A Quick Detour to Spring…

Enough of all this winter weather talk! Let’s switch gears for a minute… or a blog post.

As we enter April with the “official” start of spring in a few weeks, the chance for thunderstorms is slowly increasing. For the New England area, thunderstorms are common from June to September. They can occur any time of the year, so its never too early for a lesson and how you should prepare for a thunderstorm.

A thunderstorm is defined by the National Weather Service as “a rain-bearing cloud that also contains lightning”. There are also dry thunderstorms, which still has precipitation, but the rain evaporates before reaching the ground.

Single-celled thunderstorms occur in three stages throughout their lifetime. The schematic below represents these stages, along with arrows depicting the updrafts (red) and downdrafts (blue) that compose them. Convection from diurnal heating leads to air being heated at the surface, which will then rise and if it’s warmer than its environment, the air will continue to rise. In unstable environments, air parcels rise and lead to vertical cloud development. This is demonstrated in the cumulus stage of the thunderstorm. As this process continues, air will keep rising until it reaches a stable environment, which leads to the parcels spreading out in a large anvil head shape. Once precipitation becomes too heavy for the cloud, the cloud will precipitate out in the form of heavy rain (or hail depending on the temperature and strength of the updrafts) in the downdraft region (mature stage). This will continue until the downdraft dominates the system, which will lead to the dissipating stage of the storm.

Source: atmo.arizona.edu

Wind shear is also a major component of thunderstorms, which is the change in wind speed with height. Unstable environments with strong wind shear can lead to the development of severe thunderstorms. This includes the development of new updrafts, which introduce new storms. Supercell thunderstorms are much more complex, as depicted by the graphic below.

The main concept is still the same! A supercell requires a rotating updraft for storm maintenance. There is still a dominant downdraft, which is called the Forward Flank Downdraft. Notice behind the updraft is the Rear-Flank Downdraft. This is another, but smaller, downdraft that forms as a result of a strong updraft. Several other ingredients are necessary for a supercell thunderstorm. Sufficient background buoyancy (or Convective Available Potential Energy, CAPE) is needed for development, along with the wind shear mentioned above. If the wind shear is too strong with very little instability, the shear can prevent storm development.¬†Supercells can dissipate when the outflow “chokes off” the storm. The outflow can cut off the source of warm, moist air in the storm with cold, dry air. This would then make the storm “outflow dominant”.

The most common dangers associated with thunderstorms are lighting and flash flooding. Tornadoes are also possible with the development of supercell thunderstorms. While these weather phenomena occur only once or twice a year in Massachusetts, tornadoes occur across the country with 142 confirmed tornadoes nationwide already in 2018!

The Storm Prediction Center (SPC) is located in Norman, Oklahoma, which is an office focused solely on the prediction and forecast of storms. Each day, the SPC creates Convective Outlooks for that day (Day 1), the next day (Day 2), the day after (Day 3), and an outlook for the next four days (Days 4-7). Below is the convective outlook for today with a small chance of thunderstorms for eastern Colorado and southwest Kansas, almost all of Florida, and south Texas. The SPC also puts out a very detailed forecast discussion about what is occurring in the atmosphere now and what they expect for the day all across the country. On Day 1, a tornado outlook is also produced with a similar map. While today the outlook says there is a less than 2% probability for tornadoes, the SPC will update their graphics throughout the day. This graphic could even change if the chance for thunderstorms increases. Mesoscale Discussions (MD) are also produced from the SPC if storms in a particular area seem to be developing.

To prepare for thunderstorms’ most dangerous aspects, the National Weather Service has created some catchphrases as a reminder of how dangerous weather can be. “When Thunder Roars, Go Indoors!” is about the dangers of lightning and to find shelter as soon as thunder can be heard. Lightning can strike as far as up to 10 miles away. The thunder from this lighting sometimes isn’t heard yet, but that doesn’t mean that lighting can’t reach you.

Another phrase is “Turn Around, Don’t Drown!”, which is concerning flash flooding. Flash flooding can occur within a matter of minutes. Only six inches of fast-moving water is necessary to sweep away an adult. 12 inches of rushing water can sweep away an automobile, while 2 feet of rushing water can sweep away most other vehicles.

 

Thunderstorms are beautiful and complex weather phenomena. They are also very dangerous. Now that you have a better understanding of how they form and develop, you can stay safe while also basking in their glory.

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