From: The Texas Chaos Storm Team
Last evenings events were hard to think about and very tough to track. It wasn't the quantity of tornadoes that made this a news headline, but rather the quality of each individual thunderstorm. Many areas in north Texas, around the Dallas/Ft. Worth Metroplex were rattled by widespread severe thunderstorms. In particular, there were two towns that were hit the hardest: Granbury and Cleburn.
The original atmospheric set up did not call for such destruction. However, as the afternoon and evening wore on, the combination of an upper level low pressure system, low level heat & humidity, as well as a quick moving jet stream, came together to hash out this event. The key was the upper level low. While it was spinning over west Texas, the low was quite disorganized. However, during it's move to north Texas, the circulation became tighter and strengthening was the result. Along with the strengthening low, very cold air was associated in the upper levels, allowing for thunderstorms to grow higher, and in turn become severe with large hail, strong wind and generating tornadoes.
Some of the images were just stunning. Check out this picture of hail in Ganbury, TX that I found on facebook last night.
From spike to spike, the hail stone measures approximately 4" in diameter. That's equivalent to an average grapefruit. I cannot image something like this falling from the sky. What this illustrates is that the updraft (or upward motion) in the thunderstorm was so strong that it could support the weight and growth of this monster. The updraft speed must have been over 100mph to keep this suspended for a long period of time before crashing down to the ground.
Not only was the above picture incredible, but what meteorologists were able to view on Doppler Radar. Looking at the structure of the precipitation was a good indication that rotation was occuring in two different supercells just joust of the Metroplex last night.
If you took this image for what its worth, you would think that these were rotating supercells that look like prolific lightning, rain, and hail producers. But if you take a deeper look at the "Doppler" part of the radar, you will notice a tight circulation in the "velocity" image (illustrating speed and direction of the wind).
Here, you are looking at the southern cell that ripped though Granbury and Cleburn. This is a velocity image that helped to pinpoint the location of strongest rotation, or where the tornado was spinning. Green colors denote wind moving towards the radar site, while red shows wind moving away from the radar site. When you get these green and red colors tightly together, we call this a "couplet," indicating that there is a strong rotation in the storm with a possible tornado. Its these images on radar that support a Tornado Warning issued by the National Weather Service. Unfortunately, a radar cannot see what is happening on the ground, therefore, storm spotters, and law enforcement are extremely important in reporting whether there is a tornado or not. In this case, there was and for a time, this tornado was reported to be one-mile wide while in Granbury.
Overall, this was a very destructive event that unfolded last night and at this time the National Weather Service (NWS) in Ft. Worth is assessing each storm where a tornado was reported. Based on the damage in each area, the NWS will qualify whether a damage was from a tornado or not. If damage was from a tornado, it will be rated based on the Enhanced Fujita Scale (EF-Scale).
Because the wind was not directly measured during each storm, the rating is used as an estimation of how strong the winds were. From the survey, the NWS will say, "it will take this wind speed to cause this type of damage." Based on that estimated wind speed, an EF rating will be placed on that particular tornado.
From this morning and afternoon's survey, the NWS has rated a couple of tornadoes already. Each of the following assessments are for individual tornadoes.
Cleburn: EF-3 tornado with max winds of 140mph, and a width of 0.6 miles.
Granbury: EF-4 tornado ----preliminary
Cleburn: EF-0, minor damage with a max wind of 85mph.
For the other reports, you can click here to see the latest from the National Weather Service. As of this afternoon, there are preliminary 10 reports of tornadoes from last night. This number can change.
In closing, this was a horrific event with many reports of destroyed homes, injuries, and fatalities. When the weather looks like its going to take a turn for the worse, make sure to use your resources to stay in the loop. Your mobile devise, computer, tablet, and television are the best ways to know whats going on. Additionally, buying a NOAA Weather radio will instantly let you know if severe weather will threaten your location. Finally, your local television meteorologist will always give you the newest information so you and your family can make the safest decision.
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