Irene, Katia… and a developing tropical depression in the Gulf

When it rains it pours.

This bit of folk wisdom, like so many, reflects an underlying reality. The Earth we live on does much of its business through extreme events. What we call continental drift manifests itself on the Earth’s crust as an unremitting series of disruptive earthquakes. Ecosystems manifest population explosions and collapses. Human history is punctuated by revolution and disruptive change. But nowhere do we see this truism more clearly than with respect to patterns of rain and snowfall over the Earth.

The proverb seems to hold on all time scales. A couple of weeks ago, at the Grand Canyon, the park ranger told us that most of the work done by the Colorado River in carving out the canyon over a six-million-year span was done brief periods of flood and large (and abrasive) sediment load driven by seasonal snow melt and weather extremes. She said such down-cutting of the canyon has been temporarily slowed by the building of the Glen Canyon dam, but that within a hundred years or so the dam will likely fail, and the carving of the Grand Canyon will resume. [And she seemed to be looking forward to that day!]

Closer to home, Hurricane Irene has served as the most recent reminder, especially for those of us living in the Middle Atlantic and New England states. We’ve all experienced with dismay the destruction occasioned by half of foot and more of rain across the Northeast. Kind of makes you think, that proverb should be extended, to say something like “when it rains, it pours…and when it pours, that’s just the beginning.”

Obscured a little by all this is something more subtle: the contribution of storms such as Irene to annual average precipitation. Prior to Irene, for example, the Washington DC area was running about five inches behind the annual average precipitation for the region. That is, so far this year, we’d gotten only about twenty inches of rain versus the more-normal figure of perhaps twenty-five. Today we’re an inch above/ahead of normal.

The same sort of story plays out worldwide. Estimates of tropical storm contributions to global precipitation vary, but the following figures are probably representative. Worldwide? Tropical storms maybe contribute 2-3% of the total (pretty remarkable, considering the storms are seasonal and are confined to lower latitudes). In the subtropics? Maybe 10-15% of the total. And then there are some more localized geographic areas where tropical storms provide as much as 20-40% of the annual average rainfall.

One such geographic area where tropical cyclone rainfall matters? It’s close to home – the southeastern United States. Here such storms may contribute something like 10-40% of the warm-season rainfall. This is only half the year…but it’s the half of the year that matters most for agriculture. Thus the year-to-year variability in the number and extent of land-falling storms may look to those of us in urban settings like corresponding fluctuations in weather hazard. But to many across this region, it also looks like just the opposite. It appears as year-to-year benefit to farmers, orchard growers, foresters, ranchers, et al.

A lot of folks in New England are still mourning the loss of loved ones, coping with power outages, and extended detours around all those washed-out roads, trying to put their lives back together. The rest of the country might well be breathing a sigh of relief for dodging the bullet. This should include nearly all urban New Yorkers, who can be thankful they didn’t lose high-rise windows or experience subway flooding – this time around.

But there is a next time…and that next time might be sooner than some of us might want. A tropical storm seems to be developing within the Gulf of Mexico…one that might prove to be fairly large in areal extent, relatively slowly moving, and on the verge of dumping a lot of rain on New Orleans and environs.

And out there in the Atlantic, Katia continues to move west and develop. And each day, it looks slightly more like it might reach the east coast, depending upon whose models you believe.

So let’s all remember two things: (1) when it rains it pours, and (2) these extremes are not simply curses, but a mixture of curse (to some) and blessing (to others).  

Oh…and maybe we should keep in mind a third thing: (3) we haven’t built our homes and places of work, our schools and our hospitals in locations, or in such a way, as to be immune to these extremes. So, until further notice, we need to keep a weather eye out for approaching danger, and be prepared to act accordingly.

Stay safe.

Want to track the progress of either or both of these developing storms? Click here.