April 23, 2020

A Powerhouse Supercell Managed To Travel 300 Miles Across Three States

A real powerhouse of a supercell thunderstorm rolled across hundreds of miles of southern woodlands on Wednesday evening, ramping up in east-central Texas on Wednesday afternoon before finally petering out nearly eight hours later over the Mississippi River near Natchez, Mississippi. The supercell produced multiple tornadoes along its path, even prompting a tornado emergency as it approached Fort Polk, Louisiana.

Wednesday saw a classic springtime severe weather event across the southern Plains and lower Mississippi Valley. A line of tornadic supercells developed along a dry line in central Oklahoma, including a tornado that killed two people and grew to frightening strength in a matter of seconds.

The environment was just as ripe for severe weather farther to the south in Texas. A single supercell managed to survive for eight hours as it tracked from College Station, Texas, to Natchez, Mississippi. The video above shows a radar loop (video by me, radar by College of DuPage) of the storm along its track from College Station to Natchez. It's the southernmost storm that begins on the bottom-left and ends just right of center in the frame.
The thunderstorm near College Station that'd grow into the long-track supercell. (Gibson Ridge)

A thunderstorm began to develop broad rotation near College Station, Texas, around 3:00 PM CDT, slowly growing more organized over the next couple of hours as it moved east of the town. Things got going in a hurry as the supercell moved south of Lufkin, Texas, around 5:30 PM CDT, producing the first in a series of tornadoes that would touch down between there and central Louisiana through the evening.

A tornado east of Jasper, Texas. (Gibson Ridge)

The supercell neared peak strength once it got into far eastern Texas, cycling through stronger and weaker phases as it moved into Louisiana. The radar image above shows the storm around 7:30 PM CDT, as it approached the Texas/Louisiana border. It's broken down into four panels:

  • The top-left panel shows reflectivity, or precipitation.
  • The top-right panel shows velocity, or wind. Green shows wind blowing toward the radar (to the east), while red shows wind blowing away from the radar. When you have a strong, tight cluster of red and green right next to each other, it shows strong rotation within a storm.
  • The bottom-left panel shows correlation coefficient. The radar beam can tell us how similar in shape and size the objects are that it's intercepting. Uniform objects like raindrops have a high correlation coefficient (red/purple), while mismatched objects like tornado debris have a very low correlation coefficient (blue). When you see a dark blue dot in the same spot as tight rotation, you're looking at debris swirling around a tornado.
  • The bottom-right panel is normalized rotation, or NROT. This is an algorithm within Gibson Ridge's radar program that helps you quickly spot areas of rotation. Higher NROT values can indicate stronger rotation.
A tornado approaching Fort Polk, Louisiana, on April 22, 2020. (Gibson Ridge)

This storm appears to have produced several strong tornadoes across eastern Texas and central Louisiana. The National Weather Service in Lake Charles, Louisiana, had to issue a tornado emergency for Fort Polk, a military training site that houses thousands of troops and their families, as a confirmed tornado moved very close to the heart of the base.

The storm continued producing possible tornadoes as it passed southeast of Alexandria, Louisiana, before the updraft weakened and the storm finally started falling apart as it crossed the Mississippi River south of Natchez, around 11:00 PM CDT. The storm powered along almost uninterrupted for eight hours (!!!) before it finally fell apart.

A supercell is powered by a rotating updraft. Strong wind shear can cause horizontal rolling motions in the atmosphere. If a strong thunderstorm can develop in that environment, the updraft will push that rotation into the vertical and the updraft itself will begin to rotate. This rotating updraft then tilts downwind, allowing the storm to ingest unstable air and vent cooler air without choking itself off like a "normal" thunderstorm would.

This was a rather unusual testament to the endurance of a supercell when it's in a near-pristine environment. We usually don't see this kind of a marathon run outside of major outbreaks.

We're used to hearing about supercells after a tragic tornado or major hailstorm, but the vast majority of supercells are relatively—relatively—weak and don't last for very long. Supercells can be extremely fragile, falling apart if their outflow moves too quickly or if nearby storms contaminate the environment ahead of them with stable air. It's quite something to watch a supercell on radar practically disappear in just a few frames.  But there was nothing in this storm's way and it took full advantage of the situation.

Update: I changed the headline from "Tornadic Supercell Travels Farther In One Day Than You've Gone In The Last Two Months" to "A Powerhouse Supercell Supercell Managed To Travel 300 Miles Across Three States" because...well, do I really need to explain? 

You can follow me on Twitter or send me an email.

Please consider subscribing to my Patreon. Your support helps me write engaging, hype-free weather coverage—no fretting over ad revenue, no chasing viral clicks. Just the weather.

April 19, 2020

Another Dangerous Severe Weather Outbreak Is Possible In The Southeast On Sunday

Another severe weather outbreak is possible in the southeast on Sunday, affecting many of the same areas hit by severe weather just last weekend. A moderate risk for severe weather is in effect from Louisiana to Georgia, but just about everyone in the region is at risk for dangerous storms on Sunday and Sunday night. All modes of severe weather are possible, including the risk for strong, long-track tornadoes.

Source: Tropical Tidbits

A low-pressure system will move across the southeast during the day on Sunday. Thunderstorms will fire across the risk areas as the low pushes east through the day. Strong instability and wind shear will allow the storms to grow severe. The above model image for Sunday afternoon shows winds at the 850 mb level (about 5,000 feet or so), showing the low-level jet that will allow some storms to grow into supercells and pose a significant tornado threat.

This map looks similar to last weekend's severe weather threat The Storm Prediction Center paints a threat for severe thunderstorms across most of the southeast, with the greatest threat centered on Louisiana, Mississippi, Alabama, and Georgia. A moderate risk for severe weather covers many of the same areas that were under the greatest risk last week; this Sunday's threat covers a greater area, from northern Louisiana to central Georgia.

Storms will ramp up in a hurry during the afternoon and continue east through the nighttime hours. This is going to be another sleepless night for folks in Georgia and parts of the Carolinas.

Forecasters will refine these forecasts with updated information through the day on Sunday. Beware the sharp cutoff on the northern edge of the risk areas; it wouldn't take much of a shift to bump the threat for dangerous storms a few dozen miles to the north.

The most serious threat is tornadoes. The greatest threat for tornadoes exists in and around the moderate risk area, but any severe thunderstorms in the region have the potential to produce tornadoes. Remember that these percentages seem rather low, but we're talking the probability of tornadoes—the red shading means there's a 15% probability of a tornado within 25 miles of any point in the shaded area. The black hatching indicates the risk for strong, long-track tornadoes.

While tornadoes grab all the attention, there's a threat of large hail and damaging winds with any of the storms that form across the risk areas on Sunday. The strongest storms could produce hail the size of golf balls or larger.

There's also a significant wind threat on the eastern portion of the risk area in Alabama and Georgia. Storms will eventually coalesce into one or more squall lines capable of producing intense straight-line winds of 70+ MPH. This is a heavily wooded area, so communities are susceptible to widespread tree damage if a well-organized line can get going. It's a good idea to make a mental note of large trees or limbs hanging over your home and avoid those rooms when storms are on the way.

One caveat, like always, is that storm mode will determine who sees which hazards. Discrete thunderstorms will have the best opportunity to take advantage of the strong wind shear and live up to their full potential. That's what happened last week in Mississippi, where a big, messy cluster of storms to the north produced extensive wind damage while storms farther south had free rein to produce intense tornadoes.

As I pointed out last week, never count on the forecast flopping. Be prepared for the worst and hope for the best. Keep an eye on the radar and make sure you have multiple ways to receive severe weather warnings. Check that emergency alerts are activated in your phone's settings. Don't rely on tornado sirens for tornado warnings—sirens are outdoor warning systems that aren't designed to be heard indoors, and these aging systems prone to failure due to glitches, power outages, and high winds simply drowning them out.

Oh...one more thing. If you follow plenty of weather folks on social media, you might see chatter throughout the day on Sunday that the Storm Prediction Center might (or should, depending on their point of view) upgrade some areas to a high risk, or a full 5/5 on the scale used to measure the threat for severe weather. Don't worry about the difference between a moderate risk and a high risk. Some of the worst severe weather days have occurred during moderate risks. No matter how you phrase it, Sunday could be another significant severe weather day in the southeast and anyone in the region should be glued to the weather until the threat clears out.

You can follow me on Twitter or send me an email.

Please consider subscribing to my Patreon. Your support helps me write engaging, hype-free weather coverage—no fretting over ad revenue, no chasing viral clicks. Just the weather.

April 11, 2020

A Significant Severe Weather Outbreak Is Possible Across The Southeast On Easter Sunday

A multi-day severe weather event will peak on Sunday with the possibility of a significant severe weather outbreak across the heart of the southeastern states. A moderate risk for severe weather is in effect on Sunday for parts of Louisiana, Mississippi, and Alabama, with the threat for dangerous storms radiating out from there to include communities from eastern Texas to eastern Georgia. Conditions appear favorable for storms to support strong, long-track tornadoes, as well as widespread damaging winds and some instances of very large hail.

The Setup

Source: Tropical Tidbits

This weekend's severe weather setup has its roots in the same upper-level low that brought so much rain and snow to California over the last week. Parts of California have seen more than half a foot of rain at lower elevations and many feet of snow at higher elevations as a result of the stalled system.

California's storm got pinned over the West Coast by a large ridge of high pressure over the central United States. A trough dipping south out of Canada pushed the ridge out of the way, unblocking the upper-level low and allowing it to move east across the United States. The trough generated a low-pressure system at the surface over the southern Plains, setting the stage for the severe weather.

Sunday's Outlook

All indications point to a large-scale and potentially significant severe weather outbreak on Sunday. A moderate risk for severe thunderstorms—a 4 out of 5 on the SPC's ascending scale—exists from central Louisiana to central Alabama, with an enhanced risk (3/5) covering a huge area from the northern Gulf Coast to the Ohio River.

Storms will start across the western risk areas on Sunday morning, progressing eastward through the afternoon and evening hours. Severe thunderstorms will reach Georgia and the Carolinas overnight on Sunday into Monday.


The banner headline on Sunday is the tornado threat.

The SPC's outlook on Saturday afternoon painted a 15% risk for significant tornadoes across the moderate risk areas, with a 10% risk radiating out from there to include New Orleans, Mobile, Atlanta, and Memphis. 10-15% doesn't seem like much, but it's a relatively large risk considering we're talking about tornadoes.

This is a common setup for the middle of April. We see this type of threat at least once a year...often more than that. Tornado safety is embedded in the culture around these parts.

Not every storm will produce a tornado. Not every tornado will be enormous, nor will every tornado incur a high rating on the Enhanced Fujita Scale. But the setup on Sunday looks similar to the kind of days that produce significant tornadoes. That should give you some sense of how serious the situation could be if the event lives up to its full potential.

Source: Tropical Tidbits

If storms are able to develop during the period when wind shear is just right, they'll have plenty of "spin" to work with throughout the lower levels of the atmosphere. The graphic above shows the winds at 850 mb (about 5,000-ish feet). The area of intense southerly winds over the southeastern states is known as a low-level jet; winds in the lower levels of the atmosphere are ripping along at 50+ knots, gradually veering toward the southwest and west as you get higher up toward the jet stream. This twisting of the winds will allow strong thunderstorms to develop into supercells capable of producing strong tornadoes, large hail, and damaging winds.


As you'd expect with a significant severe weather setup, hail is going to be a big deal. The strongest storms will be capable of producing hail the size of golf balls or larger, which is big enough to total a car and cause serious damage to homes.

This kind of hail will easily shatter a window, especially when it's blowing around in wind gusts of 60+ MPH. Even "just" a severe thunderstorm warning is reason enough to get away from windows and take cover. Shattering glass from windows and skylights is dangerous enough without all the other threats thrown in the mix.


The threat for damaging winds is always the most widespread risk on a severe weather day. Wind gusts of 60+ MPH are enough to bring down trees and power lines, leading to widespread power outages. Trees are vulnerable to snapping in high winds right now because they're in full leaf and this is the first real stress they've experienced since last summer. Stay mindful of limbs and large trees that loom over your home and avoid those rooms when a storm is on its way.


Don't sleep on the risk for flash flooding from heavy rain. Several inches of rain are likely whether or not you experience any severe weather. The Weather Prediction Center's excessive rainfall outlook paints a moderate risk for flash flooding across a large portion of the southeast on Sunday. Natural waterways will quickly overflow their banks during a heavy thunderstorm, and street flooding is a high likelihood as well.


The low-pressure system will reach the Great Lakes by Monday and trigger one last round of severe thunderstorms over the eastern states before the cold front moves out over the ocean. The SPC's latest forecast shows a risk for severe storms from northern Florida to upstate New York, with the greatest risk existing along the coastal Carolinas.

Many of Monday's storms will occur early in the morning when folks are asleep, so make sure you have a way to receive severe weather warnings that'll wake you up.

Words of Caution

1) The caveats.

The best case scenario is that only a handful of severe thunderstorms form and meteorologists get hollered at for a week because people got upset and scared "for nothing." We see a few notable setups under-deliver every year. Last year was the infamous high risk over central Oklahoma; conditions looked primed for a horrendous tornado outbreak, but storms had trouble forming in the area where all the parameters for tornadic development were maxed-out.

Historic tornado outbreaks are rare and memorable for a reason. Big severe weather outbreaks have so many failure points that it's hard for a big one to live up to its full potential. Supercells can quickly merge into a squall line and rob each other of the dynamics they need to produce tornadoes. Showers and storms earlier in the day can muddle the instability and stunt storm development later on when the wind shear is most favorable for supercells to develop.

While it's hard for a day to achieve its potential, never bet on a bust. If there's a risk, assume it's going to happen and plan accordingly. It's great if it doesn't. There's still a lot to learn about meteorology. But it's a testament to how far the science has come that forecasters can raise the alarm days ahead of time when some of our immediate family members remember the days when you had to see the tornado to know one was possible.

2) Know the jargon.

You'll see a lot of insidery terms and model images flying around on social media tonight and tomorrow, often without explanation. Not all meteorologists gear their posts and language toward the general public; lots of meteorologists use Twitter to talk to other meteorologists. However, some pages, especially on Facebook, like to post this stuff because it gets your Uncle Filbert to share it with all his friends.

Source: Pivotal Weather

One of the most commonly shared images these days is "updraft helicity." This graphic appears to show tornado-like tracks moving across the at-risk areas. Updraft helicity measures the strength of the rotating updrafts in thunderstorms simulated by the model.

People who aren't familiar with these graphics tend to fixate on the exact track of the paths—not unlike spaghetti model charts during hurricane season—and go "whew, I'm in the clear" or "oh no, we're doomed." Those updraft helicity charts are best used to see where thunderstorms have the best opportunity to turn into supercells that could produce tornadoes. The stronger the signal, the more favorable the environment. It's never a good idea to focus on the specific tracks.

Source: Pivotal Weather

Another routine visitor during tornado season is the Significant Tornado Parameter (STP). Meteorologists have developed a ton of indices to boil down lots of different dynamics into one bite-sized number to quickly identify areas of hazardous conditions. The STP is one of the parameters that got traction because it's relatively easy to talk about with folks who aren't weather buffs.

The STP is an algorithm that combines different measures of instability and wind shear to highlight how favorable an environment is to produce significant (EF-2+) tornadoes. It's not perfect, but it's good for a quick evaluation of where things could get ugly if storms can fully engage with the environment around them.

You might also see meteorologists and enthusiasts talking about "CAMs," which became one of those must-use buzzwords a few years ago for people who need to prove their worth (or make themselves sound smart, but that's another issue) on social media. A CAM is a convection-allowing model, such as the NAM or the HRRR. These models allow the simulation of thunderstorms, which is helpful during severe weather events. Global models like the GFS and European can miss the small-scale features that help meteorologists fine-tune their forecasts. Meteorologists can use models like the NAM or the HRRR to help forecast the placement, motion, and type of severe thunderstorms up to a day in advance.

Safety Steps

1) Stay. Home.

The potential for a tornado outbreak during a pandemic is an unusual situation, and it's one that meteorologists and emergency managers have been dreading since the COVID-19 crisis started.

Most states are under some version of stay-at-home orders or guidance right now. Theoretically, people shouldn't be gathering at church on Easter Sunday or piling into homes for a big dinner on Sunday evening. If everyone is mindful of the safety guidelines, folks staying home instead of gathering in large crowds should limit the potential for mass casualties that would come from a tornado outbreak on a normal Easter Sunday.

That's how things are supposed to be.

In practice, though, we have widespread reports of churches intentionally rebelling against stay-at-home orders and people ignoring the orders to visit family and friends under the misguided illusion that they can't get sick or that they're not carrying the virus. That's a horrible idea given that we're approaching the apparent peak of COVID-19 infections across the country, but it's an exceptionally bad idea on a day when dangerous severe weather is possible.

Many folks live in homes that are susceptible to high winds and would be incur serious damage or complete destruction in even a low-rated tornado. I wrote about this conundrum a few weeks ago. What do folks do when they rely on community shelters that might not even be open? There aren't many good answers, but it's best to think about it now rather than when the tornado warning is issued.

2) Don't look for the tornadoes.

Seriously. The terrain in the southeast absolutely sucks for spotting tornadoes. In most areas, all you'll see are dark clouds scraping the tops of the trees until the tornado is right on top of you. I know lots of people still like to see the tornado coming before they'll dive into their basements/closet/bathroom/whatever. Not only does the terrain here obscure tornadoes until it's too late, but the atmosphere is so moist in the southeast that most tornadoes are rain-wrapped, making them impossible to spot until it's too late.

If there's a tornado warning, trust the threat is there and seek shelter like you saw it yourself.

3) Wear shoes, jeans, and a helmet.

If your home is damaged by a tornado, strong winds, or even a serious pelting of large hail, you'll have to walk through a myriad of sharp debris to get out and assess the damage. You'll need to wear shoes and jeans to walk across debris like broken glasses, jagged pieces of wood, electrical wires, pipes, and nails.

Also, it may look dorky, but wearing a bicycle or motorcycle helmet while you're sheltering from a tornado could save your life. It's unpleasant to talk about, but a leading cause of death during a tornado is a fatal blow to the head by flying debris. A helmet can't stop you from getting hurt if the worst happens, but it can keep you alive.

4) Watch the radar and listen for warnings.

This is the kind of setup that will likely produce long-lived storms. It's likely that folks in the path of the most serious storms will be able to watch them on the news for an hour or more before they approach their communities. Be proactive. Stay aware. And please make sure the emergency alerts in your smartphone's settings are still activated for tornado warnings.

You can follow me on Twitter or send me an email.

Please consider subscribing to my Patreon. Your support helps me write engaging, hype-free weather coverage—no fretting over ad revenue, no chasing viral clicks. Just the weather.

April 2, 2020

April Starts With Thundersnow And A Frigid Chill On The Northern Plains

The northern Plains is still clinging to winter as a push of Arctic air descends over the region and a round of snow to close out the first week of April. Periods of snow will bring up to half a foot of accumulation to parts of North Dakota and Minnesota through Friday before the skies clear out in time for the weekend. Parts of South Dakota experienced a full-fledged wintry thunderstorm on Thursday afternoon, complete with hundreds of lightning strikes mixed in with snow, sleet, and freezing rain.

A cold front barreling across the Plains on Thursday made itself known in a hurry. There's a 30-40°F difference between temperatures across the boundary; shorts weather earlier today turned into winter-coat-fireplace-hot-cocoa-stay-in-bed weather in just a couple of minutes once the front rolled overhead. Temperatures quickly fell into the teens across the Dakotas on Thursday, and the chill will continue toward the Upper Midwest over the next day or so.

Thursday afternoon's forecast from the National Weather Service shows a decent blanket of snow covering most of the Dakotas and northern Minnesota by Friday night. The heaviest totals are likely along the Red River between Fargo and southern Saskatchewan. Parts of northwestern Minnesota could see up to half a foot of snow by the end of the storm.

Lightning strikes across South Dakota on Thursday afternoon. | Source: LightningMaps.org

It's not uncommon to see accumulating snow this far north at the beginning of April; Fargo averages about 3 inches of snow in a typical April. But it is uncommon to see such an intense bout of thundersnow/thundersleet/thunderfreezingrain (hey, we're working with limited terms here) as folks in central and eastern South Dakota experienced on Thursday afternoon. An intense line of convection moved across the state around lunchtime on Thursday and produced a heavy wintry mix with hundreds of cloud-to-ground lightning strikes embedded within. The map above shows the history of lightning strikes as the wintry squall moved through the region.

You can follow me on Twitter or send me an email.

Please consider subscribing to my Patreon. Your support helps me write engaging, hype-free weather coverage—no fretting over ad revenue, no chasing viral clicks. Just the weather.