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
Chances are you have witnessed many thunderstorms in your life, heard the unmistakable crack of thunder, seen the bright flashes of lightning, and maybe have fallen asleep to the ever familiar sound of the rain tapping against your window. But unless you live in an arid climate, you have likely not experienced, or maybe even heard of, dry thunderstorms.
Dry thunderstorms are thunderstorms where little to no precipitation makes it to the surface. This can be common in thunderstorms that occur in deserts or areas where the lower atmosphere is very dry. Dry thunderstorms do produce rain, however, as the rain falls through dry layers of air beneath the cloud base, much of the rain then evaporates. The falling rain that doesn’t make it to the ground is also known as virga. As this rain evaporates, it cools the air beneath it causing that air to become “heavier” than the surrounding warmer air. This air can rapidly fall and cause strong winds to fan out at the surface. This phenomenon is also known as a dry microburst.
Dry thunderstorms are also an important phenomenon when it comes to fire weather. Since little precipitation makes it to the surface, dry areas experiencing dry thunderstorms are more susceptible to fire ignition by lightning strikes. In a typical thunderstorm, the rainfall can prevent lightning from igniting fires. For fire weather purposes, dry thunderstorms can be classified as producing less than .1 inches of rain, although this threshold can depend on how dry the area is as well as the amount of vegetation in the area. Both of these factors can determine how much rain is necessary to properly wet the surface and prevent lightning-caused fire ignition.
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©2019 Meteorologist Stephanie Edwards
DISCUSSION: Since severe weather season is now in full force, terms such as elevated and surface based convection will be more common. It’s important to understand the difference between the two when analyzing the possibility of severe weather. Although each one can produce severe weather, the type of weather can vary.
Elevated convection is just as it sounds, convection which is elevated. Convection can be thought of as the transfer of heat between areas of different temperature. This vertical movement of air usually results in the formation of clouds, if there is also enough water vapor present. Elevated convection occurs above the planetary boundary layer (PBL). According to American Meteorological Society (AMS), the PBL is the layer of the atmosphere that is in contact with the earth’s surface. This layer can be hundreds of meters deep and is capped by a temperature inversion. A temperature inversion is one in which the temperature increases with height, typically temperatures decrease with height. Elevated convection will usually occur on the cool side of either a cold or warm front. This is because fronts are slanted as they approach the surface. For example, behind a cold front, the air is cooler and denser. Thus, the level at which an air parcel will be lifted will be higher, and so the top of the PBL will also be higher than at a point that is ahead of the front. As shown in the 850 mb analysis image below, the front is roughly from the Texas panhandle to Dallas to near Shreveport in northwest Louisiana. This level is at about 5000 feet above the surface and is often used to detect synoptic (large scale, such as fronts) weather patterns because it is close to the top of the boundary layer.
Image courtesy of Dr. Greg Forbes
When comparing the 850 mb cold front to the surface cold front, it is obvious that the front is tilted to the northwest with height. This can easily be seen in comparing the surface image below to the previous image above.
Image courtesy of Dr. Greg Forbes
The precipitation associated with this storm system is shown below. It is evident that the precipitation is on the cold side of the front. This would be indicative of elevated convection. The severe threat is not as high with this type of convection. Precipitation is usually lighter, although stronger storms can produce small hail and occasionally damaging winds. Often, most winter precipitation results from elevated convection.
Image courtesy of Dr. Greg Forbes
Surface based convection is just the opposite of elevated convection. It is convection which results from a parcel being lifted from the PBL, instead of above it. This occurs as the surface is heated from the sun’s radiation. This type of convection arises within the warm sector of a storm system and can produce much stronger storms than elevated convection. Most storms which produce large hail, strong winds, tornadoes, and heavy rainfall is the result of surface based storms.
Although elevated convection can produce some severe weather and heavy rainfall, surface based convection is the type we most hear about most because of its greater severity. It's important to realized that both can be problematic, in their own way.
Credit: NOAA, Dr. Greg Forbes
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@2019 Meteorologist Corey Clay
DISCUSSION: On May 20th and 21st, a low-pressure system swept across the southern Central Plains and into the southern United States bringing tornadoes to parts of the Plains and Mississippi River Valley. The system began as an area of low pressure over the Colorado Rocky Mountains with a warm front stretching into New Mexico, through the middle of Texas, the most northwestern tip of Louisiana and into Arkansas. The warm front gradually moved slowly northward through Texas aided by a southerly wind which brought with it very warm moist air from the Gulf of Mexico.
The warm front interacted with the colder drier air in north Texas and southern Oklahoma that were influenced by a ridge of high pressure that was over Minnesota and Wisconsin on the morning of the 20th. The warm moist air from the South had Convective Available Potential Energy (CAPE) values above 2000 J/kg according to the soundings at locations such as Amarillo, Texas and Ft. Worth, Texas. CAPE is how much buoyancy is available in the atmosphere over a certain location with higher values leading to more severe thunderstorms, bigger hailstones and even tornadoes. The normal minimum amount of CAPE required for a tornado to be possible is about 2500 J/kg. The Storm Prediction Center (SPC) on the morning of the 20th had issued a convective outlook which included the highest risk level on their scale “High” to signify that there was a high level of confidence that widespread extreme thunderstorms and strong tornadoes are possible. The extremely high risk area covered parts of Oklahoma. According to the storm reports on the 20th, there were about 20 tornadoes reported with the strongest one being an EF-3 near Midland, Texas. There were some reported injuries but no deaths have been reported.
Then, on the 21st, the warm front proceeded to move to the South as it crossed Arkansas, Tennessee, and Kentucky towards Missouri and the Ohio River Valley. However, a cold front snuck in from the Rockies and connected with the low pressure system. The interaction between the two fronts led to the forming of an occluded front along the Kansas- Oklahoma border and grew as the cold front started to catch up with the warm front. Part of the warm front remained a warm front in Missouri into the night of the 21st, while the part of the warm front that was over Kentucky and Tennessee became a stationary front due to a blast of cooler air coming down from the Northeast as a cold front was passing through the Carolinas. On the 21st, the storm reports had tallied over 17 tornadoes across Kansas, Oklahoma, Missouri and Iowa with the strongest being an EF-3 in Bern, Kansas. One death has been reported from all these storms coming from the sole tornado in Iowa. This outbreak is just the beginning of a series of outbreaks that last through the end of May.
At the GWCC, we would like to remind you to be prepared for tornadoes. Some of our tips include getting a NOAA radio as well as batteries, flashlight, unperishable food and water. We recommend that if the National Weather Service (NWS) issues a tornado watch or warning that you either go to the basement or storm cellar, if you have one. If not available, then head to the lowest level possible in the house and get to one of the most inmost rooms without a window as a window would break and spray glass resulting in injury.
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©2019 Meteorologist JP Kalb