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.

bowecho_meted

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.

Kicking off a stormy U.S. spring

Spring in the northern hemisphere means severe weather for much of the U.S. While this year was off to a slow start in terms of tornado reports, there were several days of severe weather reported last week. torgraphDuring this time of year, the necessary ingredients for severe storms come together in the central and southern part of the U.S. These ingredients include warm moist air from the Gulf of Mexico, which are commonly separated from the warm dry air from the southwest U.S. by what we call a dry line. This boundary can be seen on radar as a thin line and can be identified on surface observations by looking at where the warm moist air from the south/southeast is separated from the warm dry air coming from the southwest. Here is an example of the dry line from March 24, 2015; the first day of active weather last week. You can see the faint blue line in the radar image as well as a computer-generated yellow line on the surface map that indicates the dry line. Notice that temperatures are similar (in the 70s and 80s) on either side of the line, while to the west of the line, dewpoint temperatures (a measure of the amount of moisture in the air) are in the 20s and 30s, while 60+ degree Fahrenheit dewpoints to the east of the line indicate moist air. Dryline_Radar_Sfc_24Mar2015 Surface air ahead of the dry line may be warm and moist, but cooler, drier air above that usually comes from the west, forming what is called a “cap.” This means that the warm air is limited in how far it can lift so something needs to push the air upward above that cap so it can reach its level of condensation. At that point, it can tap into the energy available and grow into an impressive thunderstorm. Balloons are launched twice a day, sometimes more if severe weather is expected. These balloons measure temperature, moisture, wind, and pressure. Here is an example of the data from one of these “soundings” from southwest Missouri during a time before the dry line passed through. sgf_2015032419_annotatedThe lift that’s needed to break through this cap can come from the dry line. That’s why you typically see storms developing along this boundary. Storms that develop along the boundary can displace the air above it, creating what are called gravity waves. This is similar to the way that ripples disperse from the spot on the water surface where you throw a rock. ripples In the sky, the air wants to go up, but if the air is stable (as it is out ahead of the dry line in certain layers of the atmosphere), gravity will pull the air back toward the ground, creating ripples in the sky. Where the air is rising, assuming it’s moist enough, clouds will form. On this day that we’re discussing (March 24, 2015), Karl Kischel noticed some of these gravity wave clouds over Cuba, Missouri at around 3:30 PM CDT.

WaveClouds_KarlKischel_CubaMissouri_24Mar2015_333PM

Karl Kischel, Cuba, Missouri, 24 Mar 2015, 330 PM (CDT)

Because Karl gave us the exact time and location, we were able to go back and look at the corresponding satellite imagery, where you can clearly see the extent of these clouds. The infrared satellite image gives us a sense of the temperature of the cloud tops, where the warmer colors mean warm temperatures and therefore at lower levels. Notice how these wave clouds are lower in the atmosphere than the deep thunderstorms that create the waves downstream. satellite_visir_ict_201503242045UTC_annotated Storms continue to fire off this dry line as it moved eastward. Radar imagery shows this line of storms along the boundary, extending from Missouri down into Oklahoma and Arkansas. cent_plains_201503242200cent_plains_201503250100 Matt Wing was with friends in Huntsville, Arkansas and captured an incredible view of one of these storms. This picture showcases the characteristic anvil of the beautiful cumulonimbus cloud.

Cumulonimus_MattWing_HuntsvilleArkansas_24Mar2015

Matt Wing, Huntsville, Arkansas, 24 Mar 2015

The bubbly characteristic of the middle of the storm indicates turrets of upward motion. The upward motion in these storms can be strong enough to support increasingly larger ice that can fall as hail. Indeed, storm reports on this day showed hail with diameters reaching 1-2″. stormrpts_20150324.gif As the sun was setting, David Holland was in Oklahoma City, looking at distant storms to the east.

Cumulonimbus_Mammatus_DavidHolland_OKC_24Mar2015

David Holland, Oklahoma City, 24 Mar 2015

He captured the beautiful cumulonimbus in the distance, with a curious section of cloud above the anvil. We suspect this is an overshooting top that has eroded with time. An overshooting top is a cloud directly above the updraft that penetrates through the stable layer where the anvil is seen. When the sun is shining at a low angle (like at sunset), the visible satellite can pick up on these overshooting tops, as is pointed out in this image. satellite_vis_ict_201503250000_annotated