Types of thunderstorms and severe weather.

 

Single-cell thunderstorms:

This type of thunderstorm, sometimes called pulse or airmass thunderstorm, are typically small weak thunderstorms, that don’t last very long usually lasting 20-30 minutes. They typically form due to summertime heating during the afternoon. as the name implies, this kind of thunderstorm has only one cell with a one-time updraft and one-time downdraft. Single-cell thunderstorms may produce brief heavy rain and lightning. Single cell thunderstorms typically don’t become severe.  But if they do, they can cause microburst, small to moderate size hail, straight-line wind damage, and even weak tornadoes.

Multi-cell Cluster thunderstorms:

Sometimes if the atmospheric conditions are supportive, the potential is there for more than just a single cell thunderstorm forming. This type of thunderstorm, consist of a group of thunderstorm cells moving as a single thunderstorm. The setup aloft, is such that as the first thunderstorm matures, it is carried downstream by the upper level winds, this allows a new thunderstorm cell to form upwind of the previous cell. The process continues, numerous cells in various stages of development, merging together forming a large cluster of thunderstorms that moves along the path of previous cells. This is called training thunderstorms and can lead to flash flooding. Multicell storms can produce severe weather, which can consist of microburst, small to moderate size hail, straight-line wind damage, and weak tornadoes.

Multicell Line Thunderstorm:

Multicell line storm, or more commonly called a Squall Line. Is when multi cell clusters form into a line of thunderstorms with a continuous, well developed gust front on the leading edge of the line.  Most of the time, this gust front looks like a low hanging arc of clouds, called a shelf cloud. Right behind the shelf cloud can be strong damaging winds. A squall line chiefly causes straight-line wind damage. But it can also produce small to moderate size hail, occasional flash floods and weak tornadoes. Long-lived strong squall lines are called derechos (Spanish for 'straight'). Derechos can travel many hundreds of miles and can produce considerable widespread damage from wind and hail.

Many times, radar will pick out areas along the squall line that bow out. These are known as bow echoes. A strongly bowed section along the line is an indication of very high winds behind the area of the bow echo.

 

Supercell Thunderstorms:

This type of thunderstorms is called a mesocyclone because the storm is feeding off and rotating around its tilted updraft. A supercell can last for several hours and produce very strong downbursts large to giant hail, extremely heavy rain leading to flash floods, and weak to violent tornadoes.

Derecho:

There are three types of derechos. One type is called a serial" derecho and is produced by multiple bow echoes embedded in an extensive squall line that sweeps across a very large area, both wide and long. This type of derecho is typically associated with a strong, migrating, low pressure system. An example of serial derecho with a very extensive squall line and with embedded smaller scale bow echoes like the derecho we saw on October 7^th, 2020.  The second type of derecho is called a progressive derecho and it is associated with a relatively short line of thunderstorms which can look like a single large bow echo on radar. The damage path is chiefly narrower than that of a serial type of derecho. The third type of derecho is called a hybrid derecho, and may have mixed characteristics of both a progressive and a serial derecho.

In order to get a derecho in the Northeast, it takes a complex combination of factors that have to come together at the exact right time in the exact quantity needed. If any of these factors are off you will most likely just see scattered severe thunderstorms.

Typically, we need a hot and humid airmass to be in place. Also, we need a strong intensifying upper level disturbance over Ontario, Canada, which is dragging a trough and strong cold front (sometimes a stationary front will get the job done) into the Northeast. As the trough becomes negatively tilted, we end up with a low to mid-level jet overhead.  As conditions continue to develop a cold pool of air will typically setup overhead.  With the westerly/northwesterly flow and the jet streak overhead we see instability continue to grow along with a temperature inversion (Cap) overhead. The Cap keeps the atmosphere from producing thunderstorms. Think of the atmosphere as a well shaken can of soda, ready to explode at the moment that cap is removed.  The conditions continue to build, until something like an outflow boundary or lake breeze lights the spark and breaks the Cap. With the cap no longer in place the hot, humid air at the ground with all that pent-up energy rushes upwards as a result, thunderstorms quickly form and go severe.  The thunderstorms quickly become very organized and become the monster thunderstorm complex squall line that becomes a derecho. The thunderstorm updrafts tap into the strong winds from the jet streak aloft, and mix down to the surface. Creating all the straight-line wind damage at the surface. The complex feeds off of the environment ahead of it, which like  the case on October 7^th, the derecho can track hundreds of miles producing extreme damage along it’s track.