Tuesday, September 8, 2009
Monday September 7, an Outflow Boundary, sometimes know as a Gust Front, formed north of Nashville. See image above.
An outflow boundary, or gust front, is a rush of cool/cold air from a thunderstorm. Gust fronts can last over 24 hours, and travel hundreds of miles before dissipating. Gust fronts can be seen on radar as a slight crescent moving on the map. Outflow boundaries of gust fronts can kill off the supply of warm, moist air to a thunderstorm, but they can also rapidly shoot warm air up into the atmosphere creating new, stronger thunderstorms. Monday was such a day, and had surprising results.
I was able to document the complete lifecycle of a thunderstorm created by a gust front, and it went from severe, to dropping 70+ DBz hail, to producing a mesocyclone!
At 3:30, i noticed the gust front on radar. At 6:00 PM, the storm of interest started to initiate. The storm became severe warned at 6:13 PM. 6:25 PM I began to notice some weak, low level rotation in the storm, some miles north of McMinnville. 6:27 PM, Base Reflectivity Tilt 3 shows a pocket of 70 DBz hail. At 6:30, the rotation becomes very noticeable. At 6:40, GRlevel3 decides on it, and issues a low level Mesocyclone icon right on the center of rotation! At 6:45, the National Weather Service issues a tornado warning for areas north of McMinnville, through McMinnville. At 6:47, strongest rotation appears on Base Velocity and Storm Relative Velocity. When the storm was slightly south of McMinnville, residents report a funnel cloud.
All of this from ONE outflow boundary!
The images below are in order from left to right, in the complete life cycle.
This gust front forced warm, unstable air into the atmosphere, and started up some pretty strong convection! I do not know how common this is, but I consider it rare to have not only seen a gust front make a severe storm, but produce a meso and a funnel!
For those who would be interested, this is what the 12z sounding at Nashville read. I will only be listing indices. Remember, soundings are only taken twice a day, and is only accurate for that hour. this is just an idea of what was going on.
CAPE: 1672
CINH: 24
LCL: 923
EL: 226
LI: -5.4
SI: -3.1
TT: 53
KI: 38
SW: 273
HEL: 4
EHI: 0.2
You can find a list key to most of these indices and what there mean here. http://weather.cod.edu/sirvatka/si.html
You can find out more information about Outflow Boundaries here. http://en.wikipedia.org/wiki/Outflow_boundary
You can find out more about mesocyclones here.
stormexplorer_249 on January 1, 2010
This is a FANTASTIC article !!! Very WELL WRITTEN !!!!