Severe WX 101

So what makes everyday weather, severe weather?

That’s a tough question to answer, as each person has their own idea of what severe weather is. From a pure definition standpoint from the National Weather Service, a thunderstorm is severe when the winds reach 56MPH and/or produces hail 1″ diameter or larger. Usually when that criteria has been met, damage from the storm occurs. Damaging thunderstorms are what the NWS are trying to warn the public about, and where the “Severe” part comes from. Lets talk about each part of a storm in more detail.

Lightning is a dangerous part of the storm, but ironically, the amount of lightning doesn’t indicate how severe a storm is. If you are outside and a storm starts to form, lightning is a huge factor on your activities and should not be taken lightly. If you can hear thunder, you are close enough to be struck by lightning. When thunder roars, go indoors.  Lightning in itself regardless of how frequent or intense it is, isn’t something to report, or factored into the severity of a storm. That being said, damage caused by lightning can and should be reported to the NWS.

Wind as little as 40 MPH can be enough to cause damage. Straight line winds are the most common associated with a thunderstorm. Straight line winds in excess of 50 MPH can look very violent and cause widespread damage to homes and trees. More damage occurs from straight line winds than damage from tornadoes annually, and are more likely here in the Carolinas. For example 60-70 MPH straight-line  winds are enough to roll over a mobile home, pick up debris and cause damage.

Beaufort Scale for estimating wind speed

  • < 1 MPH  is Calm
  • 1-3 MPH Smoke drifts, leaves remain still
  • 4-7 MPH Wind felt on skin, leaves rustle
  • 8-12 MHP Leaves in motion, flags unfurled
  • 13-17 MHP Small branches in motion, dust raised
  • 18-24 MPH Medium branches in motion, small trees sway
  • 25-30 MPH Large branches in motion
  • 31-38 MPH Whole trees (larger) in motion
  • 39-46 MPH Twigs broken, walking is difficult
  • 47-54 MPH Branches break, small trees toppled
  • 55-63 MPH Trees broken or uprooted, building damage
  • 64-73 MPH Widespread tree & structure damage (tropical storm force winds)
  • 74 + MPH Debris & loose objects hurled about (hurricane force winds)

Precipitation. Rainfall can vary greatly in a thunderstorm. Heavy rainfall occurs in the dissipation stage of a storm.  A typical event can start off as light rain with gusty winds, followed by a downpour. When the rainfall rates approach 1″ an hour, they could be considered severe, depending on the size of the area affected and how long the event lasts.  A large scale heavy rainfall event can cause localized flash flooding and trigger a flash flood warning.

  • Hail is a form of precipitation. Hail is formed when rain drops pass through a cold layer and freezes as at falls. Depending on the updraft of the storm, the small stones get picked back up by the updraft and carried back up into the cloud where it combines with other stones to become larger stones. This cycle continues until the weight of the stone is heavy enough to pass through the updraft and fall to the earth. Storms with a high updraft are usually severe in nature due to the large hail stones that are produced. Small hail stones are an indication that an updraft is present, and could increase and produce larger hail. For example, an updraft of 110 MPH with cold air aloft in the storm, could produce softball size hail stones. The NWS likes to receive reports of any hail at the start, and hail that is 1″ in diameter afterwards. That tells them the updraft has increased and is producing larger hail.

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