The term MESO is short for mesocyclone, which is generally a radar term. A mesocyclone is a rotation found within the tower of a strong thunderstorm, and can sometimes lead to the formation of a tornado, or a tornado vortex signature on radar.
When a radar detects a mesocyclone, it is good practice to watch for time continuity between volume scans, a diameter of two to six miles, and an extension vertically of over 10,000 feet. These characteristics can only be found in velocity imagery. Nonetheless, the mesocyclone algorithm is fairly good at making sure all of the needed criteria are roughly met. One thing that can be watched from composite reflectivity is the time period at which a mesocyclone is present. The longer the period at which a rotation is sustained within the head of a storm, the better chance there is a mesocyclone is active within that given cell.
If the NWS Mesocyclone Detection Algorithm (MDA) detected a mesocyclone-like rotation in the volume scan then this value will change to "MESO", "UNCO" (uncorrelated 2-D shear), or "3DCO" (correlated 3-D shear). If the given radar site is using the new NSSL Mesocyclone Detection Algorithm, then this field will be listed as a number ranging from 1 to 25. This is the strength rank of the MDA feature with 1 being the weakest MDA feature and 25 being the strongest. If no MDA detected feature is found within 20 km of the storm cell, it will be listed as "NONE".
Since the algorithm which detects mesocyclones is sensitive, false mesocyclones can at times be detected along frontal boundaries and squall lines, or where a definite wind shift is present, especially when the front edge of a squall line is parallel to the radar beam. Therefore, a mesocyclone on radar is not always an indication a storm is severe. Radars can also sometimes falsely detect a mesocyclone if a thunderstorm is directly over a radar site. On the flip side of that, a radar cannot detect mesocyclones well, or possibly at all, if a thunderstorm is close to the radar's transmitter. Beams are unable to extend into the updraft at such a degree from the radar site. A lesser, or possibly absent, mesocyclone classification is displayed as a result.
Mesocyclones should not be considered a formation that is viewable, as tornadoes are. Some cloud features may suggest that a mesocyclone is present in a thunderstorm, such as curved feeder or inflow bands.