Interestingly enough, no tornadoes occurred on the day the surface low was the strongest over Colorado and Kansas (March 13, not shown). This was because the system was so large and dynamic that it pushed associated surface fronts too far north and eastward, away from moisture and instability. But that moisture recovered again as surface fronts slowed for severe weather on March 14th over the Ohio Valley.
This system was a good example of how warm fronts are efficient tornado producers. The surface maps below on March 12 & 14th show that the locations of several tornadic storms were near warm fronts:
Warm fronts offer temperature gradients, increasing moisture, and enhanced low-level wind shear, which is why tornadoes seem to occur so often with storms near such boundaries. (Other tornadoes - not shown - did occur in the warm sector over Alabama during late afternoon and evening on March 14, away from boundaries.)
Note how instability and low-level wind shear combinations using forecasts of the 0-1 km energy-helicity index (EHI) from the RAP model were strong near warm fronts and tornadic storms on both days, offering good support for low-level storm rotation:
The 500mb NAM model forecasts below (roughly 18,000 ft MSL) also show how strong this system was, with a massive full-latitude midlevel trough moving out into the Plains. A "spreading" jet stream pattern (white arrows) ahead of this large trough generated large-scale lift and ascent over the areas where daytime and evening severe weather occurred on March 12 and 14 (yellow ovals):
Thankfully, no deaths occurred from any of the tornadoes with this system. But the Colorado blizzard associated with this powerful system claimed at least one life, and flooding in Nebraska from rapid snow melt and heavy rain caused at least one other death.
It is good to see that the week ahead will be much quieter weather-wise across the United States!
- Jon Davies 3/15/19