dollars' worth of damage. It was accompanied by a stationary area of high pressure in the Pacific Northwest that blocked in the low, leaving Colorado in a weeklong deluge.
This, by the way, is also the usual scenario for blockbuster snow in Denver.