Stu Ostro, Senior Meteorologist at the Weather Channel, has written a must-read post on the recent record Georgia deluges, “Off the chain without a ‘cane” (reprinted below).  He makes a key point that had not occurred to me about the devastating September rainstorms:

Usually during that month when there’s wild weather, including precipitation extremes, it’s as a result of a hurricane or tropical storm. Not in 2009.

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The main point of my post, Hell and High Water hits Georgia, was that, as climate scientists have predicted for a long time, wild climate swings are becoming the norm, in this case with once-in-a-century drought followed by once-in-a-century flooding.

Back in 2007, the NYT reported, “For the first time in more than 100 years, much of the Southeast has reached the most severe category of drought….  The situation has gotten so bad that by all of [state climatologist David] Stooksbury’s measures — the percentage of moisture in the soil, the flow rate of rivers, inches of rain — this drought has broken every record in Georgia’s history.“  So it was more than a once-in-a-century event.

As for the flooding, as one CP commenter posted, the USGS quantified that “the rivers and streams had magnitudes so great that the odds of it happening were less than 0.2 percent in any given year. In other words, there was less than a 1 in 500 chance that parts of Cobb and Douglas counties were going to be hit with such an event.”

I have called this type of rapid deluge, “global warming type” record rainfall, since it is one of the most basic predictions of climate science — and its an impact that has already been documented to have started.

But in fact this deluge is even more of a global-warming-type rain event because, as Ostro notes, unlike typical Georgia deluges, this one was not associated with a tropical storm or hurricane.  Ostro is “Senior Director of Weather Communications. Stu leads the team of tornado, hurricane, and climate experts at The Weather Channel.”  Because these extreme rain events becoming more common, and because Ostro’s excellent explanation is really too long and technical to simply excerpt, I’m going to repost the entire thing below:  [Note: For ease of reading, I’m not indenting this.  Everything below is Ostro.]

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Off the chain without a ‘cane

That’s what the weather was in the Atlanta metro area early last week, and things were wiggy in the U.S. for much of September. Usually during that month when there’s wild weather, including precipitation extremes, it’s as a result of a hurricane or tropical storm. Not in 2009.

This “ex-skeptic” hasn’t blogged about climate change in a while. For that matter, I haven’t blogged about anything for a while! Been a bit distracted, but it’s time to jump in the water again. Or maybe I should say, time to dust off my Nomex suit and put it on!

Before you fire up the flamethrower, though, let me say what this long entry is NOT about.

It’s not about H.R. 2454 (more commonly known as the Waxman-Markey bill).

And I’m not telling you that you can’t drive your SUV.

This blog is about the effect of climate change upon day-to-day weather. About physics and thermodynamics not politics.

It was two years ago last week that I first thoroughly laid out the basic premise.

Nothing that’s gone on in the atmosphere since then has convinced me otherwise, and I’ve continued to add gazillions of weather events to this PDF [56MB file, and now up to 529 slides]. My goal has been and continues to be to document and objectively analyze these cases.

There have been anomalies and extremes for as long as there has been weather on the planet; the key is to assess how they are now changing as the climate changes.

To review:

–The global climate is overall warmer than it was in the 1970s. (That shouldn’t be too controversial a statement!)

Technical talk: The atmospheric warming has resulted in an increase in 1000-500 millibar thicknesses. Those increased thicknesses are manifesting themselves primarily by an increase in 500 mb heights (particularly notable in mid-high latitudes of the Northern Hemisphere), as there has not been a similar rise in 1000 mb heights. Although there is of course natural year-to-year variability, the overall trend at 500 mb has clearly been upward.

Analogy: It’s like bread baking in the oven. As it warms, the dough expands in depth. Although the details of the science involved are different, the analogy works, which is that the depth (thickness) of a given layer of the atmosphere is increasing on average as that layer warms. Furthermore, in this case, the bottom of that atmospheric layer (1000 millibars) is not significantly changing, just as the bottom of the bread isn’t (in that case, it’s fixed by the bottom of the pan).

Technical talk: In turn, one of the ways in which those increased mean 500 mb heights are manifesting themselves is by way of individual (temporally short-term and spatially small-scale) 500 mb positive height anomalies. These positive 500 mb height anomalies are also playing a role in concomitant short-term negative anomalies.

Translation: What we’ve been observing over and over again in recent years is exceptionally strong ridges of high pressure, sometimes accompanied by strong, persistent “cutoff lows” (upper-level lows cut off from the main jet stream) to the south of the ridges.

Over and over and over again, there has been one of those ridges of high pressure bulging northward. One week it might be in Eurasia, the next in North America. Like when you squeeze a kids’ squishy ball in one place and then another, as I’ve done on the air and in talks I’ve given. For an example of a TWC segment (from
July 1, 2009), click on the screen capture below.

The upshot: many weather events/patterns in recent years which have been topsy-turvy and/or produced precipitation extremes and temperature anomalies.

The past few months help illustrate all of this.

As I recently reported in a TWC segment, it’s extremely important to look at the context of this year’s cool summer in a portion of North America including the U.S. Midwest! That was a local anomaly of blue dots (below-average temperatures) amidst a sea of red dots (above-average temps). The map is for “climatological summer,” or June, July, and August (JJA). And this was occurring in the context of the global temperature rankings.

The temperature pattern was associated with a negative 500 mb height anomaly (persistent troughs/cutoffs) “trapped” to the south of a strong positive height anomaly (strong ridging in the Arctic). Here are a conventional “cylindrical equidistant” map projection to provide that perspective, and a “polar stereographic” map which doesn’t exaggerate the area around the North Pole. Both maps of 500 mb height departures from average show the very clear and persistent pattern.

Then after JJA came September, and the atmosphere outdid itself.

First, there was the flash flood disaster in Istanbul, produced by what was reportedly the largest amount of rain there in 80 years.

The atmospheric circulation signature aloft at the time?

In the U.S. a couple days later, a non-tropical low pressure system took on some tropical characteristics and became a hybrid as it was forced northwestward (an atypical direction) from out in the Atlantic to Delaware Bay, due to a cutoff low in tandem with a strong bulging ridge aloft to the north of it. The cyclone brought wind damage to southern New Jersey from gusts as high as 61 mph, and also locally heavy rain there and other locations in the Northeast.

The pattern aloft at the time:

Then, in one of the oddest, most deviate meteorological things I’ve ever seen in my career, an upper-level cutoff low at ~45-50N latitude in mid-September not only “retrograded” to the west but it did a complete 360 degree loop over the course of several days (September 10-14), from southwest Saskatchewan to South Dakota to Wyoming and back to southwest Saskatchewan.

This was occurring at a time when there was an extreme spike in the preliminary satellite-derived globally-averaged mid-tropospheric (600 mb, or 14,000 feet) temperatures to a value which was by far the highest in the ~30-year period of record.

Just sayin’.

[Click on image for larger version. Source: UAHuntsville; NASA; DISCOVER Technologies]

Global cooling?

Fortunately that bizarro cutoff low did not have catastrophic consequences, but while that situation was happening another remarkable weather pattern was unfolding. And this system, like the one in Turkey, would ultimately bring disaster.

A persistent mid and upper-level low just sat and meandered and sat and meandered day after day after day over the ArkLaTex and Oklahoma, producing a series of localized deluges from September 9-21 and resulting in record September rainfall in Texarkana and Tuscaloosa.

The pattern during that period:

Starting to look familiar yet?

This all culminated in the flash flood calamity not far from The Weather Channel in the Atlanta suburbs, affecting some TWC employees and causing 10 tragic fatalities in Georgia including one involving a heart-wrenching 911 emergency phone call.

Rainfall quickly went off the chain, unheard of for September in Georgia without a hurricane or tropical storm. [Side note: Is El Nino totally to blame for such an exceptionally quiet Atlantic hurricane season? Hmmm.]

Many locations in the northern part of the state received more rain in one week than previously on record for the entire month of September, those prior records having been set in 2004, when the remnants of Frances, Ivan and Jeanne brought torrential downpours.

Record floods occurred on Suwannee Creek near Suwanee and on the Chattahoochee at Whitesburg.

The two images below are from not quite the end of the event, but most of the rain had fallen by then; the first shows the rainfall accumulation at one location and the second is a mapped radar estimate of the amounts.

Dewpoints, representative of the amount of moisture in the air (and indirectly the warmth) helping to fuel the extreme rainfall rates, were as high as 80 degrees, exceptionally high for late September.

The pattern at the time on September 20-21:

And last but not least, on the 22nd …

While it fortunately did not result in any new flood misery, this was the off-the-chain coup de grace meteorologically: a ridge that bulged so strongly for this time of year that it set the record for the highest 500 millibar height so late in the season so far north in the United States (5950 meters at Spokane, Washington). And for icing on the cake, there was yet another cutoff low stuck to the south.

Was all of this just an “accident”? Let’s look at the bigger picture. This has happened in the context of overwhelmingly positive 500 mb height anomalies (higher-than-average pressure aloft) so far this year, as has also been the case in recent years.

So what does all this mean? Did global warming “cause” the Atlanta flood?

Well, the atmosphere is very complex, and with any weather event there’s a combination of factors rather than a single one for an outright cause. Additionally, there’s no way of knowing what would have happened without the climate having changed. Maybe there would have been an even more extreme and deadly catastrophe by way of a landfalling major hurricane. And large-scale climate drivers didn’t determine the small-scale specifics of, for example, my neighborhood having been spared serious flooding while other Atlanta suburbs got hit hard.

Nevertheless, there’s a straightforward connection in the way the changing climate “set the table” for what happened this September in Atlanta and elsewhere. It behooves us to understand not only theoretical expected increases in heavy precipitation (via relatively slow/linear changes in temperatures, evaporation, and atmospheric moisture) but also how changing circulation patterns are already squeezing out that moisture in extreme doses and affecting weather in other ways.

While it’s important to consider what may happen in 50 or 100 or 200 years, and debate what should be done about that via H.R. 2454 or other measures, we need to get a grip on what’s happening *now*.

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