The only thing more dangerous than a tornado is a tornado that occurs at night. People have some weird need to want to see the tornado before they seek shelter or take any other action. Seeing a tornado at night is usually almost impossible, but last night it wasn't. Meteorologists at the National Weather Service in Jackson, Mississippi were able to confirm the presence of a damaging tornado last night based completely on radar data, proving the effectiveness of its new dual-polarization technology.
Tornadoes in the southern United States are notoriously hard to see even in daylight. Supercell thunderstorms that occur in this region of the country are typically considered "high precipitation," so tornadoes are often obscured by extremely heavy rain and hail. To compound the situation, many areas of the south are hilly with tall tress making up much of the landscape, making it even harder to safely spot a tornado before it's too late.
At night, you can forget it. You will not see the tornado until it is on top of you unless you have access to weather radar imagery. This was the case during last night's severe weather outbreak in the Deep South.
At 214AM CDT, the NWS office in Jackson issued a tornado warning for the storm in question. They used the term "radar confirmed tornado" rather than "radar detected," displaying their confidence that there was a tornado tearing through central Mississippi.
...A TORNADO WARNING REMAINS IN EFFECT FOR NORTHEASTERN COVINGTON AND SOUTHEASTERN SMITH COUNTIES UNTIL 230 AM CDT...
AT 214 AM CDT...A CONFIRMED TORNADO WAS LOCATED 6 MILES NORTH OF COLLINS...AND MOVING NORTHEAST AT 45 MPH.
SOURCE...RADAR CONFIRMED TORNADO.
IMPACT...FLYING DEBRIS WILL BE DANGEROUS TO THOSE CAUGHT WITHOUT SHELTER. MOBILE HOMES WILL BE DAMAGED OR DESTROYED. DAMAGE TO ROOFS...WINDOWS AND VEHICLES WILL OCCUR. TREE DAMAGE IS LIKELY.
THE TORNADO WILL BE NEAR TAYLORSVILLE AROUND 225 AM CDT.
They posted this status on Facebook shortly thereafter:
Confirmed tornado debris signature around 6 miles northwest of Collins moving northeast at 45mph. Take cover now!
Here's how they were able to confirm the tornado based on radar imagery alone.
A quick look at the radar image above shows no obvious signs of a tornado (like a hook echo). The image is called "base reflectivity," which is used to show precipitation. Cool colors indicate light precipitation, while warmer colors indicate heavy rain and hail. In this radar image, there's a destructive tornado on the ground towards the middle near Collins, but you would never know it based on the above radar data alone.
A look at the "base velocity" product, which shows the winds inside a storm, confirms what you can't see just by looking at the precipitation — there is extremely strong rotation in the storm near Collins.
A zoomed-in view of the base velocity product shows the strong rotation near Collins. The green colors depict wind flowing north towards the radar site, while red colors show wind flowing away from the radar site (in this case, towards the south). When these red/green couplets show up in a severe thunderstorm on radar imagery, it signals strong rotation within the storm and the likely presence of a tornado.
The velocity data can only tell you so much about what's happening in a thunderstorm. It can let you know that the storm is rapidly rotating that there is a good chance that a tornado is present, but it can't confirm the tornado for you.
Enter dual-polarization (dual-pol) data.
During incredibly destructive tornadoes, meteorologists often show the public a "debris ball" that shows up as the radar beam reflects off of the debris in the tornado. However, in rain-wrapped tornadoes like the one that occurred last night, debris balls can be drowned out by the heavy rain and hail around them.
There is one dual-pol product called the "correlation coefficient" that proves extremely helpful in the detection of tornado debris, and this is how meteorologists were able to confirm last night's tornado without any eyewitness reports.
The correlation coefficient is a product that uses the size and shape of the objects the radar detects to tell you how similar they are to one another, based on a scale from 0% to 100%. 100% indicates that the objects the radar detects are all completely the same size and the same shape. For instance...heavy rain is uniform, so it would get a correlation coefficient (CC) number of around 99.8%.
When the radar detects objects close to one another that are not similar in size or shape (like tornado debris), it gets a very low CC number. The presence of low CC numbers within strong rotation confirms that there is a tornado producing damage and lofting debris into the air.
Here's another look at the velocity data as the tornado moved north of Collins.
...and here's the same view using the correlation coefficient product.
The correlation coefficient image easily lets you see that there is debris swirling around in the tornado right where the strongest rotation is located.
This area of Mississippi is (thankfully) not heavily populated, and most of what the tornado hit was farmland and wooded areas. There are reports of some structures and homes being damaged and destroyed by the storm, but it's not as bad as it could have been.
You can see the progress of the tornado with each successive correlation coefficient image.
Meteorologists are surveying the damage and will issue a preliminary report tonight or tomorrow. The storm likely won't get too high of a rating, as intensity on the Enhanced Fujita Scale is based on the damage the tornado produces, and it hit mostly trees and farm structures.
UPDATE: The tornado was assigned a preliminary rating of EF-2 with estimated maximum winds of 125 MPH. The tornado cut a 16-mile path of damage through Covington County, Mississippi.
[Top image via NOAA, all others via Gibson Ridge]