May 20, 2019

A Rare 'High Risk' Tornado Outbreak and Flash Flooding Are Likely in Texas and Okla. Monday



Monday could be a high-end severe weather day on the southern Plains, with a threat of severe weather we only see once every couple of years. Multiple rounds of severe thunderstorms are possible, beginning in the morning and likely continuing after people go to bed on Monday night. This is a complicated setup that has the potential to produce significant, long-track tornadoes if the storms are able to take full advantage of the environment around them. Heavy rain from Monday's storms could also produce significant flash flooding in Oklahoma.

The Storm Prediction Center has issued a rare high risk for severe thunderstorms across parts of western Oklahoma and Texas. This is the highest category on the SPC's scale used to convey the risk for severe thunderstorms, saved only for days that feature dynamics that could produce the most dangerous severe weather outbreaks. A moderate risk—one rank lower, but still extremely serious—radiates from the high risk to include Oklahoma City, Tulsa, Lubbock, and Abilene.

It's been two years since we last saw a high risk issued by the Storm Prediction Center. The agency reserves high risk days for when they're forecasting high-end severe weather events. They're very careful not to pull the trigger on a high risk so that the term keeps its urgency and invokes a specific response from people in harm's way.

It appears the environment will be capable of supporting a high-end severe weather outbreak. It's hard for big severe weather outbreaks to occur. Lots of things have to go right and it's easy for storms to morph into a sloppy mess on a day like this. Even if the storms on Monday don't reach maximum capacity—and many likely won't!—one storm breaking through to produce one bad tornado is still a big deal.

I keep flashing on the Lee County, AL, tornado from a couple of months ago. The storms that day were largely sloppy, but one supercell broke loose and put down an EF-4. It's cliche, sure, but it really does only take one. Please take the threat seriously and prepare even if it doesn't turn into a classic tornado outbreak.

That said, the SPC's 2:00 AM EDT update on Monday doesn't mince words. Here are some of the phrases they use in their technical forecast discussion explaining why they issued this forecast:
"[...] a tornado outbreak is likely across the southern Plains. The tornado outbreak is expected to continue into the overnight period. This event should result in a significant threat to life and property."

"This will be ideal for a tornado outbreak with strong tornadoes [...]"

"[...] making conditions favorable for long-track strong tornadoes and possibly violent tornadoes."
The SPC doesn't use strong language like that lightly.

Timing

This will be a long-duration event that begins Monday morning and likely lasts through early Tuesday morning. The greatest threat for severe weather exists during the afternoon hours, but dangerous thunderstorms will be possible for a solid 12+-hour window.

It's likely that several rounds of storms are possible. The greatest tornado and hail threat will occur in discrete supercell thunderstorms that manage to engage with their environment without contamination from other storms. The greatest threat for damaging wind gusts and flash flooding will occur in squall lines and clusters of storms that train over the same areas.

Tornadoes



Monday's high risk is in effect due to the 30% probability of seeing strong, long-lived tornadoes within 25 miles of any point within the shaded area. A 30% risk doesn't seem all that big—we see a 30% chance of thunderstorms almost every day!—but when you consider your odds of seeing a tornado on any given day are down around 0%, it's extremely high. The 10% and 15% risk areas are also nothing to sneeze at. Usually you would start to raise an eyebrow when you see 10% on the chart.

Additionally, there's a risk for strong, long-lived tornadoes in and around the areas covered by black hatching on the map above.

Not everyone will see a tornado, of course, but the environment appears extremely favorable for strong tornadoes to develop in any storms that can tap into the instability and wind shear available to them on Monday. This has the potential to turn into a very serious situation very quickly.

Flash Flooding



It could turn out that flash flooding is the most significant disaster that unfolds across the southern Plains on Monday. High moisture and deep, robust thunderstorm activity will allow any thunderstorms that form to drench whoever's caught beneath them.

Heavy rain and flash flooding will occur regardless of tornadoes or damaging winds or large hail. It appears that many of the thunderstorms that form across Texas and Oklahoma on Monday will begin to train, or repeatedly move over the same areas like a train on railroad tracks. Some models are spitting out bonkers rainfall totals in Oklahoma—even approaching double-digits in just 24-36 hours.

This kind of heavy rain will quickly lead to life-threatening flash flooding, both for residents in vulnerable areas and motorists trying to cross roads covered by moving water. It doesn't take much water to carry a car downstream and drown the occupants inside. An average year sees more people die in floods than tornadoes. Take the threat for flooding just as seriously as the threat posed by the thunderstorms themselves.

Hail and Wind

Don't sleep on the threat for hail and wind. Strong supercells can produce enormous hailstones to the size of baseballs or larger. That can easily total a car and break through windows, skylights, and even flimsy walls and ceilings. It's certainly deadly if you're caught outside when that kind of hail is falling. Wind-driven hail is even more dangerous. Severe thunderstorm warnings say "stay away from windows" for a reason.

Everyone under a threat for severe weather on Monday is under a threat for damaging wind gusts. Straight-line winds can produce as much damage as a weak tornado, but over a much wider area. Downed trees are a hazard to people in cars and in homes. Power outages are a given. Structural damage is even possible if some of the squall lines can produce wind gusts above 70 MPH.

Perspective

Monday is sure to be a long, stressful day, and there's (understandably) quite a bit of anxiety about it. Many schools in Oklahoma have cancelled on Monday to keep everyone home ahead of the severe weather. Area businesses, churches, and governments will likely follow suit. The main goal is to keep people at home where they're safer than they'd be if they were in a big building or out on the roads.


This is prime time for tornadoes on the southern Plains. The last two weeks of May—and Monday's date in particular—occupy an uncomfortable period in tornado climatology in this part of the country. No two severe weather outbreaks are exactly alike. It's hard for a significant tornado outbreak to occur. But the environment is more than capable of supporting serious thunderstorms that could produce significant tornadoes. Even one storm producing one bad tornado is one too many.

Safety

The region's climate leaves most residents well-prepared for the basics of tornado safety. We all know the deal..."get to an interior room in the lowest level of the building, putting as many walls between you and the outdoors as possible." That's just the beginning, though.

Plan your day accordingly. Don't run to Walmart or another big box store when storms are on the way. Mentally map out your home, work, or school to scout out the safest place to go if a tornado warning is issued. Do you have a safe building to pull off the road if you have to go out? If you have to be in a big box store, do you know the safest place within the store? Ask the manager. Seriously. They have to have a plan for that kind of stuff in this part of the country.

Leave mobile/prefabricated homes. Spend the whole day somewhere safer—a friend's house, the library, anywhere but home. Mobile homes can start to roll and break apart in winds as low at 70 MPH. Even a small tornado can heavily damage or destroy a mobile or prefabricated home and leave you in a life-threatening situation. Hell, the bathroom in a McDonald's provides you with more protection than a mobile or prefab home.

Consider the flooding risk when seeking shelter from a tornado. If your home is in a flood-prone area, consider leaving and going somewhere safer for the day. You don't want to be put in the impossible situation of deciding whether to go to the basement—and possibly drown—or stay at ground level and possibly risk a direct strike from a tornado.

Wear a helmet, jeans, and closed-toe shoes if you have to take cover from a tornado. The most vulnerable part of your body in a tornado is your head and a helmet will spare you from at least some debris if the worst happens. Jeans and closed-toe shoes will protect your feet and legs if you have to walk over debris.

Make sure wireless emergency alerts are activated on your cell phone. These alerts have saved countless lives over the past couple of years. Some folks disabled these alerts ahead of a nationwide test last year. Keep them on! I often receive my emergency alert within a minute of my weather radio going off.

Use a weather radio. Many people in this part of the country already have weather radios. Modern weather radios are like smoke detectors for the weather. You can program them with your county's unique code so they sound a loud siren when you're placed under a watch or a warning. Most devices even flip on the weather radio feed and read the warning out loud.

Don't rely on tornado sirens for warnings. These systems are outdoor warning systems and they are not meant to be heard indoors. Tornado sirens are also vulnerable to technical failures, power outages, and wind shifts that affect where they can be heard.

Don't hide from a tornado under a bridge or overpass. It is not safe. Strong winds grow even stronger when they press under a bridge. Three people died under three separate bridges during the 1999 Bridge Creek-Moore tornado in central Oklahoma. Bridges will not protect you. Get to a sturdy building and take shelter there.

Don't hide from large hail under an overpass, either. Drivers trying to protect their vehicles from hail damage often wind up creating a traffic jam, stranding hundreds (possibly thousands) of people and emergency crews behind them. This is especially dangerous since large hail often precedes a tornado in a classic supercell, which could leave all those people trapped in the path of a tornado with nowhere to go.

Don't go storm chasing. Monday is going to be a dangerous day even for the experts and experienced weather enthusiasts, and it's magnitudes more dangerous for folks who don't know what they're doing. Just stay home. Honestly, if I was a storm chaser, my first move on Monday morning would be to book the first flight to anywhere out of Oklahoma City. I don't know why anyone wants any part of that mess, especially with the high risk for flash flooding in many of the areas expecting severe weather.

Keep up with the Storm Prediction Center's website through the day. The SPC issues severe thunderstorm and tornado watches, as well as short-term forecast discussions that can give you a heads-up of what's coming over the next couple of hours. Stay proactive on a day like this. You should know about the threat for storms an hour or two before they arrive. Don't simply wait for a warning to act.


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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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