Category Archives: Arcus

Views of the April 26 severe storms from above and below

“Well that looks ominous” said Meredith O’Neill Muminovic as she took this photo of an approaching storm on 26 April 2016 in St. Louis, Missouri. The shelf-like appearance of the storm’s leading edge indicates strong winds as rain-cooled air lifts warmer, moist air out ahead of it. At the time of this photo, a Severe Thunderstorm Warning was in effect as 60+ mph winds were reported in the area, as well as hail 1″ in diameter covering the ground in some locations.

Shelf Cloud, Meredith O'Neill Muminovic, St. Louis, Missouri 26 Apr 2016

Shelf Cloud, Meredith O’Neill Muminovic, St. Louis, Missouri 26 Apr 2016

 

The corresponding radar image from around this time shows that the storm Meredith photographed was part of a line of storms moving across Missouri, referred to as a squall line. The red and orange areas in radar reflectivity indicate the heaviest rain, with weaker but widespread rainfall following behind the leading edge. The yellow box around St. Louis indicates the area under the Severe Thunderstorm Warning, which is aligned where the squall line appears bowed.

Radar Reflectivity, St. Louis, Missouri, 26 Apr 2016 2:07 PM CDT

Radar Reflectivity, St. Louis, Missouri, 26 Apr 2016 2:07 PM CDT

 

The bow echo is commonly associated with strong, often damaging winds at the surface. Much research has gone into studying bow echoes, leading us to understand how they form and the resulting weather they cause. The bow structure is strongly related to the wind shear of the environment these storms form in, meaning how the winds change direction and speed with height.

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Underneath the storm, turbulent motions are also present, as nicely captured by this video by Billy Reed in St. Louis around the time of Meredith’s photo.

 

Above, the clouds are deep and, individually, take on the classic structure of a cumulonimbus. In this schematic from the National Weather Service’s online school, JetStream, you can see that where the radar reflectivity shows the heaviest rain with the reds and oranges, the cloud is deep. Warm, moist air flows into the storm from out ahead of the squall line, fueling the strong updraft which hits a stable layer aloft, usually the tropopause, and creating an overshooting top. Within those strong updrafts, large hail can grow as supercooled liquid water freezes upon ice. Smaller Ice crystals can be carried outward to form the anvil of the cumulonimbus or fall and melt behind the updraft, contributing to the heavy rainfall at the surface and lighter rain extending behind the main leading line.

Schematic showing a vertical cross section of the cloud, precipitation, and air motion associated with the radar image of a squall line (from NWS)

Schematic showing a vertical cross section of the cloud, precipitation, and air motion associated with the radar image of a squall line (from National Weather Service)

 

The rain cools the air near the surface relative to the surrounding environment. This rain-cooled air rapidly moves outward away from the rainy core. The leading edge of this dense, cool air is referred to as a gust front.

Labeled schematic of a squall line storm from University of Illinois Urbana-Champaign

Labeled schematic of a squall line storm from University of Illinois Urbana-Champaign

 

Warm, moist air that’s flowing in towards the storm is lifted up and over this denser, colder air along the gust front, leading to new cloud formation, and sometimes the shelf cloud that extends outward from the main line of storms, as was shown in Meredith’s picture above.

This multi-cell nature that allows these storms to persist can be seen in this photo from  western Oklahoma on this day, when Jack Christian also had his eyes to the sky. The anvil of this series of this multicellular storm over northern Texas extended far across the Plains, with newer cumulus congestus clouds forming in its vicinity. Notice the tilt in these cumulus congestus clouds, as the strong wind shear indicates increasing winds with height, but turning in direction from the tops of these clouds to the top of the cumulonimbus as the anvil spreads out in the other direction.

Cumulonimbus, Multicell, Jack Christian, Elk City, Oklahoma, 26 Apr 2016 5 PM CDT

Cumulonimbus, Multicell, Jack Christian, Elk City, Oklahoma, 26 Apr 2016 5 PM CDT

 

So we’ve taken a good look at these storms from below, but what about above? Matt Barto was flying over Oklahoma later that afternoon and was treated to this spectacular view of the storms from above. Look at the classic structure of this cumulonimbus, with the anvil spreading outward from the bubbling core.

Cumulonimbus, Matt Barto, over Oklahoma 26 Apr 2016

Cumulonimbus, Matt Barto, over Oklahoma 26 Apr 2016

 

We live in the era where 1-min visible satellite data is available and it’s incredibly valuable for looking at the evolution of these storms. Here’s a 30-min loop showing the storms over Oklahoma and Texas where you can see the bubbling nature of the individual clouds, with the overshooting tops clearly visible, the anvils spreading outward, and gravity waves resulting from the displacement of mass in the atmosphere by these massive storms.

GOES 14, 1-min Visible Sector 26 April 2016 2220- 2250 UTC

GOES 14, 1-min Visible Sector 26 April 2016 2220- 2250 UTC

 

At the end of the day, not only where there very strong wind reports (blue dots) from the squall lines, but over 30 reports of tornadoes (red) and hundreds of reports of hail (green) including some baseball-sized.

26 April 2016 Severe Reports

26 April 2016 Severe Reports

 

Did you experience severe weather this day? We’d like to hear your story and see your cloud photos.

A “Bore”ing, “Glory”ous Morning over Oklahoma

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

 

On the morning of 17 January 2016, Pamela Heinselman captured a beautiful sight over Norman, Oklahoma: A long, extensive roll cloud associated with an undular bore.

 

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

 

What exactly creates these clouds? An undular bore is a type of gravity wave (meaning gravity is the restoring force) in the atmosphere (particularly the lower levels near the ground) that occurs when a low-level boundary such as outflow from a thunderstorm or a cold front reaches a layer of cold, stable air. This “disturbance” of the air is similar to when you disturb water by throwing a rock in the pond, creating ripples on the water’s surface. The disturbance of the low-level air by the front leads to ripples in the air. If the air is moist, clouds will form where the air is rising along this wave-like disturbance.

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Idealized drawing of an undular bore created by a cold front. From Hartung et al. (2010).

 

Bores have been described in the scientific literature since as far back as 1950. With advancements in our observational capabilities through ground-based and satellite-borne instruments, the formation, properties, and impact of these bores have been studied in great detail since then. The drawing above is adapted from one of these studies, in which a group of scientists described an undular bore associated with a cold front that moved through Oklahoma in 2006, much like the one photographed by Pamela in this blog. In that study, they showed that there was a strong temperature inversion near the surface in the morning, meaning that temperature increased with height indicating stable air ahead of the cold front.

Shown below is data from instruments released on a balloon that tells us how temperature, moisture, winds, and pressure change as you go up in the atmosphere. On the morning of 17 January 2016, not long before Pamela took her photos of the roll cloud, the ballon data showed a strong inversion near the surface with the temperature at the ground below freezing, indicating that the air over Norman, Oklahoma was cold and stable ahead of the cold front.

Screen Shot 2016-01-18 at 3.17.52 PM

 

This cool, relatively stable air trapped near the surface was lifted ahead of the advancing cold front. Due to the inversion, that lifting air was trapped, leading to the oscillating, wave-like pattern that resulted in the undular bore seen in the photos over Norman. Not only did this bore show up as a roll cloud in photos by Pamela and others, the roll pattern of clouds associated with the bore showed up in visible satellite imagery, looking like ripples in a pond. Notice how far these clouds extend across Oklahoma!

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Visible satellite image at 1437 UTC (8:37 AM) on 17 Jan 2016 showing the undular bore over central Oklahoma

 

When these bores pass over an area, not only can you visibly see them as beautiful, long rows of clouds, they are also associated with shifts in surface observations, particularly gusty shifting winds and rising pressure. Data from a station at Norman shows a sharp increase in pressure along with a quick increase in wind speed and change in direction around the time these photos were taken! The cold front passed through later in the day, leading to another shift in the winds to northerly and decreases in temperature and moisture.

meteogram_Norman

Surface data from Norman, Oklahoma showing the surface response to the bore.

 

While many studies of bores have occurred in Oklahoma due to many instruments available, they are not unique to this area. Examples from Iowa have shown bores on radar and webcams, as well as examples off the coast of Texas. Perhaps the most well-known example of undular bores throughout the world is a roll-cloud formation in Australia called the Morning Glory.

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The view from a glider of a morning glory cloud in northern Australia (Credit Al Sim – file photo)

 

Triggered by sea breezes near the Gulf of Carpentaria, the length, persistence, and smoothness of this cloud band attracts scientists and gliders (as seen in the photo above) who like to “surf” these atmospheric waves. The Morning Glory Cloud is such a common feature in the Spring in this area that there’s even a festival named after this incredible phenomenon.

Shelf clouds, mammatus, gust fronts, oh my!

 

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Shelf cloud courtesy of Kevin Sheely (Warrenville, Illinois, 8/25/2014)

What an ominous sight heading your way! Kevin Sheely captured this incredible shot of a shelf cloud near Warrenville, Illinois. On this day, numerous storms moved into and formed within Illinois.

To understand what creates this shelf-like appearance, we first need to understand that the rain that is falling from the core of the storm cools the atmosphere. This cold air is denser than the surrounding air so it sinks to the ground and then spreads outward from where the rain is falling. This cold air spreading outward is referred to as “outflow” and the leading edge of this rapidly outward moving air is called an outflow boundary or gust front. This boundary (like other weather fronts) separates air of different temperatures/densities: in this case, the cold air from the storm and the warm air surrounding it.

Arcus_Shelf_KevinSheely_WarrenvilleIL_25Aug2014_annotated

Same picture as above from Kevin Sheely with the cold outflow shown as blue arrow spreading outward and the warm moist air being lifted out ahead shown in red.

The warm, moist air ahead of it is less dense so it is lifted up and over the spreading cold air. This air is lifted and cooled, to the point where it condenses and forms the shelf cloud extending out ahead of the main storm along this outflow.

gustfront

Diagram showing shelf cloud formation along gust front

Notice in that diagram that the warm air out ahead is being lifted into relatively stable air above. This leads to the layered characteristics of the shelf cloud as it extends outward instead of continuing to grow upward like the parent storm.

This rapidly expanding air is often responsible for strong, potentially damaging winds at the surface. As these storms, with their shelf clouds, passed through Illinois on the 25th, downed trees were left in their path. These storm reports from the Storm Prediction Center show a cluster of blues in northern IL, indicating strong winds.

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So while the shelf clouds themselves aren’t dangerous, if you see them coming, you can expect strong, cool gusty winds shortly after, followed by heavy rain and possibly even hail.

Luckily these outflow boundaries can also be detected by radar to help warn of this impending gustiness. As the cold air lifts up the warm air ahead of it, it’s also lifting up dust, insects, birds, etc. These can be detected by radar, but the power return is much less than the heavy rain falling. So while the heavy rain in these storms appear orange and red on these radar images, you can see the “fine lines” of blue ahead of these storms indicating the location of the gust front. Notice in these series of radar images that storms produce these boundaries, which then go on to produce new storms that then produce their own gust fronts. And so the cycle continued on this day…

Radar_boundaries_25Aug2014_KLOT

Series of radar images from the Chicago National Weather Service radar (KLOT) showing numerous storms and their outflow boundaries.

 

We’ve learned that these shelf clouds are seen at the leading edge of the storm, indicating cold gusty winds to come, but what about behind the storm? Well, storms can only reach a certain height in the atmosphere, at which level they spread out horizontally. This is often seen as an anvil. The air below the anvil is typically drier and therefore sometimes mammatus clouds can form underneath. These bulbous beauties were seen on this day in other parts of Illinois by Bill Morris.

Mammatus_BillMorris_GrundyCountyIL_25Aug2014

Mammatus over Grundy County, IL courtesy of Bill Morris (8/25/2014)

And that’s not all! Moving ahead to the next day (8/26/2014), another round of storms moved through Illinois, allowing for another opportunity to photograph shelf clouds. This picture was submitted to us by Melissa Godbee. Even though this is looking at the storm from the side compared to the head-on view shown by Kevin, can you still see the resemblance?

Arcus_Shelf_MelissaGodbee_IL_26Aug2014

Shelf cloud (Melissa Godbee, Illinois, 8/26/2014)

Have you seen these ominous, yet beautiful, clouds where you live?