Thursday, May 16, 2013

Comments on Last Night's Tornadoes

Ft. Worth Star-Telegram
I have spoken with a number of reporters today (USA Today's article here) about the storms in Texas last night. There are a couple of thoughts I'd like to pass along to my readers:

Cleburne Tornado's Change of Direction

Yes, tornadoes change directions. There is an excellent animation (look at the lower left hand corner) here. But, keep in mind the geographic scale: This is a shift of 2-3 miles. Two points:

  • The NWS Central Region attempt to declare "tornado emergencies" for highly specific locations may fail in a surprising number of cases because we don't know how to anticipate these shifts. 
  • That said, the tornadoes stayed within the warning polygons. Emergency managers often justify sounding the sirens over entire counties (well beyond the polygons) because "you never know which way they are going to go." We do know on the scale of the polygon. Sirens should be sounded only in the polygon and a small margin for error. We need to stop over warning. 

Intensity of the Tornadoes

In the case of the Southeast tornado outbreak of April 27, 2011, or the Kansas-Oklahoma outbreak of April 14, 2012, we knew it was coming for days and said so. Like Joplin, yesterday's storm ingredients came together quickly. The map below depicts the atmospheric "helicity" (tendency to rotate) at 7pm from ground to 1 kilometer above the ground. Significant tornadoes are more likely to occur with values above 150. You can see values were above 300 last night. 
When we saw the ingredients coming together, we started to sound the alarm hours in advance. 

These geographically limited tornado outbreaks, for the forseeable future, will probably not be forecastable days in advance. But, keeping in mind that it only takes a short time to save your life in a tornado, the fact there was a timely, advance tornado watch and a timely warning, we consider it a success. 

Comparison to Greensburg

I mentioned this while I was live-blogging the storms. The one-mile wide tornado after dark looked like the Greensburg storm and the intensity of the Doppler wind measurements (very strong) were similar. 
Winds are about 120 mph blowing toward and away from the radar. Very, very strong tornado indicated and the NWS, after examining the damage today, rated this tornado EF-4. More from the National Weather Service here

ADDITION: I just came across this video on Facebook. Between the continuous lightning and the tornado's appearance, it is very similar to Greensburg.

More tornadoes are likely this weekend and Monday. Please check back from time to time. 

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.