Tornadoes, Dust Storms and Floods: What the Hell Happened This Week?

Below, we answered are some key questions about this wild weather week.

What the heck kind of storm was this, anyway?

The storm system that helped cause the tornadoes and the flooding is known as a “closed low,” or a low pressure area in the upper levels of the atmosphere that has been cut off from the atmosphere’s steering currents and left to meander on its own. Think of it as a storm that the jet stream cast aside, for unknown reasons.

In any event, the closed low meandered above the High Plains, as the jet stream took on a bizarre, sinuous S-like shape across the U.S. and Canada.

The jet stream barely changed throughout the week, keeping the closed low locked in place. The firehose-like feed of Gulf of Mexico moisture blasted northward on its eastern flank, resulting from the counterclockwise flow of air around the low pressure area.

This moist air was one of the main ingredients that helped spark the tornado outbreak on April 27 and 28, as well as the flash flooding on April 30.

Interestingly, the closed low that spun its way around the High Plains was mirrored by another closed low off the East Coast, in a weather pattern that seemed to have been engineered for the sole purpose of producing extreme outcomes across the U.S.

Why did the storm move so slowly?

As happens from time to time — although some suspect it is happening more frequently now — the weather pattern got stuck in place this week. An unusually large and intense area of high pressure across southeastern Canada helped ensure that the closed low above the Plains had nowhere to escape, as the S-shaped jet stream slowly slithered its way eastward toward the East Coast, like a snake slowly digesting its prey.

Blocking patterns such as this one often lead to extreme weather events, especially temperature and precipitation extremes.

For example, a blocking pattern across Europe and Russia in 2010 led to the deadly Russian heat wave that killed thousands and contributed to massive wildfires, as well as the disastrous Pakistan floods that occurred around the same time. Another blocking pattern resulted in the deadly 2003 European heat wave, which killed an estimated 40,000 people.

In the U.S., meteorologists generally view closed, meandering, almost drunk weather patterns like the one from this past week with a sense of foreboding, since they can instigate and prolong severe weather outbreaks. In other words, closed lows usually mean trouble.

How might global warming have contributed to this storm?

Given the sharp increase in greenhouse gases such as carbon dioxide, the atmosphere now contains more moisture, on average, and is warmer, on average, than it used to be. Therefore, any weather system that occurs does so in an altered setting. However, that doesn’t really tell us much, so we have to pick this event apart into its many components.

First, let’s take the blocking pattern itself. Some meteorologists and climate scientists suspect that global warming is leading to more amplified, or wavy, jet stream patterns like the one we saw this week. This can prolong weather events and lead to more extreme events. One such scientist, Jennifer Francis of Rutgers University, has published several studies arguing that rapid climate change in the Arctic, where temperatures are increasing twice as fast as the rest of the world, is behind the jet stream changes in the northern midlatitudes.

However, many atmospheric scientists are not yet sold on this hypothesis, and see little evidence of detectable jet stream changes at all. Nevertheless, this remains a subject of intense ongoing research.

Workers repair Euclid Street near 12th Ave. in Pensacola, Fla. that was washed out due to recent flooding, Thursday, May 1, 2014.

Image: John Raoux/Associated Press

Next, let’s look at the tornado outbreaks. This is another subject of ongoing research, with evidence from computer modeling studies so far pointing to projected increase in the number of severe thunderstorm days in a warmer world, but a possible decrease in the number of tornado days as one of the key ingredients for tornadoes, wind shear, becomes more scarce across parts of the U.S.

Wind shear occurs when winds vary in speed or direction with height, or both. When the tornadoes struck Arkansas, Mississippi and Alabama this week, wind shear was extremely high, with low-level winds blowing from the southeast, and winds a few thousand feet off the ground coming out of the southwest. This helped create the spinning motion that eventually resulted in the tornadoes.

According to the AP, data shows that the number of days with at least one significant tornado in the U.S. has been declining since the 1970s. Yet at the same time, the number of tornado outbreak days, with 30 tornadoes or more, has increased.

"Something has been happening and we're not sure yet why," tornado expert Harold Brooks of the National Severe Storms Laboratory told the AP.

This transition to a “boom or bust” tornado regime is consistent with some climate studies showing that even if wind shear declines, it will still be present on some days, leading to potentially larger, but less frequent, outbreaks.

Lastly, there’s the heavy rainfall and flooding to consider. Here, at least, the scientific evidence is clearer — global warming is already leading to an increase in heavy rainfall events in the U.S. and elsewhere, and this is expected to continue. The reason for this is that warmer air holds more water vapor, which provides added fuel for storms.

According to the National Climate Assessment report, which is to be released on May 6, every region in the country (except Hawaii) has seen an increase in heavy precipitation events since 1991.

This means that we better get used to events like the one that occurred in Pensacola, where total 24-hour rainfall amounts approached two feet in some spots.

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