On Tuesday morning, my colleagues and I anxiously awaited the Storm Prediction Center’s “Day One Outlook,” a product that shows the severe weather potential in the United States for that day. Most of us had a hunch on what was going to happen that day, and unfortunately, it was one of those feelings where you knew whatever happened would be the headline in the news tomorrow.
SPC's Day One, Severe Weather Outlook: High Risk (Pink), Moderate Risk (Red), Slight Risk (Green)
The Storm Prediction Center issued a “high risk” of severe weather for the eastern half of Oklahoma and southeast Kansas. My heart just sank. A “high risk” threat is issued a handful of times per year and only when a severe weather outbreak is anticipated that day. Sure, there have been days when a “slight or moderate risk” has been issued and the forecast falls through because all of the elements didn’t enter the mixing bowl at the right time, but when a forecaster with years of experience in Norman, Oklahoma issues a high risk of severe weather, he or she certainly means business.
An area of low pressure tracking out of Colorado and into the northern Plains was the primary cause of the severe weather outbreak. Ahead of the low, a counterclockwise flow (southerly winds) pulled moisture-rich air northward from the Gulf of Mexico. To give you an idea what I’m talking about - at six o’clock that evening, the same time frame the storms were developing and dropping tornadoes, the temperature in Oklahoma City was 83°, while the dewpoint clocked in at a muggy 72°. At 3,000 feet above the ground, the temperature was even warmer, in the middle 80s. This warm air layer above put a cap on the air near the ground, allowing the water vapor to pool. It’s just like placing a lid on top of a boiling pot of water. Moisture builds up inside the pot when the lid is on, but once you take the lid off, the vapor rises quickly toward the ceiling. This same concept applies to how these severe thunderstorms formed. As a dry air mass nudged into Oklahoma and Kansas, the atmosphere became very unstable. The dry air eventually eroded the cap away, allowing the water vapor to soar high into the atmosphere and condense to form clouds. By turning water vapor into droplets, the condensation process released latent heat into the atmosphere, priming the environment even further for thunderstorm development. But one could have all the moisture in the world racing to the higher levels of the atmosphere and you still wouldn’t have a major severe weather outbreak. That’s where another critical element comes into play: wind shear.
In atmospheric science, wind shear is the change of wind speed and direction with height. A good example of wind shear is at the ground, winds are southerly at 15 miles per hour, while at jet-cruising altitude, the winds are westerly at 90 miles per hour. The turning and increasing speed is what causes the spinning in the atmosphere that leads to tilted, long-lived updrafts, large supercells and tornadoes. All the elements needed to form a violent sky entered the mixing bowl at the right time last Tuesday, and it kept meteorologists, emergency management and the public on edge for hours.
To look at one of the storms from a meteorological standpoint, we’ll turn to our friends at the National Weather Service in Wichita, Kansas. Below are two radar images of a severe thunderstorm that produced a tornado in south-central Kansas at 4:35 P.M. that day.
Reflectivity Image of tornadic storm over Kingman County, KS - 10 May 2010, 4:35 P.M.
Velocity Image of tornadic storm over Kingman County, KS - 10 May 2010, 4:35 P.M.
The first image is the base reflectivity of the storm, which is the ordinary radar you see on TV forecasts and online. The second image is the velocity of the winds inside the storm, measured by electromagnetic radiation emitted by Doppler radar. As you see in the first image, there’s not only a lot of red coloring, which indicates heavy rain and potentially large hail, but there’s also a curl or hook shape at the south side of the storm. This is known as a hook echo, and a hook echo like this is a red flag for rotation. Looking at the second image, you’ll notice there’s a couplet of bright red and bright green colors in the same location as the hook echo. This is a tornadic vortex signature (TVS) and the opposing colors tell a meteorologist the winds are quickly changing directions at one point in the spectrum. If you think about how a tornado works, the flow around it is always changing direction, from southerly, to easterly, to northerly, to westerly. If you think if it that way, you may be able to look at the velocity image and understand my explanation.
Tornado in Kingman County, KS. Photo courtesy of Jeremy Lundin
At the ground, witnesses saw the actual tornado caused by the thunderstorm, including Jeremy Lundin, who captured this image (below) in south-central Kansas. This tornado reached EF2 intensity (winds up to 135 mph) as it moved across the Kansas landscape, damaging homes and other structures. As of May 12, 2010, there were 11 confirmed tornadoes and 42 reports of tornadoes. It is likely the number of tornadoes confirmed will go up once the National Weather Service surveys all of the damage.
In the Coulee Region, we’ve been quite fortunate over the years. We do not live in an area that witnesses a severe weather setup as often as the central U.S. Nevertheless, there are statistics to share!
- Climatological speaking, May and June are our busiest tornado months (June is #1).
- Since 2004, we’ve only had 1 May tornado in the NWS-La Crosse service area, but 32 in June! August has had 11 followed by March with 6.
- Minnesota averages 24 tornadoes a year; Wisconsin averages 21, and Iowa averages 48
- On May 25, 2008, 13 people were killed when Parkersburg / New Hartford, IA were hit by an EF5.
- Last F5 in our service area = 1968 (Charles City/Oelwein, IA)
- Last F4 in our service area = 1971 (Elma/Waucoma, IA)
- Last F3/EF3 in our service area = 2004 (State Line)
- Last EF2 in our service area = 2009 (Austin)
Information courtesy of the National Weather Service - La Crosse
We certainly hope these stay as records rather than a reality, but you and I know sooner or later, a severe thunderstorm will spawn a tornado in the local area. What really matters is knowing where to go and what to do in the event a tornado moves into your community. Thank you for reading and have a great day!
News 19 Meteorologist Nick Grunseth
Posted under Science, Weather
This post was written by ngrunseth on May 12, 2010
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