RASMUSSEN TECHNIQUE


Once supercells form, it's crucial for the public's safety to find out where they will be headed.  At first, they move with the mean cloud-layer wind; however, as supercells intensify, they take a turn to the right.  Here's a radar loop showing the motion of the various supercells and their associated tornado warnings in the red boxes on the afternoon of April 4.  As you can see, many of the thunderstorms are moving northeast; however, as they strengthen, they turn more to the east or slightly north of east.

The Rasmussen Technique serves as a guide for us to predict the motion of these right-moving storms.   The image below is a diagram I drew on a hodograph demonstrating this technique using information I got from this sounding from Jackson, Mississippi just prior to these supercells.  First, plot the wind speed between the ground and 500 meters, just under 10 kts, and then at four kilometers, 45 kts. Then, draw the shear vector between these two points.  After that, locate the point that lies 60% of the distance between the tail and the arrowhead and from there draw a line representing 17 kts at a 90 degree angle.  Last, draw a line between the origin and the end point of the previous line segment.  This line represents the approximate motion of these right-moving storms on April 4 using the Rasmussen Technique.  In this case, the storm motion vector is 25535, meaning that the supercells should move from the west-southwest at 255 degrees at about 35 knots. This matches up well with the previous radar loop.


This is a diagram representing the Rasmussen Technique.  It predicted the storm motion for the right-moving supercells on the afternoon of April 4,
2008.  In this case, the motion was from the west-southwest at 255 degrees at 35 knots.

Even though some of the mesoscale analysis I documented were only marginally unstable for tornado producing supercells, for example MLCAPE and 0-1km SRH, the surface and upper-air parameters worked together to make for a sizeable outbreak; therefore, there were 14 reports of tornadoes on April 4, mostly in the watch area from central Mississippi to central Alabama.  Obviously, the SPC was right on target in issuing this tornado watch.  With so many variables to examine before issuing these watches, it's amazing how often they're right.


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