Thunderstorms come in a variety of packages. They can organize into mesoscale convective systems (here's a radar image of a mesoscale convective system that produced severe weather over the northern Middle Atlantic States in early June, 2008) or thunderstorms can remain discrete.
Discrete convection can take the form of a single-cell, multicell or supercell thunderstorms. By and large, vertical wind shear determines the type of discrete convection, with single-cell storms forming in environments with weak shear, multicell storms in environments characterized by moderate shear, and supercells in environments marked by large vertical wind shear. There's a lot of meteorology in that last sentence, folks, so I'll just focus this essay on supercells. Actually, I should say a specific supercell.
Late in the afternoon on February 18, 2009, a long-lived supercell produced golf-ball sized hail and straight-line damaging winds from the town of Livingston, Alabama, to Montgomery, Alabama. Above, the 2307Z base reflectivity from Maxwell Air Force Base (KMXX) shows the supercell pretty much due west of the radar site at the time. Please hold this thought for a moment.
By way of background, a supercell is a highly organized thunderstorm that's characterized by a rotating updraft. The original branding of a thunderstorm as a supercell probably was meant to reflect its longevity (the average lifetime is one to four hours, with a few senior-citizen supercells lasting as long as eight hours). Modern mesoscale meteorologists, however, define a supercell as a thunderstorm that possesses a deep, persistent mesocyclone.
You can infer the presence of a mesocyclone in the schematic of a supercell below; the curving shape of the updraft suggests rotation (most, but not all, mesocyclones rotate counterclockwise in the Northern Hemisphere).
The most useful observation we can make from this schematic is that the supercell's updraft and forward-flank downdraft are essentially separate. In this configuration, the downdraft doesn't destructively interact with the updraft. As it turns out, the lack of interaction paves the way for supercell longevity. Why do the supercell's updraft and forward-flank downdraft remain separate? Not surprisingly, fast winds at high altitudes associated with strong vertical wind shear carries precipitation particles downstream away from the updraft, where they fall on the storm's forward flank, dragging air downward with them and creating the forward-flank downdraft.
Although supercells are probably the least common type of thunderstorm, they produce almost all the strong to violent tornadoes (EF-2 or higher on the Enhanced Fujita Scale) and essentially all hail with diameters greater than two inches (roughly golf-ball size) ... which brings me back to the long-lived supercell that produced golf-ball sized hail over parts of south-central Alabama on February 18, 2009.
Before I tackle the specifics of how supercells erupted on this day, I emphasize that the lift required to initiate discrete supercells is typically weak to, at best, moderate. If you're recoiling in disbelief, I point out that strong lift (strong low-level convergence, strong upper-air divergence, etc.) often results in storms organizing into mesoscale convective systems. In other words, any outbreak of discrete thunderstorms tends to quickly evolve into organized ensembles of thunderstorms.
The synoptic-scale environment in which the Alabama supercell developed was pretty typical. On the morning of February 18, red flags went up in my brain when I looked at computer guidance (the 12Z model runs). The upper-air forecasts from the Canadian Model (American models had similar a solution) showed an area of 850-mb confluence (see the merging 850-mb streamlines on lower-left panel below).
Here's a close-up in case you can't quite see the 850-mb confluent zone. Confluent conditions also prevailed near the ground, suggesting an area of relatively weak convergence near the earth's surface. The confluence zone formed in concert with a sprawling area of low pressure centered over the Great Lakes region and a return flow from the Gulf of Mexico ahead of a cold front (check out the 12Z surface analysis).
By 21Z, solar heating paved the way for Convective Available Potential Energy (CAPE) to increase to 1000 Joules per kilogram and higher in the vicinity of the confluent zone. Here's the analysis of 21Z analysis of CAPE from the Storm Prediction Centre in Norman, Oklahoma (red contours indicate lines of constant CAPE). For the record, CAPE is a measure of instability. More specifically, CAPE represents the area between the path of an air parcel (after it reached its Level of Free Convection) and the local environment's temperature. Read more about CAPE.
Meanwhile, the vertical wind shear in the layer between the ground and an altitude of six kilometers was as high as 70 knots over the region (see 21Z analysis below).
In a nutshell, there was relatively weak lift at the surface, strong vertical wind shear, and adequate instability ... a perfect recipe for discrete, fairly long-lived supercells to form.
And form they did (2325Z regional radar reflectivity). The supercell west of the Maxwell Air Force Base first caught my eye later that afternoon when I looked at the radar reflectivity and saw a honkin' hail spike while also watching the supercell split into two cells (check out the 2238Z radar reflectivity below). Folks, the topic of splitting supercells will be fodder for another essay, but the spurious hail spike obviously motivated me to write this piece.
I say 'spurious' here because the radar echoes associated with the hail spike (sometimes called a flare echo) were phantom targets. To explain, first note the relatively small area of 70 to 75 dBZ reflectivity near the core of the storm. Such extremely high reflectivity corresponds to large, wet hail suspended high in the supercell (the updraft speed likely exceeded 60 knots). In such situations, suspended, large hail backscatters a lot of energy transmitted by the radar. Some of this scattered energy hits the ground, which, in turn, backscatters energy back to the suspended hail, which, in turn, backscatters some of the energy back to the radar. This multiple 'ricochet' of microwave energy takes extra time for the signal to return to the radar. The radar then interprets this 'extra time' associated with the returning signal as echoes that lie at a greater distance from the radar. These phantom echoes then appear on a display of radar reflectivity as a spike that extends away from the supercell.
Inherently, spurious hail spikes are compelling, but they also serve to alert forecasters of the presence of large, potentially damaging hail.