So far, so good! There have been no severe storms in the Tennessee Valley this morning; however, as the low-level clouds begin to separate and allow some sunshine through, the temperature will warm into the upper 80s, and that will destabilize the atmosphere significantly for the afternoon and evening hours.
Today’s slight risk of severe storms comes from the center of low pressure that was once Hurricane Isaac; it is spinning eastward across southern Illinois and Indiana today, and as little pressure troughs (extensions of the surface low) pass through this warm, unstable air in the afternoon, we expect thunderstorm development over West and Middle Tennessee first. Then, the storms will move southeast into North Alabama after 2 PM today.
The first trough to move through the Tennessee Valley has brought some heavy tropical downpours along with some occasional thunder and lightning in Madison, Jackson, Lincoln and Franklin Counties over the past three hours.
Multiple mesoscale models that we get fresh information from every hour suggests new thunderstorm development will start in West and Middle Tennessee by 1 PM. Initially, those storms will be moving ENE at 25-35 MPH. By 3 PM, the entire complex of storms will be shifting southeast into North Alabama, but individual storms within the complex will be moving ENE still at 20+ MPH. If that seems confusing, think of a bus full of students traveling down I-65. The over-all motion of all students is SOUTH, but the individual students might be shifting around the bus making their NET motion seem different than the motion of the bus.
Here’s one of those mesoscale model’s output for 6 PM this evening. The red circle indicates where there is a greater mix of severe weather ingredients (shear and instability) at that time:
Here’s what all of this means to you:
- Be alert! A Severe Thunderstorm or even a Tornado Watch is possible this afternoon. A WATCH means that conditions are favorable for the development of severe weather (58 MPH winds, hail larger than a quarter, and tornadoes) are possible in the watch area. If a WARNING is issued for you, it’s time to take action. The weather office at WHNT News 19 will be staffed throughout the evening, and if there is something you need to know about, we will be there on TV to make sure you get the message.
- Have a reliable source of weather warning information close at hand. NOAA Weather Radios are the best way to get a warning at home. WHNT News 19 offers multiple web and mobile applications for iPhone, Android, and iPad if you are on the go. There are also some great weather radio apps out there like MyWarn and iMap Weather Radio, but you’ll have to pay for them. Baron Services has Baron Saf-T-Net for free; all you have to do is sign up. Here are the links:
- The odds of any one community getting severe weather this evening are less than 10%. The over-all chance of rain is high, and some of us may get over an inch of rain, but severe storms are going to be “isolated.” That means we cannot tell you if a tornado is going to hit a specific town or neighborhood hours in advance. If we could, we’d do it, but it’s just not possible. That’s why warnings only last 15-30 minutes; we forecast storms for individual communities on a much smaller time scale than the 7-day forecast because storms get strong and weaken over minutes instead of hours.
- If you have outdoor plans for the rest of the day, you need a back-up plan if it goes through 5 to 8 PM. The best chance of storms will run from around 3:00 PM to 10:00 PM in North Alabama.
- Above all, don’t sit around and worry about this all day. The threat is low, so go about your business. Just make sure you have a way to get quick information if severe weather happens to approach your area.
- We use social media all the time to communicate short-term weather information. Sometimes we cannot get to a particular question in a timely manner because there are (1) too many at a time or (2) we just don’t see it for some reason. If you ask us a question on social media, be patient and understand that we’re not ignoring you. If you don’t get an answer, ask again…just remember, we’re not going to be able to tell you if severe weather will definitely impact a specific town…the answer is always “there’s a chance” on a day like this.