Generally, when you think of thunderstorms, you think of spring and early summer. You're probably wondering why I'm talking about thunderstorms in the fall. I mainly want to communicate to you how you can tell whether thunderstorms will form or not without the aid of forecasters or forecasting experience. This I want to explain because next spring, long-range forecasters are predicting a violent severe weather season for the Ohio Valley and the Lower Great Lakes region. They are expecting an even greater threat for severe weather around here than last year, with the possibility of a very strong EF-3 or EF-4 arriving in the Northern Indiana forecast area by mid-April, which could create some major problems for this area. I want to people to prepare for next spring, as all the signs look just right for a damaging severe weather season in the United States.
First I want to talk about the life cycle of a typical thunderstorm (air-mass variety), as compared to a severe thunderstorm's life-cycle. Back in the 1940's, there was a study done to show the life-cycle of a typical thunderstorm, so that meteorologists could make better forecasts on thunderstorm development and to help save lives. The first part of the cycle that they found was the convective stage or cumulus stage. During this stage, superheated air from the surface rises and cools to saturation, forming a cumulus cloud. This updraft continues to build and causes the cumulus cloud to build, as long as the temperature of the updraft is warmer than surrounding air (instability). Soon the cumulus cloud becomes large enough that it is called a cumulus congestus, which in Latin means “large heap”. As you can tell, these cumulus clouds are very large and are nearly as large as a cumulonimbus cloud. At the cumulus congestus stage, light rain may fall, but it is very rare. Usually, a cumulus congestus cloud has a very dark base, giving it an ominous look. In the second stage of the cycle, the updraft strengthens causing the cloud to build to heights of 40,000 feet or greater. In this stage, the cloud can be properly called a cumulonimbus cloud. Soon the large cloud begins to produce heavy downpours, lightning, and if conditions are right, high winds, large hail, and tornadoes. The heavy rain falling out of the cloud causes a weak downdraft to form, strengthening as the rain becomes heavier. This downdraft descends out of the base of the cloud, and if it is strong enough, it will lift warm, humid air at the surface into the cloud, strengthening the storm for a short while. Soon, the downdraft gets so strong that rain begins to fall into the updraft. This leads us to the third stage, the dissipation of the storm. As rain falls into the updraft, it cools it, causing its temperature to be closer the surrounding air. This reduces atmospheric instability and the updraft weakens, and dies out. At this part, the downdraft has taken over the entire storm and at this time, the rain is at its heaviest as all the cloud droplets are forced downward. As water content in the storm drops, the downdraft is able to warm when it descends, causing a drop in relative humidity and the storm's cloud droplets begin to evaporate. This is why storms dissipate from the bottom to the top. These are the three stages of a typical air-mass thunderstorm.
Many times, the different types of thunderstorms will be associated with the different environmental conditions. The main environmental conditions that we will looking at in this article are wind shear and atmospheric instability (there are other environmental conditions in association with thunderstorms but these are less significant than the ones mentioned. In this section, we will ignore the fact that there could be an atmospheric cap, which would limit thunderstorm activity.). There are two types of wind shear, directional and speed. In directional shear, the winds change with height and in speed shear, wind speed increases with height. There are generally several different types of situations that are encountered that affect the type of thunderstorm. In the first situation, there is a low amount of speed shear and directional shear, with a low amount of instability. In this situation, thunderstorms will not form unless, a cyclone comes into play. In that kind of a situation, the thunderstorm will form above the friction layer and it will generally in the form of a line of thunderstorms just before a cold front passage. If you are in this kind of situation, do not expect much severe weather, but if the cold front is slow moving and there is a lot moisture available, expect heavy rainfall and flooding. In the second situation, speed shear is low and directional shear is low, while instability values are very high due to afternoon heating of the lower atmosphere. Generally, the type of thunderstorm that develops will be a typical air mass thunderstorm with possibly heavy rain, lightning, and even some hail if the storm lasts long enough. They will occur on most hot summer days, and are usually scattered to isolated in nature. If the atmosphere above your town has a significant amount of speed shear and directional shear, but not much instability (because it is the dead of winter), expect widespread thunderstorms and maybe a few severe thunderstorms. This type of situation occurs usually north of a warm front during the late winter, in association with a strong mid-latitude cyclone. When this situation occurs and especially if the mid-levels of the atmosphere are unstable, expect dynamic lifting of air above the friction layer, which will cause widespread heavy thunderstorms and embedded severe thunderstorms, with an isolated tornado possible. If the speed shear is very high while directional shear is low, and atmospheric instability is high, you can expect the formation of a major squall line by early evening (depending on other conditions as well). If this situation occurs with a mid-latitude cyclone, the squall line could be very large and damaging and could cause a widespread severe wind event over an area. Heavy rain and the isolated tornado could occur within the squall line also. If speed shear if virtually non-existent, and directional shear and instability are high, expect the formation of strong air mass thunderstorms. They will be similar to regular air mass thunderstorms, but they will have a twisting component to them. A tornado is possible during these situations, but the tornado has to form before the storm weakens quickly. In the final situation, all three factors are high, expect a major severe outbreak, including large, violent tornadoes, extremely large hail, and very strong gusty winds. The predominant storm type that will develop in this case, will be a large supercellular thunderstorm. This situation usually occurs in the Midwest and Southern United States at least 5-10 times a year, which is why during these situations is when most of the United States' tornadoes form. This usually occurs in tandem with a strong low pressure center, a dry line, and strong low-level jet stream, and a strong polar jet stream in the upper levels of the atmosphere. One of the most famous cases of this happening was on April 27, 2011, the day of those prolific tornadoes that killed nearly 340 people. If you are ever caught in this situation, prepare for a wild day of severe weather, because odds are that a major tornado outbreak will result.
If you would like to know more about what will happen on a day when thunderstorms are possible, read NWS forecast discussions, at: http://forecast.weather.gov/product.php?site=IWX&issuedby=IWX&product=AFD&format=CI&version=1&glossary=1.
The forecast discussions give the main reasons for the why the forecasts are the way they are. The meteorologists at the National Weather Service explain what factors went into this forecast. So if you are ever worried about what kind of thunderstorms will form on a certain day, read the forecast discussion and you will receive what you need to know about what will happen.
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