View of Marlin cell from Chilton looking almost due east (Photo courtesy of Claytia Doran)

Bill Purcell, a storm chaser from Houston targeted the area of east central Texas on Mar. 21st and was rewarded with this unusual event!
See some of his photographs here.


0.5 deg base reflectivity from GRK nexrad at 6:05pm CST
There was a persistent but narrow zone of convective activity across northern Bell, southern McLennan and northern Falls counties from noon through 6pm CST. The images below (at 30 minute intervals) show how the convection waxed and waned several times.


0.5 deg base reflectivity from GRK at 1:00 pm CST.


0.5 deg base reflectivity from GRK at 1:30 pm CST.



0.5 deg base reflectivity from GRK at 2:00 pm CST.


0.5 deg base reflectivity from GRK at 2:30 pm CST.


0.5 deg base reflectivity from GRK at 3:00 pm CST.


0.5 deg base reflectivity from GRK at 3:30 pm CST.


0.5 deg base reflectivity from GRK at 4:05 pm CST.


0.5 deg base reflectivity from GRK at 4:30 pm CST.


0.5 deg base reflectivity from GRK at 5:00 pm CST.


0.5 deg base reflectivity from GRK at 5:30 pm CST.


0.5 deg base reflectivity from GRK at 6:00 pm CST.
The images above also show that comparatively strong and widespread convection covered the area southeast of Falls County for most of the afternoon, although the activity gradually drifted eastward. Surface analyses (below) at Noon, 3pm and 6pm CST show southeasterly winds and cool, moist conditions in the area east and southeast of Falls County as a result of that activity.

Plots made with Digital Atmosphere

   Three sets of three maps each (for 18Z, 21Z, and 0Z) showing (1) raw surface plot, (2) Theta-e and (3) wind streamlines. In each set of maps, Falls County (Marlin is the county seat) is highlighted on the raw data map. The Theta-e analyses and the streamline analyses are a product of what is called objective analysis (OA). OA means that the analyses were not influenced by human intervention. The raw data plots are just that (i.e. no analysis has been performed). A good way to think of Theta-e (equivalent potential temperature) is that is represents a combination of heat and humidity (both important components for thunderstorms). The wind streamline analysis is way of representing the flow of air parcels via the surface wind.
 
 


12 Noon CST



3pm CST


6pm CST


12 Noon CST


3pm CST


6pm CST


12 Noon CST


3pm CST


6pm CST
The following are zoomed images from GRK nexrad at about the time of the tornado. I have added some annotations to assit in identification of important features:
 
 


0.5 deg GRK base reflectivity at 6:05pm CST. Note the small reflectivity notch just above the "n" in Marlin.



0.5 deg GRK storm-relative velocity at 6:05pm CST. Height of the beam at Marlin ~3600 ft. Note the coincidence of the inbound/outbound couplet compared to the notch inthe preivous image.


1.5 deg GRK storm-relative velocity at 6:05pm CST. Height of the beam at Marlin ~8200 ft. Inbound (green)/outbound (red) couplet showing rotational velocity of ~26 knots.


0.5 deg SRM at 6:05pm (non-smoothed)


1.5 deg SRM at 6:05pm (non-smoothed)


2.5 deg SRM at 6:05pm (non-smoothed)