Brian and I made it back from Hamilton. What a long day for the people in Tennessee. And we now have weather to deal with, too. I need to borrow some coffee from JB. This event is not over. Tornado warning in effect for Lamar county, and that storm approaching Pickens county has developed strong rotation also, so I wouldn’t be surprised to see tornado warnings on that storm soon if it continues.
Looking at the SPC mesoanalysis, the strongest wind shear is moving farther to the northeast into Kentucky now, as surface low moves toward southern Illinois. But, keep in mind that the 0-1 km storm-relative helicity values tonight in Tennessee (up to 900 m2/s2) are about the highest I have ever seen in a storm environment. It only takes about 150 for storms to rotate, and we still have 400 m2/s2 here in central Alabama.
Storm-relative helicity is simply winds increasing in speed and changing direction with height, creating spin about a horizontal axis, like a well-thrown pass. If a lineman tips the ball at the line of scrimmage, the ball will still spin, but about a vertical axis. The updraft of a storm plays the role of the lineman tipping the spin from horizontal to vertical, producing storm rotation. The helicity measures the horizontal spin relative to the storm inflow.
It has cooled off some, with temps near 70. But, the upper-levels are cooling off, too, and the air is still humid. So, we have instability over central Alabama, with CAPE still near 1000 J/kg…plenty to maintain storms.
The bad news for us is that the storms have still not gotten together in a single line, but there are instead discrete storms out ahead of the line. With the high wind shear, this may continue all the way to I-65 or further.
