Kevin Myatt

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About Kevin

Kevin Myatt grew up in Arkansas to the tune of tornado sirens and the rhythm of hailstones, aspiring to be a meteorologist before his studies and career were turned to journalism instead. Though he often chases storms, he prefers living in the cooler, more tranquil weather of the Blue Ridge. He moved to Roanoke in 1999 to take a job on the copy desk of The Roanoke Times; writing headlines and editing copy is his principal work for the newspaper today.

Each May, Kevin assists Pulaski County High School / Virginia Tech meteorology instructor Dave Carroll in leading college and high school students to the Plains to observe severe weather firsthand. The accounts of many of his storm chases can be found here on the storm chasing page of his weather blog on roanoke.com.

Kevin was an editor for "Hurricanes and the Middle Atlantic States," a book written by D.C.-area weather enthusiast Rick Schwartz and published by Blue Diamond Books that documents hurricanes striking the mid-Atlantic states since colonial times.

The Weather Journal column began in 2003 and appears on Friday's Virginia section front in The Roanoke Times. The Weather Journal blog began in 2006 and follows weather day-by-day between the larger columns.

Subtleties in system spawned twisters

In the big picture, there was little indication that Monday's storms would produce a tornado outbreak in southeast Virginia.

Sure, there was a strong cold front pushing into warm, moist air, but that happens dozens of times each year with little or no tornado activity.

Just before noon Monday, the Storm Prediction Center, the nation's nerve center for severe thunderstorm and tornado forecasting, described the amount of shear as "modest" and instability as "weak."

Shear refers to changing wind directions with height, while instability refers to the ability of warm, moist air near the surface to bubble into colder air high in the atmosphere. Both are important ingredients in long-lived, rotating thunderstorms, called supercells, and the tornadoes they can spin off. Neither factor was overly impressive late Monday morning.

"Locally gusty winds or brief tornadoes will be possible from stronger cells," the SPC said in a midday statement. There's certainly nothing in those words that's unusual for a late April cold front passage, and nothing portending an episode that would wreck neighborhoods and injure hundreds.

But subtleties elevated Monday's rather routine risk of severe thunderstorms into a destructive tornado outbreak.

About midafternoon, forecasters picked up on a small but very important feature near Richmond. A miniature low pressure system, or mesolow, had developed. A mesolow is a localized area where the barometric pressure has fallen slightly.

The tiny low was just enough to assert a slight counterclockwise rotation which ever-so-slightly shifted surface winds from southerly and southwesterly to a more southeasterly direction.

Anytime there are winds blowing in different directions at different layers of the atmosphere, there is the potential for the updrafts that power thunderstorms to rotate. But updrafts have a greater potential to rotate more rapidly and for longer duration when the surrounding winds help them spiral.

This spiraling is enhanced as winds gradually shift in direction from one layer to the next: southeasterly at the surface, southerly a little higher up, southwesterly in the next highest layer, west at the highest levels.

The potential for winds to spiral upward is called "helicity." That term is based on the word "helix." Those of you familiar with biology know that a DNA molecule in a cell is referred to as a "double-helix," or two intertwined spirals.

The slight wind shift effectively doubled the helicity over southeastern Virginia on Monday afternoon. The potential for storm updrafts to rotate rapidly increased dramatically.

A second factor that contributed to the tornado outbreak was the amount of instability near the surface.

Through the entire atmosphere, instability was not very impressive, a fraction of that commonly found in conditions that produce the monster supercell storms common in the central United States.

But in the lowest layer of the atmosphere near ground level, there was an extreme amount of instability. Warmth and moisture had pooled near the surface and were ready to shoot upward into somewhat cooler air above the surface layer.

So, for a few hours Monday afternoon in southeast Virginia, updrafts of warmth and moisture from the surface exploded rapidly into the middle levels of the atmosphere. The helicity present from the winds gradually shifting with height spun those updrafts in a spiral.

The thunderstorms rotated, and in some cases, those rapid updrafts wound tightly into tornadoes.

On a large scale, it is relatively simple for forecasters to find broad regions where there will be a threat of severe thunderstorms. The Storm Prediction Center highlighted the risk for severe weather across eastern Virginia and eastern North Carolina three days before Monday's outbreak.

What is much harder is to pinpoint the atmospheric details that can make the difference between a line of storms with a few limb-breaking gusts and a structure-smashing tornado outbreak.

Forecasters, both at the SPC in Oklahoma and at the weather service office in Wakefield, were just ahead of the weather Monday. Suffolk was under a tornado warning an hour before the storm arrived, and downtown Suffolk was specifically named as being in the path of a tornado 15 minutes before being hit.

No one died in the tornadoes. There are probably many factors involved in that, but early warning may well be one of them.

Sniffing out the subtle features is all-important in tornado forecasting. And that's why, starting May 11, I will again be helping lead college and high school meteorology students on a trip to gain field experience forecasting and tracking severe thunderstorms and tornadoes.

Our success will be determined by how well we pick up on the little things. What the students learn both in success and failure could help save lives.

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