The First Alert Weather team has declared this Sunday, April 12th, 2020, as a First Alert Weather Day due to the potential for a significant severe weather outbreak.
We first warned you about this threat on Tuesday when we put the First Alert out, and the latest trends continue to strengthen with that thinking. A low pressure system will develop and bring a first round of severe weather over the Southern Plains Saturday and that will strengthen and mature overnight Saturday into Sunday.
As the system moves into the Valley it will bring periods of showers and storms to start the morning Sunday. As the day goes on we will see some heavier rainfall along with strong storms. Rainfall will be widespread and heavy at times and rain totals could be as high as 2 to 4 inches in some spots. Inside stronger storms we could see localized heavier rainfall amounts which would increase the threat of flash flooding. For perspective, our average rainfall in Huntsville for the month of April is 4.32″. That means we could see nearly half if not all of our monthly normal in one day.
Impacts: Sunday’s severe weather threat includes of all forms of severe weather; Damaging wind, large hail, & tornadoes. Some of those tornadoes could be violent and long tracked if they develop. This means we could see tornadoes of EF-2 intensity or stronger. For a tornado to be rated as EF-2 it has to have estimated winds off 111mph or stronger. Damaging straightline wind gusts will have the potential to reach 60 mph or greater and hail of 1 inch in diameter or larger is possible as well. The strongest severe storms could have the potential to drop hailstones the size of hen eggs or larger, which is 2 inches in diameter.
Not only will we have to worry about wind, hail, and tornadoes, but there will be very heavy rain inside these storms as well. In fact, the Weather Prediction Center has placed the Tennessee Valley in their highest risk for potential flash flooding on Sunday, placing us in a moderate risk. This is because storms could drop anywhere from 2 to 3 inches of rain for much of the Valley, with localized heavier amounts closer to 5 inches.
Timeline: Showers and storms will develop overnight Saturday into Sunday and will increase in coverage over the Tennessee Valley through the early and mid morning hours. These storms will bring periods of heavy rainfall, but should NOT bring a severe weather threat. However, as mentioned above, these storms may play a big part in our threat for stronger storms during the afternoon. The longer that they stick around and keep our atmosphere cooler and contaminated/disturbed the lesser the chance for stronger storms during the afternoon.
The morning storms are projected to clear out by midday as the warm front move north through the Valley and that will increase our warmth and energy across the Valley for the afternoon. Our threat for severe storms looks to increase after 2 to 3 PM Sunday afternoon and will extend into the evening and early morning hours on Monday when the cold front finally completely passes through.
WHY?: When forecasting severe storms, we look for many atmospheric ingredients. Similar to baking a cake, you need the right combination of each ingredient for the outcome to be perfect. There are 4 main “ingredients” that we look for when forecasting strong/severe storms. They go by the acronym, S.L.I.M. Shear, Lift, Instability, & Moisture. While Sunday’s set up looks to have a amount strong of all four ingredients there is still some uncertainty in one of the biggest ones, instability. Generally when forecasting severe storms, especially significant severe storms, you look for convective available potential energy (CAPE) values of 1,000 J/KG or greater. When that number exceeds 1,000 J/KG the potential for severe weather starts to skyrocket. The larger that number gets, the higher the likelihood for larger hail as well as violent tornadoes. As seen in the pictures, CAPE will be our one question mark as computer models are having a hard time coming to a consensus on this. Some models have a limited amount of instability while others are not lacking at all. This will really be determined my the morning showers and storms and how long they stick around through the morning. As mentioned above, the earlier they clear out, the greater the likelihood that our afternoon storms will be severe.
Atmospheric shear/rotation will be our strongest ingredient. Shear will not be in question as we will have strong southerly winds at the surface this morning with gusts of 15 to 25 mph out of the south. There will be also be stronger southwest winds around 1 mile up in the atmosphere which helps to promote low level rotation in these storms. Lift shouldn’t be an issue either as there will be several boundaries left over from the morning showers and storms that have already formed along the cold front to the west. There will be quite a bit of moisture in the atmosphere to work with as humidity climbs through the day. Dew points are forecasted to climb into the low to mid 60s which would provide an ample amount of moisture for these storms.
REMINDER: Storms will likely lead to areas of power outages so we stress that you have multiple ways of receiving your weather warnings. This includes the 48 First Alert Weather App, a NOAA Weather Radio, Television, livestream, etc. We will be pushing out video updates on your 48 First Alert Weather App throughout the next 48 hours to keep you updated with the very latest information. To make sure you are getting these updates you will need to switch your alerts to “on” in your 48 Weather App. You can do that in the settings tab which is on the top right of the App. A video breakdown of that can also be found here. Once storms form and become severe, we will have continuous coverage live on-air with coverage, but we will also be streaming live on our app, waff.com/48now, and Facebook Live. Please be safe and check back for more info as we go throughout the day today and this weekend!