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Severe storm definitions

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There is often an overwhelming media blitz of terms that identify unique weather. An example was the winter of 2014 featured that the Polar Vortex. This has been a long standing definition of this among meteorologists, but it sounding catchy and helped describe the incredible intensity and long duration of cold air impacting much of the US.

Another storm event that became cliché is a derecho. The June 29, 2012 event that extended from Chicago through metro Washington DC and central Maryland with 600 miles of destructive weather. Since it impacted large metropolitan areas, it became widely known, but these events have had a frequency of striking with a lot less press coverage.

I wanted to compile a list of terms describing severe and extreme storms for the warm weather season, in hopes to clarify what is what. Not every intense storm is a derecho, but there will be others. Misusing it might give the false impression of what is on the way. While the fear of damaging storms may raise awareness for more people to take it seriously, I would prefer to continue to use the proper descriptions.

This is not every term available for storms or associated topics. Rather this is the most commonly used for storms themselves. Many of the terms listed below are directly from NOAA. Others have been simplified.

Thundershower- A brief rain event with lightning and thunder. Often these last only a few minutes.

Thunderstorm – A longer lasting rain event with lightning and thunder. These can include gusty winds to produce some damage, but under 57 mph gusts. Small hail may be part of this as well.

Severe Thunderstorm – A thunderstorm with winds reaching 58 mph and or associated with large hail of 1 inch in diameter or larger. These are often referred to as ‘quarter size’ hail stones or larger.

Most thunderstorms that form with afternoon heating have a life cycle up to 1 hour.

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Severe storms on the way?

Watch: Issued for severe thunderstorms or tornadoes for a large area that has the conditions that might help these events develop. It is NOT a guarantee that it will happen., but may during the duration of roughly 6 hours.

Warning: Issued for a severe thunderstorm or tornado that is occurring at that time. This alert would be for a specific area, such as a county with certain towns or cities identified. This would usually be issued for 30-45 minutes blocks. That does not mean the storm will last that long, just track in the path listed during that time.

Supercell- A severe thunderstorm with the entire structure rotating. This is a small area of Low Pressure that allows the storm to extend longer than a typical thunderstorm with a constant supply of warn air feeding energy in, and a way to evacuate the colder air from clogging the system. These produce larger hail, and can drop tornadoes but that is NOT a guarantee.

Mesocyclone- A small area of Low Pressure, but a large complex of thunderstorms like a supercell that can last for many hours.

Gust Front – A push of cold air ahead of an approaching storm. As rising air builds a thunderstorm, colder air at the top of the cloud structure gets forced out in front of the system, and since it is more dense that the warmer air, will dink rapidly. This is a downdraft. This can produce a drop in temperature just before the rain arrives. It is often responsible for developing shelf clouds as well.

*Downburst - A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.

Microburst - A small, concentrated downburst affecting an area less than 2.5 miles across. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

Straight-line Winds - Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds. Damage can be similar to a tornado with winds over 60 mph, but the wind pushes in one direction, rather than twisting and turning around a concentrated center.

Derecho - (Pronounced deh-REY-cho), a widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

MCC - Mesoscale Convective Complex. A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

  • Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more.
  • Duration: Size criteria must be met for at least 6 hours.
  • MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

MCS - Mesoscale Convective System. A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Tornado - A violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. A condensation funnel does not need to reach to the ground for a tornado to be present; a debris cloud beneath a thunderstorm is all that is needed to confirm the presence of a tornado, even in the total absence of a condensation funnel.

Ranking tornadoes based on the Enhanced Fujita Scale (EF Scale)

  • F0 (weak): 40- 72 mph, light damage.
  • F1 (weak): 73-112 mph, moderate damage.
  • F2 (strong): 113-157 mph, considerable damage.
  • F3 (strong): 158-206 mph, severe damage.
  • F4 (violent): 207-260 mph, devastating damage.
  • F5 (violent): 261-318 mph, (rare) incredible damage.

*Landspout - [Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Waterspout - In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.

Flooding is also considered severe weather.

Flash Flood- Quickly rising flood waters. Sometimes this can occur as water flows downstream for a storm in another location.

Aereal Flooding: Widespread high water due to region wide heavy rain.

Coastal Flooding: This can be from heavy rain; rising water from tides, surge, or sloshing; or a combination.

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