Wednesday’s Forecasted Severe Weather – What Happened?

Several days in advance the National Weather Service, media outlets, and other weather partners began to discuss the potential for a major severe weather event across central and southeast Illinois for Wednesday afternoon and evening.   The Lincoln NWS office did special multimedia weather briefings and conference calls with state and local emergency management officials discussing the likely scenario. Widespread severe weather, particularly damaging straight-line winds, were expected to move across the region. We highlighted the threat in our forecasts on our website, Twitter, Facebook, and Youtube. Many people took measures to prepare for the worst.

 

Storm Prediction Center Catagorical Severe Weather Outlook - Issued 11:26 AM June 12

So what did we get? A few isolated thunderstorms – none severe. Severe weather did occur across northern Illinois into northern and central Indiana, but for our area not one report of severe weather. Storms originally developed across eastern Iowa and northern Illinois during the middle to late afternoon as expected, but never developed into the severe line of storms that we forecasted to impact central and southeast Illinois.

Severe Weather Reports - June 12

We were wrong. We were flat out wrong.   
 
So why were we wrong?
 
First lets discuss the ingredients needed for the development of thunderstorms: moisture, instability, and lift.
 

Moisture

Dew Point Temperature analysis at 1 PM Wednesday (Graphic courtesy of the Atmospheric Sciences Department at the University of Illinois at Urbana-Champaign)

 
The moisture was definitely in place over the area as anyone will tell you who had to be out in the muggy conditions on Wednesday. Dew point temperatures, a measure of how much humidity is in the air, were in the upper 60s to lower 70s which is well above normal for middle June. Lack of moisture to fuel the storms was not a problem on Wednesday.
 

Instability

Warm air rises. Once a bubble of warm air begins to rise, if it remains warmer than the air it is rising through it will accelerate upward. This is known as an updraft. If the bubble becomes cooler than the air that it is rising through it will slow down and may eventually begin to descend. Once the air starts to descend it is called a downdraft. Thunderstorms are made up of updrafts and downdrafts.   The stronger this up and down motion in a storm is the better are the chances for severe weather.

 A strong (fast) updraft is required to hold ice up in the cloud until it becomes large enough to fall as a large hail stone. The strong updraft is also necessary to develop the processes needed to cause a rising column of air to begin to rotate eventually becoming a significant tornado.
 
A strong downdraft is necessary to produce strong damaging winds at the surface. As the downdraft rapidly descends and then hits the ground, it spreads out producing the strong winds. Think of a drop of water hitting the ground. The ground stops the drops downward progress and is spread horizontally along the ground.
 
Instability is the term used by meteorologists to describe the temperature difference between a rising bubble of air and the temperature of the air it was passing through. The higher the instability, the more potential there is for strong updrafts and downdrafts.
 
On Wednesday, the atmosphere was very unstable. Temperatures of a rising bubble of air were calculated to be around 15 to 20 degrees warmer than the surrounding air by the time it reached 15,000 feet or so above the ground. This type of difference would normally lead to very strong and fast updrafts and downdrafts. Instability was definitely present.
 

There was one “little” fly- in-the-ointment however. There was a layer in place on Wednesday about 5,000 feet above the ground where temperatures were actually a few degrees warmer than our rising bubble of air from the surface. This meant that there was a zone that the updraft would pass through that would cause the bubble to actually slow down and possibly even begin to descend before it could reach the big differences in temperature further up in the atmosphere. This is what happened much of the day Wednesday and is why we didn’t have thunderstorms earlier in the day. This was not unexpected.  It just meant that the hot and humid air near the surface would need to wait for a strong nudge to push them up through this warm layer. That brings us to lift.

Lift

To get a strong updraft started, we must first have something to start moving our bubble upward. This mechanism can be a difference in air characteristics such as a front (which is what triggered the storms to our north and east), or the spreading out of downdrafts from previous storms (known as a “gust front”). This latter process is what we expected on Wednesday afternoon and evening to start the hot air moving upward over our area.

Once thunderstorms develop, the updrafts and downdrafts interact with the environment around them. Sometimes they will work together and downdrafts will combine to produce a large rush of air at the surface such as a gust front, other times they will impede each other as cooler air from a neighboring storm gets sucked into a storm’s updraft and decreases the instability (speed at which the air can move upward) and weakens the storm.
 
Think of it like a barber shop quartet. If one of the singers is a bit off on his timing, the song sounds awful, but if everyone is working together and the timing matches up, the voices resonate together and harmony is made. Storms can be like that too. If the downdrafts from several storms interact in a positive way they can produce a large gust front (or “Bow Echo”) containing widespread strong and damaging winds. As this air pushes into the hot and humid air ahead of it, the cool air wedges underneath and the hot air is lifted. If there is instability (the rising lifted air is warmer than the air it is moving through) it will accelerate upward and develop into thunderstorms that produce downdrafts that further reinforce the large area of damaging winds.  The process feeds off of itself producing a long track of large-scale wind damage. That is what we expected to happen on Wednesday.

However, on Wednesday the quartet never quite got their timing right. We never got downdrafts from several storms to interact in such a way that they combined into a strong gust front that could push south into central and southeast Illinois. We did have a few individual downdrafts that pushed into the Interstate 74 corridor, but those downdrafts were not strong and deep enough to push the hot air upward through the warm layer several thousand feet above the surface and to tap the instability above.

The result – a few isolated thunderstorms and most of us remaining dry.

Radar, Mean Sea Level Pressure Pattern, and Winds from 7 PM Wednesday Evening 



Return to News Archive

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.