visible satellite 1945 UTC 7/6/01Tornadic profiles with very large low-level CAPE and relatively weak shear
Environments with small CIN, a low LFC, and very large 0-3 km CAPE (around 200 J/kg or more) are often associated with relatively weak or marginal amounts of SRH and vertical wind shear. Possibly because of increased potential for parcel accelerations near the ground, such "weak shear" situations with large low-level CAPE can produce "surprise" tornadoes, although rarely will they be stronger than F1 or F2 in intensity. Pre-existing boundaries with significant vertical vorticity seem to play an important role, and when SRH and deep layer shear are at least marginal, interesting tornado events can happen.
Example 6. This environment has small CIN (<
20 J/kg), a low LFC, and around 200 J/kg of 0-3 km CAPE, which is very large. With
at least marginal shear support and several boundaries in the area to provide vorticity
and focus, this large low-level CAPE environment was capable of supporting a storm with a
damaging and significant tornado (rated F2).
(Eta analysis profile at Myrtle Beach, South Carolina, 18 UTC 7/6/01,
updated by 19 UTC surface ob at MYR):
visible satellite 1945 UTC 7/6/01
Observed: F2 tornado with $8 million damage at Myrtle Beach |
total ML CAPE 3477 J/kg 0-1 km SRH 86 m2/s2 |
| parameter value | support for supercell tornadoes? | why? | comments | |
| 0-1k EHI | 1.9 | marginal/ok | near 2.0 | shear-CAPE combinations and |
| BL-6k shear | 30 kts | marginal | >= 30 kts | vertical shear are |
| LCL height | 872 m | strong | < 1000 m | at least "marginal", while |
| low-level thermodynamic | ||||
| CIN | 19 J/kg | strong | < 50 J/kg | parameters are all "strong" with |
| LFC height | 1121 m | strong | < 1500 m | low-level CAPE quite large |
| 0-3k CAPE | 201 J/kg | very strong | > 90 J/kg, very large |
description and key for examples
operational low-level thermodynamic parameter guidelines