Wednesday, July 04, 2012

Weather Whys Wednesday: Not All Thunderstorms Are Created Equal

With the word "derecho" thrown into the local meteorological vernacular in the last week, I thought I'd kick off our "Weather Whys" segment by giving a little insight to how we define thunderstorms. We know in reality that thunderstorms are not all alike. But by definition, not all thunderstorms are created all.

The Thunderstorm Spectrum, from the University of Illinois
Thunderstorms generally come in three varieties: Single-cell, multi-cell, or supercell. In a nutshell, your single-cell storms can be your typical summertime "pulse storms" that flare up quickly, move slowly, but also weaken quickly. Occasionally they can go severe, but these are more often garden-variety. Obviously, supercells are the strongest and most powerful type of thunderstorm cell you can have, with rotating updrafts, large hail, strong winds, and tornadoes of all strengths and sizes. A more detailed overview on the differences in structure and development between the various types of storms can be found from the University of Illinois.

Let's talk about multi-cells. They fall into two sub-categories: Clusters and Lines. Multicell clusters are often what we see in the Northeast, where you get lines of thunderstorms with individual cells flaring up and pulsing down within it...but continuing for long periods of time. It's fairly common for these to evolve into a squall line, which means it often transitions from the multi-cell cluster to multi-cell line category. But the majority of the "lines" of thunderstorms we hear about in spring/summer are associated with these multi-cell clusters.

Plains Squall Line (Credit: Wikipedia)
As multi-cell thunderstorm clusters become better organized and become more severe than not, they can develop into squall lines. The image at left from Wikipedia shows a typical squall line in 2008 trailing from a storm system in the Plains. They're generally long, linear, and skinny. They can "bow" out forward at times due to windflow within the line of storms, becoming on radar what meteorologists refer to as "bow echoes."

Squall lines generally become big enough to be classified as "mesoscale convective systems" or an "MCS." According to the American Meteorological Society's Glossary of Meteorology, an MCS is defined as:
 A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction.
By definition, an MCS is just a multi-cell grouping of thunderstorms that is consistent over 100 km (~ 60 miles) in any direction. So, while a lot of people use the term MCS to describe large complexes of severe storms, it's actually a fairly common event. However, when an MCS gets very large, it can transition into what we call a "mesoscale convective complex," or an "MCC." 

An MCC is a large MCS, that has to meet certain size and intensity definitions (as measured by satellite). These generally peak in the afternoon and evening, maintain some measure of intensity overnight, but primarily become rain producers in that timeframe. The common theme is that they like to travel in the fast westerlies and toward the equator generally, which is why you often see them traveling huge distances and moving east and south in most cases here in the US. More good info on MCC's can be found from Wikipedia

When MCC's dissipate, they can leave behind a remnant circulation called a "Mesoscale Convective Vortex," or "MCV." As MCC's tend to release a lot of latent heat, their interiors can warm over time and pressures lower. You're creating a small area of low pressure basically within the cluster of storms. A lot of times, this is why when we see these large complexes of storms winding down on radar and satellite, it almost has the appearance of a hurricane...and the term "land hurricane" can be tossed around (albeit too loosely sometimes) to describe an MCV. Some great details and examples of MCV's can be found here.

So you've got MCS, MCC, and MCV. But Friday night you had a derecho, so where does that fall in all this? By definition a derecho must have a damaging wind swath of at least 240 miles, including a band of 58 mph or greater wind gusts on the leading edge that lasts as a system for at least six hours. You can have "serial derechos," which have at least a couple "bow echoes" embedded within the complex. Friday night's derecho would be classified as a "progressive derecho," which develops as a small line of storms and increases into one large bow echo. There are also "hybrid derechos," which can have characteristics of both serial and progressive derechos.

Friday Night's Derecho, Credit: CIMSS Satellite Blog
I have yet to find anything that says definitively that Friday night's derecho was an MCC, though I think it met the definition. It certainly was, by definition, an MCS. Regardless of what it was, it was impressive. The University of Wisconsin CIMSS Satellite Blog published an awesome, awesome satellite recap of the derecho yesterday. It's worth checking out, especially for the nighttime snapshots of lights, showing power outages, as well as for the event-long loops. 

Hopefully this clears up a bit of the meteorological lingo involved in thunderstorm definitions.