What happened to Monday’s Severe Weather? 20 March 2012

Tornado Reports are Reports, Not Tornadoes 

By BlueWolf

On April 20, 2012 · Leave a Comment April 14, 2012 reports to the Storm Prediction Center. Red, Tornado; Blue, Wind; Green, Hail. Tornado watchers know the map well. The one with red, green and blue dots all squeezed up close to each other. It’s not like it’s terribly hard to decipher either. It says “reports” seven times. While the April 14, 2012 tornado outbreak weighed in with a formidable number of tornadoes, enough to put it in the top rung of April one-day events since 1950, far too many continue to write* or talk** about tornado reports as if they are each separate tornadoes (or in some cases, actually a tornado to begin with!). As of recent counts, the outbreak caused at least 60 tornadoes, and perhaps as many as about 75. Signaling on the event was excellent, and warning was at least passable if also increasingly confusing — i.e., tiered warnings and/or one warning type now meaning different things depending on the office that issued it. Most fortunately in the whole event, almost all of the tornadoes occurred in very rural areas, and there were many eyes on each storm. This is by no means an exhaustive analysis. But, to take a quick look at the difference between a tornado report and an actual tornado, we’ve pulled out a few of the more notable long-track supercell events on April 14 to get an idea of how many times an individual tornado — or groups of tornadoes — was called in to the National Weather Service (NWS). Map by Katie Wheatley. The one confirmed violent tornado of the day, to the southwest of Salina, Kansas, was the subject of many a video. It also received, by our count, at least nine individual raw (unfiltered) reports compiled by the Storm Prediction Center (SPC). Almost miraculously, the supercell producing the large and dangerous tornado cycled enough for the tornado to lift as it approached Salina. Later, additional and weaker tornadoes touched down to the northeast of the city. The latter pair of EF-1s picked up at least another six reports in the SPC log. That makes 15 reports for three tornadoes. Map by Katie Wheatley. As nightfall approached, another tornadic supercell entered into Kansas from northwest Oklahoma. This storm produced at least eight additional tornadoes in areas nearby to, and including, Wichita. These tornadoes that crossed the Wichita area produced well over two dozen tornado reports, or about 3-4 for each tornado, if averaged. Granted, while dual reports are common, there are plenty of instances — particularly days with isolated activity — where the only official report equals one tornado. It’s also just as easy to imagine a situation where a long-track tornado like that to the southwest of Salina lifts and drops multiple times. Still, as attention-grabbing a headline as 100+ tornadoes strike is, it’s wrong. This was a hefty event, though in terms of strong to violent tornadoes, and in terms of impact to humanity, it could have been much, much worse. As we’re reminded often in situations that can cause catastrophe, it only takes one. An important part to reducing the risk of life from tornadoes is to better understand them. Within that understanding is the ability to keep events, whether large or small, in context. So, when reporting on reports, remember… they’re just reports. While tornado reports on the SPC web site have remained stable since the maps above were created, it’s still possible they will change. The same goes for tornado tracks and/or potential additional tornadoes. The information here is presented as an example and not necessarily a final look at this event. However, we allowed enough time to pass that we feel any changes will be minimal enough overall to not impact the idea presented. Storm report data from the Storm Prediction Center. Survey data obtained through NWS Wichita. Base map data via ESRI. Notes… * NYT piece written April 16, when reports were close to 100 compared to today’s 146. **AccuWeather mentions preliminary tornado reports of 544 as tornadoes this year, while SPC specifically says not to do that on their page. What happened to Monday’s Severe Weather?
20 March 2012 Last Updated on 20 March 2012 0 Comments

Monday's Severe Weather Outlooks with Storm Reports

This graphic shows the severe weather outlook from the Storm Prediction Center for Monday, March 20. A standard risk of severe weather was in effect from Wichita Falls southwest to Lang-try and points east. An enhanced, significant risk of severe weather was in effect from Southeast Oklahoma southwest into North and Central Texas. Overlaid on the outlook are official storm reports which show hail larger then one inch, winds over 58 MPH, and tornadoes. We see several reports of large hail, damaging winds, and even a few tornadoes in the southern portion of that significant risk area, but what happened in North Texas and Southeast Oklahoma? There are only a few reports of marginally severe hail and damaging downburst winds. The sun was out and instability was there, wind shear was very favorable for a severe weather outbreak, yet most folks only received garden variety thunderstorms with heavy rainfall. In terms of severe weather for the northern half of the significant risk area, our forecast was a bust. I want to explain what came together to cause the severe weather not to occur. We were always saying the highest tornado risk was in South Texas, where tornadoes did occur, but we expected large hail and damaging winds up north. We still got a few reports of those events, but the coverage was expected to be much greater. Lets go ahead and dive into some details and explain what factors came together to prevent widespread severe weather.

North Texas Radar - 4:04 PM CDT

I saved this reflectivity image at 4:04 PM on Monday for North Texas. The white line on the screen is an outflow boundary, which is essentially the leading edge of rain-cooled air. To the west of this line, thunderstorms are elevated above the surface and temperatures were in the upper 50s. To the east of this outflow boundary, temperatures were in the upper 70s with dewpoint values between 63°F and 67°F. Those living in those areas know how humid and muggy it was on Monday. The airmass ahead of the outflow boundary was very unstable and as many of you could tell yesterday from the surface winds, we did have a lot of wind shear in place. So why did we not receive severe weather in North Texas? Take a look at that radar image again and notice where all the rain/thunderstorms are located in relation to the outflow boundary. All the rain and thunderstorms at the time of this radar image were behind, or west, of the outflow boundary. That means those storms were ingesting rain-cooled air and were elevated above the surface, meaning they were not rooted in the boundary layer of rich, unstable air. If that outflow boundary had not pushed out ahead of the thunderstorms, it’s likely they would have been quite strong to severe as they pushed eastward across North Texas. So why did nothing form out ahead of this line in the warm, humid air mass?

Weather Balloon Data - 1 PM Fort Worth

The National Weather Service in Fort Worth launched a weather balloon at 1 PM on Monday. This allows us to actually see what is going on in the atmosphere. I know there is a lot of data on this graphic, but I want to focus your attention on the black circle I’ve placed on the screen. Do you see how the red line quickly rises? That red line actually shows the temperature around 6,000 feet above sea level and that rise in temperature is indicative of a strong inversion, or cap, in the atmosphere. That cap prevented thunderstorms from developing ahead of that line. Based on the data this weather balloon provided, it appears temperatures would have needed to be near 90°F in order for thunderstorms to break the cap. Obviously we didn’t get near that yesterday.

Further south in areas south and west of San Antonio, they were warmer and the upper level forcing associated with the storm system was closer to them, thus the cap was weaker and discrete thunderstorms were able to form out ahead of the line of thunderstorms. These storms quickly became supercellular and produced giant hail and tornadoes, one of which did appear to be rather significant. Luckily, these occurred outside major metropolitan areas, but did approach San Antonio. Yesterday goes to show that no matter how well you think you understand a weather setup, mother nature will always throw something unexpected into the mix. Don’t think just because yesterday didn’t end up being a major severe weather day that future forecasts will be so lucky.