DISCUSSION: One way to map lightning strikes is to use ground-based networks that sense radio frequency emissions from lightning. This data is highly accurate, but the areal coverage may be limited. Furthermore, some ground-based networks are primarily sensitive to only cloud-to-ground lightning. But most lightning occurs cloud-to-cloud or within a single cloud. Hence, these networks may miss a large portion of total lightning flashes.
Another way to observe lightning is from satellites. In particular, the Geostationary Lightning Mapper (GLM) was first launched in 2016. There are currently two GLM instruments in orbit that combined provide constant coverage over nearly the entire western hemisphere. GLM is sensitive to optical emission from lightning. As long as the top of the cloud is illuminated, GLM can observe all types of lightning, including those that don’t strike the ground.
The U.S. National Weather Service uses three products that are derived from GLM data, and the picture (credit: NOAA/NESDIS/Scott Rudlosky) above illustrates these products. The bottom-right panel is an IR satellite image (not from GLM) that shows where the clouds are. In particular, the reds/blacks/whites show where the highest, coldest cloud tops occur. The panel in the top-left shows the flash extent density product derived from GLM (reds and yellows indicate a higher flash density). This product shows where lightning is occurring and how many flashes are occurring in a given area per time. This product can be used to identify when convection begins and where the strongest storms are located. In particular, a higher flash density generally corresponds with stronger convection (in this case, the coldest cloud tops). In addition, prior research indicates that severe weather (e.g., strong winds, hail, and/or tornadoes) are sometimes preceded by a strong increase in flash density with time. Thus, another use of the flash extent density product is to help provide severe weather warnings. The panel in the top-right illustrates the total optical energy product (magenta and yellowish colors indicate more energy). This product basically shows how dim or bright the cloud is. Brighter areas typically indicate deep clouds with lightning, while dimmer clouds are shallower clouds that are illuminated indirectly by lightning in nearby deeper clouds. Finally, the bottom-left panel provides an indication of the area covered by flashes. Typically, smaller flashes represent new or rapidly intensifying convection, while large flashes indicate dying convection.
In summary, GLM complements observations of lightning from ground-based networks. Products derived from GLM have numerous applications including severe weather nowcasting, distinguishing intensifying from weakening convection, identifying and warning areas where it may not be raining but may still be at risk of lightning, etc.
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©2021 Meteorologist Dr. Ken Leppert II
DISCUSSION: Tornadoes are one of the most dangerous and destructive meteorological phenomena on Earth (e.g., tornado pictured above damaged parts of Tuscaloosa, AL in April 2011). However, tornadoes are small-scale features that are difficult to observe. The primary means of observing these features are with radar and visual confirmation. Given the limited number of fixed radars and the fact that radar beams increase in height with distance from the radar, tornadoes rarely occur close enough to a radar such that their associated surface circulation can be directly observed. In addition, in places like the southeast United States with some terrain and abundant tree cover, sightlines to a tornado may be limited. Thus, it would be helpful if there was another way to detect tornadoes more reliably and at greater distances.
A research group at the University of Mississippi developed microphones that are sensitive to infrasound (i.e., sound with frequencies too low to be detected by the human ear). The original purpose of these microphones was to detect infrasound emitted from illegal nuclear weapons tests. However, the research group recently discovered that tornadoes or the thunderstorms that produce them emit infrasound that can be detected by these microphones up to 50 miles away.
There is promise that these infrasound microphones could be an additional tool to detect tornadoes, but many questions need to be answered before this tool could be used operationally. For example, what specifically is producing the sound, the tornado or some other aspect of the parent thunderstorm? Do non-tornadic thunderstorms also produce infrasound? Can infrasound produced from tornadoes be confused with other sources, or do tornadoes have a unique acoustic signature? If there is a unique tornadic signature, do all tornadoes emit such a signature?
In summary, microphones initially designed to detect acoustic signals from nuclear weapons explosions may provide another tool to help detect tornadoes that is complementary to existing detection methods. These microphones may be able to provide earlier detection of tornadoes which may lead to warnings with greater lead times, potentially helping to save lives. Ongoing work seeks to learn more about the application of this tool to tornado detection before the method can be used operationally.
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©2020 Meteorologist Dr. Ken Leppert II
With the summer months approaching, we will start to see the increased probability of supercell thunderstorms developing. Supercells are strong thunderstorms that give us the majority of our tornadic severe weather. They form in environments with rising air and moderate directional and speed wind shear. The interaction of these factors and the relative strength of the two can affect which type of supercell is formed. The National Weather Service, NWS, defines wind shear as how the wind changes speed and/or direction with height. These two elements are essential for supercell development. The traditional and most common supercell development is referred to as a classic supercell. Supercells differ from traditional thunderstorms because they have a mesocyclone. A mesocyclone is a rotating updraft within the storm. Also, classic supercells can show a clear “hook echo” on radar which can be evidence of a tornado developing. A hook echo is when the mid-level mesocyclone wraps rain around the updraft. On radar, you can identify this by looking for a strong hook shape within the supercell. The presence of a hook echo does not necessarily indicate a tornado is present but shows mechanisms that are important for tornado genesis are present. Classic supercells can also come in two variations: Low Precipitation (LP) and High Precipitation (HP).
Low Precipitation supercells are supercells that produce relatively low amounts of rain. With (LP) supercells, there are high upper-level winds that push the rain far from the base of the supercell. This shows in the storm structure visibly as the storm can be tilted in the horizontal more than the traditional supercell. They are traditionally low moisture environments that aid in there not exhibiting large amounts of rain. However, hail is not uncommon with this type of supercell. The available water can suspend high in the cloud and rain out as hail. Tornadoes are unlikely with this type of supercell because the base of the storm is typically too high up in the atmosphere for a tornado to extend to the surface. They have this elevated base because rising air must go higher up in the atmosphere before clouds can form due to its dry environment. However, if one does produce, they are highly detectable because there are no strong amounts of rain to obscure it. They are common in the Texas and Oklahoma areas because of the lower amounts of moisture.
On the opposite side of the supercell spectrum, high precipitation supercells produce large amounts of rain. They have lower upper-level winds, approximately 35 knots or less. This heavy rain-fall can obscure different features like tornadoes and wall clouds. Obscure tornadoes can be particularly dangerous because they can’t be spotted by the human eye, and observers would not know when the tornado was approaching. They occur in high moisture environments and high CAPE environments, which means there are high areas of rising air. If there is high wind shear and high CAPE, you can have dangerous flooding, tornadoes ,and hail. Tornadoes formed in (HP) supercells can be extremely wide in diameter as the base of the storm is lower than normal. The El Reno tornado in 2013 had a width of 2.6 miles as the entire mesocyclone touched the surface. High precipitation supercells can have high impacts such as flooding, hail, lightning, and tornadoes.
Photo Creds: Supercell Diagram- Weather Underground
Low and High Precipitation Supercells - University of Illinois at Urbana
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©2020 Weather Forecaster Dakari Anderson
Wearing of the Green- Are Green Skies usually associated with Severe Weather? (Photo Credit: Jennifer Shoemake)
Shelf cloud taken from Albuquerque, New Mexico's west side on July 26, 2018.
Across the Great Plains, folklore says that if the sky turns green it’s time to head inside. But, is there truth to this saying? While it is not uncommon for green skies to accompany severe weather, there appears to be no direct correlation between the two.
During the day, the sky appears blue because the shorter-wavelength, bluer end of the light spectrum bounces off of air molecules better than the long wavelength red end of the spectrum. As the sun shifts lower in the sky (and after the peak heating of the day), the spectrum of direct sunlight is shifted from blue wavelengths of the sky to the red, orange, and yellow of the sunset due to the longer trip of the sun’s rays through the atmosphere. A thick cumulonimbus is composed of many water droplets and ice particles, resulting in air molecules scattering and attenuating. Since water reflects blue and green light better than red, there appears to be a green-ish tint to the sky. The same phenomenon is present when you place a glass of water with blue food coloring in front of a glass with blue that will produce the same green tint that the light in the sky transmits.
A study from Pennsylvania State University concluded that the relative contribution of hail to the green color was actually quite small. It isn’t the presence of hail needed to produce a green sky, but the size of the droplets/particles in the cloud may dictate the shade of green (for example, smaller drops may lead to more of a blue-green sky than larger drops which may produce more of a yellow-green color).
These intense thunderstorms that produce green-tinted skies also have the potential to generate large hail, damaging winds, frequent lightning, flash flooding, and even tornadoes, which may make the green color appear threatening. But, this is not always the case. Either way, it is best to head indoors when thunder roars.
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©2020 Meteorologist Sharon Sullivan
DISCUSSION: Atmospheric rivers are long, narrow corridors in the atmosphere through which vast amounts of water vapor are transported. The strongest atmospheric rivers can carry up to 15 times more water than what flows through the mouth of the Mississippi River. These rivers are best depicted in satellite images that show water vapor in the atmosphere, but can also sometimes be seen via cloud cover. For example, the clouds extending from the bottom-left corner of the satellite image above (from NOAA’s GOES West satellite) into the west coast of North America represents an atmospheric river from 14 February 2019. Atmospheric rivers often make landfall along the west coasts of continents and can bring much-needed rainfall. They can also bring torrential rains that cause flooding/landslides and may be associated with high winds and associated damage.
Traditionally, a quantity known as integrated vapor transport (IVT) has been used to assess the intensity of atmospheric rivers and to assess their impacts. IVT incorporates measures of both water vapor and winds. Thus, it is possible that two atmospheric rivers have the same values of IVT, but one river has high moisture combined with relatively low wind speeds, while the other river has relatively low moisture with high wind speeds. According to the IVT measure, both these atmospheric rivers have the same intensity, but they can have very different impacts.
Scientists at Stanford University separated IVT values into their water vapor and wind components for historical atmospheric rivers. They then used this information to classify the rivers into four categories: wet atmospheric rivers have high moisture with low wind speeds, windy rivers have relatively low vapor contents with high wind speeds, wet and windy atmospheric rivers have both high vapor content and high wind speeds, and neutral rivers have average values of moisture and wind speeds. The Stanford University scientists studied the impacts of the four types of atmospheric rivers and somewhat surprisingly found that windy rivers tend to result in higher precipitation totals in the western United States than wet rivers. It might be expected that atmospheric rivers with higher moisture contents would bring more rain/ snow. However, in order to get precipitation, moisture-laden air must be forced to rise and cool. Windy atmospheric rivers may force more air up the slope of the mountains in the western United States causing more of the moisture in the river to precipitate out. Research is continuing to try to better forecast each type of atmospheric river in order to better anticipate and prepare for particular impacts of a given event.
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©2020 Meteorologist Dr. Ken Leppert II
The National Weather Service, NWS, defines a tornado as “a violently rotating column of air touching the ground, usually attached to the base of a thunderstorm”. So, what does this actually mean to those who know nothing about weather? Fundamentally, a tornado is a funnel-like cloud that descends from the base of a cumulonimbus, which is the type of cloud that defines thunderstorms. When a cumulonimbus cloud is strong enough, air beneath the cloud, within the updrafts and downdrafts, can begin rotating strongly enough to pull some of the cloud down towards the ground. When the rotation at the cloud joins the rotation at the ground, and debris is picked up, you have a tornado. While most tornadoes only last a few minutes, the damage can be devastating. Winds can reach up to 300 miles per hour, and the path of the tornado can be a mile wide and 50 miles long. Whenever you are alerted to a tornado in the area: take shelter IMMEDIATELY!
So, what can you do if you’re in the path of a tornado? Well, that depends on where you are. If you’re indoors, stay away from windows. The pressure drops drastically near a tornado, and the harsh change can shatter windows in a matter of seconds. You should also try to stay on the lowest level of your home, in the basement if you have one. If not, hunker down in a bath tub with a mattress over your head to protect yourself from flying debris. There’s also the chance that you are outside driving in your car. The first rule of tornado safety while driving is to never try to outrun it! Tornadoes move quickly and often unpredictably, so trying to drive away from it could cause you injury. If you see a ditch on the side of the road, get out of your car and lie face down in the ditch. This will protect your face and eyes from flying debris as the tornado passes. And speaking of passes, overpasses are not good places to seek shelter from a tornado. Winds are much stronger there, as the overpass creates a wind tunnel, and debris could hurt you even more. Once the tornado has passed, assess damage around you and any injuries to yourself. Seek help as soon as it’s safe!
©2019 Weather Forecaster Sarah Cobern
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When lightning flashes across the sky, it is usually associated with the booming sound of thunder. Specifically, in the southern U.S, lightning is sometimes seen as a faint flash with no sounds of thunder and not the traditional lightning strike we are accustomed to. This is commonly referred to as heat lightning. However, heat lightning is a myth and is just lightning from a distant thunderstorm. The National Severe Storms Laboratory defines lightning as “a giant spark of electricity in the atmosphere between clouds, the air, or the ground.” The sound we traditionally call thunder is caused by lightning. Lightning heats the air to extreme temperatures causing the air to expand, creating the sound of thunder.
With lightning that has been called heat lightning, there is a variety of reasons why you may not hear the associated thunder or see the storm where it originated from. Topography, like mountains or hills, can affect your view of the storm and inhibit you from seeing the actual lightning strike. Also, the curvature of the Earth can affect it as the sound wave can bounce off the surface before it reaches the individual. Lastly, unless you are within close proximity of the thunderstorm, there is a strong possibility the sound of thunder will not reach you due to the distance between you and the storm. Whether you hear the thunder or not, lightning remains one of the most interesting and captivating weather phenomena.
Photo Credit: Farmers' Almanac
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©2019 Weather Forecaster Dakari Anderson
As the relatively calm days of summer are behind us, it is that time of year once again that sharper frontal systems begin to march their way southward across the U.S. to deliver crisp and refreshing Fall-like weather across much of the country. This gradual shift in the day-to-day weather pattern is also a catalyst in the return of organized convective systems across most of the central and eastern parts of the country. As cooler air is reintroduced equatorward, the sharpening temperature gradient favors an increasing possibility for strong to severe storms to impact areas closest to this boundary. At times, meteorologists will discuss the topic of “convective modes” and how some modes can favor certain types of severe weather under the right atmospheric conditions. So what exactly are these convective modes that are brought up on occasion?
It’s important to first understand the significance of the classification of certain convective modes prior to diving into some of the specifics. A convective mode is simply just the set-up of storm cells in any particular environment. There are many different factors that govern exactly what a favored convective mode will be leading up to a severe weather event. In short, the primary ingredients that are needed to distinguish different convective modes are moisture, instability (such as convective available potential energy, or CAPE), and lift, the primary ingredients for development and sustenance of organized severe convection.
Discrete convection is the type of convection in where cells develop in an isolated environment with no immediate interference between other storms in close proximity. This is considered to be somewhat more dangerous in the sense that discrete convection tends to lead to the development of more robust supercells. A paper by Thompson et al. (2003) showed that roughly 90% of reports for 2” or greater hail diameter are from supercellular storms which most often are discrete in nature. Moreover, Thompson and Mead (2006) relate the greater probabilities of significant tornado development to discrete supercells in that significant tornadoes are about four times as likely to form as a result of a discrete storm as opposed to any other convective mode. Most of the time, although not always, discrete storms exhibit more of a surface-based nature. That is, parcels of air begin to be buoyant closer to the surface as opposed to elevated off the ground. This in turn favors more robust updrafts and a longer CAPE profile and, coupled with favorable shear parameters, increase the likelihood of discrete and more severe convection. Examples include the 2011 April 27 tornado outbreak across MS and AL, where most storms were discrete in nature and many were capable of producing long-track, violent tornadoes that ravaged both states.
On the other hand, one must consider the development of a multi-cellular, or multi-modal, convective mode. In this particular mode, the atmosphere favors the conglomeration of individual storms under a moderate to strong shear profile confined to the lowest 3 km of the atmosphere. In addition, a stronger cold pool and surface convergence of the air also aid in the development of multi-modal convective mode. The end result is a long line of storms containing many of the severe weather hazards, although flash flooding and high winds become the primary risks as opposed to significant tornadoes. A prime example of multi-mode convection was during the “high risk” severe weather day which led to the flooding that was seen across portions of north and central Oklahoma on May 20th of this year (2019). Discrete cells from earlier in the day moved northward across the Oklahoma City metro area and merged with storms to the north of a warm front to become one “organized” complex of storms, which some meteorologists would argue as being a “messy” system. Several cities and towns in both the Oklahoma City and Tulsa metro areas received north of 4” in just that one event alone with locally higher totals as well, highlighting the significant flash flooding risk that multi-modal storm complexes pose.
Convective modes can be a tricky forecasting aspect for meteorologists and researchers, especially given the pressures of trying to forecast them in a realtime setting. However, field campaigns and more sophisticated modeling techniques have allowed for a more comprehensive insight into the unpredictability that lies within convective mode forecasting. All hazards are certainly possible with any convective mode, but the ability to forecast them will always mean the difference between positive and negative outcomes for both life and property.
Here are the paper references from this article in case there are interests to read further into this topic:
Thompson, R. L. and C. M. Mead, 2006: Tornado failure modes in the central and southern Great Plains. Preprints, 23rd Conference on SLS, St. Louis, MO, AMS, 59, #3.2.
Thompson, R.L., R. Edwards, J.A. Hart, K. L. Elmore, and P.M. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.
Photo Credit: National Severe Storms Laboratory
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© 2019 Meteorologist Brian Matilla
Why do Thunderstorms Often Occur on Summer Afternoons? Credit: NOAA National Severe Storms Laboratory / The Weather Prediction.com
Thunderstorm on May 26th, 2012 in Alliers, France
Thunderstorms are a weather phenomenon that occur and develop due to high amounts of moisture in the air along with warm air that is rising. These storms typically last less than thirty minutes and occur within a 15-mile radius. According to NOAA, in the United States nearly 100,000 thunderstorms occur each year, with ten percent of these storms becoming severe thunderstorms. Thunderstorms occur most often in the afternoon and evening of the spring and summer months, and bring with them thunder, lightning, heavy rain, and the potential risk for flash flooding.
A thunderstorm forms when warm moist air is unstable and begins rising. As this warm air rises the water vapor within the air cools and releases heat. Condensation then occurs as the air condenses creating a cloud, that then grows until it forms a towering cumulonimbus cloud. Ice particles within the cloud holding both positive and negative charges create lightning when leaders extend from these charges within the cloud. These negatively and positively charged particles within the cloud connect through a channel with the opposing charges of electricity rising up from the ground, creating a strong electric discharge. Lightning is followed by thunder after the lightning heats up the surrounding air causing it to expand rapidly. This expansion creates sound waves that make a loud cracking sound after the lightning strikes.
Thunderstorms occur more often in the afternoon and evening because in order for there to be high amounts of moisture in the air along with warm rising air, there must be instability in the atmosphere. During the warmer months the humidity is much higher. On days with less clouds in the sky temperatures can also rise to very high values. Because of this daytime heating throughout the day, the late afternoon and evening hours are when radiational heating and instability are at their highest points, and thus there is a steep temperature gradient between the mid-levels and the Boundary Layer. This daytime heating is often strong enough to completely overcome significant capping inversions, thus triggering Convective Available Potential Energy or CAPE that can spur up even severe thunderstorms. The intense heating that can occur during the daytime of the spring and summer months is very conducive for afternoon and evening thunderstorms. As the summer comes to a close, be sure to be aware of the potential for afternoon thunderstorms and the risks that come along with them.
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©2019 Weather Forecaster Christina Talamo
Mississippi River Flooding 2019: Surpassing the Great Flood of 1927?
It’s no secret this year has been a wet one for much of the eastern half of the country. In fact, Mississippi alone has already surpassed the annual average rainfall and there is still 5 months left of 2019. Many of the communities settled along the river in the Mississippi Delta have been underwater since the record rainfall that occurred in early March and, unfortunately, they may not be returning home anytime soon after all the rain from Hurricane Barry. The Mississippi River has flooded before, during the Great Mississippi Flood in April 1927.
The Great Mississippi Flood of 1927 is still considered one of the worst natural disasters in the history of the United States. Heavy rainfalls were to blame, and levees all down the river had given way. Some places were submerged under 30 feet of water and it would be two months until the people of those communities would be allowed to go back. About 250 people died during this tragic event and even more went missing. The picture above is in Arkansas City, Arkansas.
This year can compare quite scarily to this event as the Mid-South faced nonstop heavy rainfall in March and April of this year, bringing the Mississippi to dangerous flood stages that hadn’t been reached in years. The highest crest during this event was near 40 ft, which would deem the river being at “Moderate Flood Stage”. While still under the momentous 49 ft crest during the flood, this is still significant as times have changed and population has grown substantially, and urbanization has increased. Thankfully, Barry did not drop as much rain as anticipated, with most areas only getting at most around four inches. What was forecasted for areas along the Mississippi would have surely brought the river back to record flood stages and leave the people whose communities are still underwater with no hope of getting home anytime soon. The river has not receded back to moderate levels and is expected to continuously decrease for the remainder of the 28-day forecast that the Lower Mississippi River Forecasting Center has put out. The picture below is a picture from Chickasaw County in Mississippi during the rainfall in March and many parts of the county underwater to this day.
For residents along the Mississippi River, this is good news as the river continues to decrease with size. Many forecasters in the National Weather Service in Memphis were quite concerned that the river could peak to stages close to that of 1927 as the rain was relentless this spring, but it stopped just in time to save what it could. Many farmers across the Mid-West and the Mid-South have lost most, if not all, their crops for the year. However, as floodwaters decrease, people will be able to move back to their communities and begin to rebuild and recover from the awful start to 2019.
Meteorologist Ashley Lennard
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