The first significant severe weather outbreak of 2019 is likely to unfold on Saturday across parts of southern United States. An approaching storm system will allow for a dangerous environment in which thunderstorms can grow severe in a hurry. The Storm Prediction Center characterized the threat for Saturday as a "significant tornado and wind damage event" in their Friday night forecast. The severe weather threat will develop during the late morning hours and continue through nightfall.
The same upper-level trough that brought unusual snow to southern California and the desert southwest on Thursday will serve as the catalyst for the severe weather we'll see on Saturday. A surface low-pressure system was already developing in northeastern New Mexico on Friday night. This low will continue to strengthen as it moves across the southern Plains and into the Midwest through Saturday evening.
A combination of warm air, muggy dew points, and an advancing cold front will trigger the development of strong thunderstorms across the Deep South on Saturday afternoon. The storms will likely turn severe as they interact with robust wind shear passing over the region.
Friday's late-night forecast from the Storm Prediction Center paints a moderate risk for severe thunderstorms across parts of Mississippi, Alabama, and Tennessee, which is a 4 out of 5 on an ascending scale measuring the threat for severe weather across a region. An enhanced risk for severe thunderstorms—a 3 out of 5 on the scale—stretches from northeastern Louisiana to central Tennessee, including Memphis, Nashville and Jackson, Mississippi.
The moderate and enhanced risks are in place due to the threat for strong, long-lived tornadoes and damaging straight-line wind gusts in excess of 70 MPH. The threat for these two significant hazards isn't confined to the areas shaded orange and red on the map, but it's where the SPC expects the environment to be most conducive for the most dangerous thunderstorms to develop.
Storms are likely to fire-up across the western risk areas in the late morning hours on Saturday, continuing to grow in strength and extent as they move east through the afternoon and evening hours. Many areas in the eastern part of the risk areas could deal with their severe threat after nightfall, which adds an extra element of danger for folks who insist on looking outside (which, let's face it, is pretty much everyone) before diving for the basement or interior bathroom.
It's tempting to focus solely on the bullseye in a severe weather forecast, but it's important to remember that severe thunderstorms are also possible in the marginal (dark green) and slight (yellow) risk areas. Everyone between Little Rock, Baton Rouge, Birmingham, and Evansville should stay alert tomorrow and prepare to take quick action if a warning is issued.
Damaging straight-line wind gusts are most likely in storms that form into squall lines, while the threat for strong tornadoes and large hail are more likely in individual storms that are able to fully interact with the environment without contamination from nearby storms choking off their supply of unstable air.
There's also a risk for flash flooding across the affected areas. Flooding adds insult to injury when there's a threat for major severe weather. Parts of Mississippi and Alabama have seen 8-10 inches of rain in the last week. Many of these same areas will see even more heavy rain with the storms on Saturday. It's a good idea to stay off the roads (and avoid storm chasing!) to avoid running into flooded roadways when you might have to outrun dangerous thunderstorms.
Safety Tips
Here are some tips to remember on Saturday, especially if you're visiting this region and you're not accustomed to preparing for severe thunderstorms.Remember that a tornado watch means that conditions are favorable for the development of tornadoes over the next couple of hours. A tornado warning means that a thunderstorm is capable of producing a tornado based on strong rotation detected by Doppler radar, or that someone spotted a tornado in progress. A watch means to watch for storms. A warning means you need to act immediately.
Tornado warnings are represented on most weather maps and radar imagery as a red polygon. The threat for the tornado is greatest within that polygon, but areas just outside of a warning polygon should keep an eye on the storm. Polygons are drawn based on the speed and motion of a thunderstorm. Storms can change direction and speed up or slow down without much notice.
Make sure you have a way to receive severe weather warnings the moment they're issued. Enable the emergency alerts function on your cell phone. Wireless Emergency Alerts are free push notifications sent out by the National Weather Service using cell phone towers to triangulate your position and determine if you're in a tornado or flash flood warning.
Do not rely on tornado sirens for tornado warnings. These aging systems are unreliable and they're only designed to be heard outdoors. People have died in tornadoes because they relied on tornado sirens that they never heard. If you don't have a cell phone—or if you live in an area with an unreliable signal—listen to television, radio, or NOAA weather radio for severe weather updates. Local news stations will run wall-to-wall coverage if tornado warnings are in effect, and many radio stations in the south will stream those severe weather newscasts.
The best place to ride out a tornado is in an underground basement or dedicated tornado shelter. If you can't get underground, your best bet is an interior room on the lowest level of the building. The goal is to put as many walls between you and flying debris as possible. An interior bathroom works really well since there are pipes in the walls and (usually) a bathtub to crouch in.
Wear closed-toe shoes like sneakers or boots if you're under the threat for tornadoes. You don't want to get caught walking through broken glass and wooden splinters barefooted or in flimsy flip-flops.
Wear bicycle or motorcycle helmets if you have to take shelter during a tornado warning. You might look like an idiot, but the fashion police are taking cover, too, and the helmet could save you from head injuries if the worst happens.
Since we're getting into severe weather season, it's a good idea to make the Storm Prediction Center's website a part of your daily routine if you live in a part of the country prone to severe thunderstorms. The SPC's forecasts are tremendous and they can keep you informed of a potential severe weather outbreak days in advance.
(I updated this post at 2:00 AM EST to mention the threat for flash flooding across the areas expecting dangerous thunderstorms.)
(I updated this post at 2:00 AM EST to mention the threat for flash flooding across the areas expecting dangerous thunderstorms.)
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