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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On July 23, 2010, a hailstone that shattered records fell in Vivian, South Dakota. This hailstone weighed roughly 1.9375 lbs and was 8 inches in diameter. A typical hailstone is nowhere near this large, so how did a phenomena like this occur?
On July 13th, 2019, Tropical Storm Barry made landfall on the coast of Louisiana in the early afternoon hours. After briefly becoming a Category 1, Barry weakened as it approached the coast and proceeded to pose numerous risks to those in the southeastern portion of the United States. After landfall, Barry moved into the interior United States at a slow pace, which led to flooding threats into the states that were in its path. Barry’s impacts were felt as far away as Toronto, Ontario, Canada, when they reported seeing about 60 millimeters of rain on July 17th, when the remains of Barry were just to the south of the city.
This storm, at landfall, was a tropical storm and not powerful enough to be measured on the Saffir-Simpson Scale. However, Barry’s impacts were known before making landfall. For example, New Orleans, Louisiana received a total of 6-9 inches from this storm of the city to flood. This disrupted travel and caused some businesses to shut down. The flooding was magnified due to abnormally high water levels of the Mississippi River. New Orleans started to experience flooding a few days prior to landfall because of Barry’s asymmetrical shape.
Furthermore, Gulf Coast coastal cities experienced life-threatening storm surge due to the movement of water caused by Barry. For example, Biloxi, Mississippi had to deal with 2-4 feet of storm surge, which prompted the National Weather Service to issue a storm surge warning to anyone within their area of coverage. While 2-4 feet of storm surge may not seem like a whole lot, it was powerful enough to cause the sand on Biloxi’s beaches to wash across the road and shut down some of the roads near the coast.
Once Barry made landfall, the threats of flooding continued to move inland. The storm did not go back out to sea, but it dumped all of its energy out as rain inland. In Tennessee, the rainfall totals from the remains of Barry totaled around 2-8 inches, which prompted flash flood warnings throughout the state and only increased the damage from this year’s high counts of flooding.
Overall, it is estimated Barry caused between roughly 500-900 million dollars worth of damage Weaker storms like Barry can and will have life-long effects, even though they may not be as powerful as major hurricanes. The key thing to remember is that a tropical storm, no matter the strength, can have impacts that extend far beyond the coast where it made landfall.
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Sources: https://www.nhc.noaa.gov/archive/2019/BARRY.shtml?,https://upload.wikimedia.org/wikipedia/commons/thumb/5/5e/Barry_2019_track.png/800px-Barry_2019_track.png, https://www.weather.gov/images/lix/5day_BarryRainfall_2019_07_16.png
©2019 Weather Forecaster Shannon Sullivan
Weather forecasts are important in dictating our plans for the day, week or even longer. We are reliant on forecasts and timing of potential storms to figure out when we should leave our homes. Simple tasks can turn troublesome such as grocery shopping, a commute to work or a day at the beach due to unexpected rain. Unfortunately, people are quick to criticize (often broadcast) meteorologists — whether storms are in the forecast for the evening but turned out to be a brief shower that cleared up quickly, or sunshine was forecasted but a stray storm passed by unexpectedly. The importance of monitoring forecasts throughout the day is critical for safety and preparation, but also to create a low-stress environment for your plans.
Photo: Thunderstorm structure with emphasis on warm air being the necessary fuel to form clouds and storms (Courtesy of Encyclopedia Britannica).
Convective storms are common types of thunderstorms we are used to that can be capable of producing damaging winds, large hail, or even a tornado. These storms, similarly to rain showers can occur at any time of the day and often dictate potential changes during the day. One of the main ingredients that is necessary for storms to form is heat, usually from the sun (solar heating) that warms the surface to generate the rising air necessary for storms to form. While there are more ingredients involved for storms, heat is the main driver in this situation.
Evening convection is the most common type of convection during summer months since it follows peak heating times (allowing for rising air to fuel storms). Forecasters and storm chasers alike look toward the evening hours to find supercells or lines of convective storms in their region during the early to late summer months. This is the most common type that we see, but that doesn’t mean it’s the only one.
Since evenings are seen as the most common time for storm generation, we may get into the routine of seeing rain in the forecast and expecting it in the evening. The timing of a storm and different atmospheric changes can inhibit the generation of evening storms.
Consider the situation of expecting storms at 5pm (1700) local time, but an earlier line of storms passed by around 2pm (1400) that resulted in significant cooling (due to rain cooled air). Because of the cooling that occurred after the first storm, the expected evening storms may not be nearly as severe, if they occur.
Consider another example: storms have been passing through the area in the early morning and cloud cover has lasted until noon, but storms are still in the forecast for the evening. The cooler air caused by an earlier rain mixed with the lack of solar heating could greatly impact a forecast for storms in the evening. This results in a lack of heat to fuel the storm and thus reduces the severity of the storm.
Multiple factors impact the chance of evening convection aside from lack of solar heating from cloud cover and earlier storms or rain. These are only a few instances where the heat necessary to fuel a storm is taken away. Atmospheric changes can play a factor in an adjusted forecast like a cold frontal passage or cool lake breezes. These factors can actually help a storm form or destroy it. A cold front can pass through a hot and humid air mass that will force the hot air upward and aid storm generation. Similarly, a lake breeze that creates a frontal boundary can meet a warm frontal boundary that helps creating a rising motion. Depending on the strength of these cooler boundaries, they can inhibit the formation of storms by cooling the air in a region where a storm is headed, thus causing the storm to continuously lose heat and energy it needs to strengthen or keep its current strength.
Incoming lines of storms seen on the radar for hours can easily lose energy when coming in contact with a cooler environment. A gust front or outflow boundary is often present with a larger line of storms and act as a cold front. In turn, the storm will move into a cooler environment and quickly lose energy. As you watch the storm move closer to your area and notice it lacks the same energy it had hours before, then there is some environmental factor playing a role.
Overall, forecasts are important during all parts of the day. Keeping a close eye on changes and understanding the multiple factors that go towards a forecast can help you understand why a call for severe weather may not happen as planned. These ideas hopefully expanded your understanding of the expectation of rain throughout a day and provided insight on some of the changes that can occur.
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@2019 Meteorologist Jason Maska
If you’ve never heard of the phrase, “Colorado Magic,” it simply means that Colorado has shown to be quite the hot spot in storm and tornado development. It all starts with the Rocky Mountains. Based on a 2017 article by The Weather Channel, from 1950 to 2016 the county ranking number one in the most tornadoes occurring over this 67-year period is Weld County, Colorado (see the first link below the article for more details on how this statistic was obtained).
This is where the “magic” begins. The reason this location including Eastern Colorado is so prone to such frequent storm development is based on the mid to upper level winds and how these winds cross over the mountains. As westerly winds cross over the Rockies the air sinks and warms due to subsidence where air descends, compresses, and warms in the process. This leads to the development of lee cyclogenesis in which the lack of cooler air at lower levels combined with the descending upper level-air causes the leeward side of the mountain to become less stable than its surroundings. This sets the stage for storm development as a cyclonic circulation develops downwind of the mountain range.
The formation of such storms begins with warm, moist, unstable air (typically from flow off the Gulf of Mexico) being forced upslope along higher terrain, which is a common process towards the Western Plains where orographic lift is especially enhanced. There exists a specific region of higher terrain, oriented West to East across central and eastern Colorado known as the Palmer Divide. It stretches approximately 80 miles from the front range of the Rockies into central Colorado towards Limon, Colorado and is known for its significant synoptic and mesoscale impacts.
A phenomenon also known to have effects on storm development in this area is known as the Denver Convergence Vorticity Zone (DCVZ). This is yet another topographically induced mesoscale feature, oriented North to South and categorized by convergent winds in northeastern Colorado due to the interaction of southerly, low-level flow with the Palmer Divide. These convergent winds often help initiate storm development especially during the convective season as convergent boundaries such as these often create natural zones of vorticity, or localized areas of spin to create rotation.
If you remember that the basic ingredients for a thunderstorm are lift, instability, and moisture (while adding in the right amount of shear for supercells specifically), it’s no doubt that this area provides all the right ingredients to create such explosive development, and this merely scratches the surface as to what this terrain can do!
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@2019 Weather Forecaster Christine Gregory