HUNTSVILLE, Ala. (WAFF) – Today has been declared a first day weather alert for the potential for strong and severe thunderstorms during the afternoon and evening.
We issued the first alert for this potential on Monday afternoon. Overnight Thursday through Friday, the Storm Prediction Center placed the entire Tennessee Valley at increased risk (level 3 out of 5) for severe storms on Saturday.
Weather Team 48 agrees with this growing threat. Weather models are beginning to agree that the parameters will be robust enough to lead to the potential for significant severe severe weather on Saturday afternoon and evening, but there is still some uncertainty. Below is a breakdown of the timeline of these storms, the impacts, and what could impact our storm risk.
Impacts: While all forms of severe weather are possible on Saturday, there are a few hazards that pose a greater threat. Damaging wind gusts of 60-70 mph and tornadoes, some of which can be strong and violent, are the two main concerns. Embedded tornadoes will be a concern later in the evening within the moving squall line. The Storm Prediction Center has highlighted the entire Tennessee Valley as an area with a greater likelihood of strong, violent tornadoes of EF2-EF5 intensity. Although not as concerning, these storms will be capable of producing large hail. Severe criteria for hail are any stone 1 inch in diameter or larger, which is the size of a quarter.
In addition to these dangers, we will have the threat of flash floods. Huntsville Airport has already recorded 1.5 inches of rain this week, but some other communities have seen around 4 to 6 inches of rain. We could see an additional 1-3 inches of rain with Saturday’s thunderstorms, which will only increase the potential for flooding in some areas. Some communities may see locally larger amounts inside stronger storms. Flood potential will increase significantly if storms train/pass through the same areas for an extended period of time. In the photos above, you can see the rainfall potential and the Weather Forecast Center’s forecast for potential flash floods.
Chronology: The morning should start warm and windy across the valley, but we will see some isolated showers and thunderstorms to start. These will not be severe, but they could influence the afternoon thunderstorms. The Moment of Concern will open around or just after 2 p.m. for areas along and west of I-65 and I-565. There will be two different parts to the threat. The first threat will be to individual cells developing upstream of the main cold front and line of conduct. These will have the potential to develop into supercells that pose the greatest risk of strong tornadoes. It will also be the best hail threat. These cells could form as early as 2 p.m. for areas along and west of I-65 in northwest Alabama. These will be a problem from around 2pm to 7pm or 8pm. It will probably be moving from the southwest to the northeast.
The second piece or part of the threat will develop to our west. These supercell storms may merge into a line or join a line of storms already developed before the cold front. This will at some point become a line of storm squalls heading west to east. It will start in the shoals and northwest Alabama as early as 5 or 6 p.m. and move quickly east. The line could sometimes move at over 50 mph as it crosses the valley. This is when the threat turns more into a straight line wind threat. This line of storms will likely move out of northeast Alabama and Sand Mountain between 9 p.m. and midnight. Everything will depend on when these storms start.
What should happen: Just like baking a cake or a meal at home, you need the right atmospheric ingredients for storms, especially severe storms, to thrive. There are many factors or “ingredients” that we meteorologists look at when forecasting severe weather, but the four main elements are SLIM, which stands for shear, lift, instability, and humidity. There will be plenty of moisture for these storms, along with shear with strong southerly winds and a strong jet stream at the low and mid levels. The lift should be there with moving borders, but that might be limited if temperatures stay cooler.
If we remain cloudy or see more intense showers in the morning and midday, this could limit our daytime warming, which also inhibits the development of instability. However, it looks like right now we will see some sunshine at times during the day on Saturday, which will allow temperatures to warm up in the east into the mid 70s. If so, we should have enough instability for severe storms to develop. Below is an image showing our projected/expected “CAPE” which stands for Convective Available Potential Energy. The higher these values, the stronger the storms will be if they develop. As you can see, the values are well above the “500 J/KG” threshold that is typically needed for severe storms to develop.
Another thing to think about is that there are still outside decorations going on right now due to the holiday season. If you are a family/household with these inflatables, today may be a good day to remove them if you haven’t already. An additional note, given that it is the holiday season, some of you may be visiting town or have family from out of town who do not have knowledge or understanding of our area and extreme weather conditions. You yourself may have just moved to northern Alabama and you may not know the area as well. Below are some tips and reminders on what to do on a bad weather day, along with a map to help YOU! Knowing what county and city you are in as well as other cities/counties near you could help you stay safe on Saturday.
As always, you can get more information as well as the latest forecast on our 48 First Alert weather app which is free in your device’s app store. As the storms develop on Saturday, we will be posting coverage on our Facebook page as well as on our streaming apps wherever you stream us. You can find us on Roku, Apple TV and Amazon Fire TV devices. Stay safe and alert!
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