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 Langtry 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.
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?
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.