Another day of storms in the central U.S.

In our last blog post (, we shared some photos from the stormy start of the severe weather season in the central U.S. (24 March 2015). Large cumulonimbus grew over portions of Oklahoma, Arkansas, and Missouri, producing large hail in some locations. On the next day (25 March 2015), another round of severe weather would bring the first reports of tornadoes for the year.


There has already been an excellent summary created for this event that describes the atmospheric conditions and storm timeline:

Earlier in the day, before the storms formed, mammatus clouds were observed over the National Weather Center in Norman, Oklahoma. Dena Grose shared with us her excellent photo, showing these bulbous clouds that can form when the air is much drier below the cloud deck.


Dena Grose, Norman, Oklahoma, 25 March 2015 (2 PM CDT)

While there were severe storms later in the day, these mammatus were not associated with any storms. This is confirmed by looking at the corresponding radar image from this time.


As time went on, a cold front provided the necessary lift to produce storms later in the evening. Matt Wing shared with us a picture of mammatus clouds, this time over Tulsa just prior to when a tornado warning was issued. In this case, the mammatus were indeed associated with severe storms.


Matt Wing, Tulsa, OK, 25 Mar 2015

Post-storm damage surveys indicated several tornadoes that moved through the Tulsa area. The strongest tornado was an EF-2 reported in nearby Sand Springs. Here’s a summary of the damage survey from the National Weather Service.

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While Oklahoma experienced the worst of these storms, this unsettled weather provided beautiful views of turbulent skies over nearby regions. Shauna West sent us this picture from Pittsburg, Kansas.


Shauna West, Pittsburg, Kansas, 25 Mar 2015

Further south, over Arlington, Texas, Whitney Coker Terrell shared her turbulent view beneath a storm that same evening.


Whitney Coker Terrell, Arlington, Texas, 25 Mar 2015

Thanks for all the beautiful pictures!

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.


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.


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.


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

Incredible Seattle sunset

Last night, Seattleites were treated to a spectacular sunset. I, Angela, was attending the Seattle Sounders game and admit that I was quite distracted by the clouds, as were many others at the stadium. Here was one of the views I had from my seat:
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Angela Rowe
Location: Seattle, Washington
Date: 14 July 2014

What causes these vibrant colors? First, let’s take a look at the visible spectrum. The sun’s rays are made up of a range of wavelengths and therefore colors. The cool colors (blues) are the shorter wavelengths, while the reds are the longer wavelengths.

This simple diagram from the National Weather Service online education course (JetStream) shows what happens to the sun’s rays as the sun sets in the sky. At lower angles (B and C), the sun’s rays have to pass through more of the atmosphere, therefore more of the shorter wavelengths are scattered by the air. This leaves the longer wavelengths (the reds and oranges), which can be reflected off of clouds in the mid- and upper-levels of the atmosphere.

While there are many colorful sunsets (and sunrises) in Washington and beyond, the conditions last night were particularly conducive for a colorful show due to the mid-level clouds present over much of the area. Earlier in the evening, water vapor satellite imagery showed an upper-level low pressure system sitting off the coast, spinning counter-clockwise and pumping moisture into the area, seen as the pink colors in this image.

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This moisture was primarily concentrated in the mid-levels, as could be seen as a thickening altostratus layer covered the sun over the city. Note there are some darker clouds underneath as moisture continued to move in from the southwest.
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Showers and thunderstorms were prominent to the south, especially over northern Oregon, but the lower levels over Seattle and much of NW Washington remained dry. Every day, twice a day, the National Weather Service launches instruments on balloons called radiosondes that measure the temperature, moisture, pressure, and winds as the balloon ascends into the sky. Here is an example of what this balloon data looked like last evening around this time, with the red line indicating temperature and the blue dewpoint. The dewpoint is the temperature at which the air would have to be for saturation to occur. The higher the number, the more moisture in the air. Where the dewpoint and temperature are far apart indicates dry conditions (low humidity), while areas where they are nearly the same are where you might expect clouds. So, if you look at this example, you’ll see that most of the atmosphere near Seattle was very dry, with the exception of right near the surface and in between pressure levels of 400 and 350 mb. This corresponds to heights above the surface between roughly 7.5 and 9 km, where many of the clouds were observed on this particular evening.

So as I entered the stadium around this time, I quickly became distracted by the mammatus clouds that were forming overhead. The dry air below the moist levels helped create these brief, but beautiful bulbous clouds and it became increasingly difficult to pay attention to the game.

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In order to keep track of what was going on around the stadium, I kept an eye on the radar. Some of the showers from the south looked like they were making their way toward Seattle, as can be seen in this radar image during the game.

At the stadium itself though, some light drizzle could be seen, but overall the rain was evaporating before reaching the ground. This is referred to as virga and can be seen in these pictures that I took from the stadium around this time.
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So as you might have guessed, I was quite distracted by the sky throughout the game. The Sounders ended up beating Portland in an exciting 2-0 game, but I bet a large portion of the 64,000+ fans at the stadium were also paying attention to the sky. In fact, social media sites exploded with pictures of this fiery sunset, from the stadium and throughout other areas near the Puget Sound (

Later on at night, lightning flashed in the distance as storms persisted over the southern Olympic Peninsula. The low was moving onshore, bringing with it cold air aloft that created an unstable environment, in addition to the moisture available, to maintain these storms. Living in a place like this, where there aren’t a lot of thunderstorms, I’ll certainly take what I can get.

This morning, I was curious to see how much rain did end up reaching the ground across the area last night. The CoCoRaHS network ( is a volunteer organization where people report rain at their house. This map shows the reports for the 24-hours ending this morning, with most locations near Seattle receiving less than one-hundredth of an inch!
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And finally, for your viewing pleasure, here’s a link to time lapse video of the clouds and sunset as viewed looking west from the University of Washington’s Atmospheric Science building rooftop. Can you see the different cloud layers before the colorful finish to the day? Click here to see this time-lapse video!

Mammatus over the UK

Mammatus clouds form on the underside of other clouds as the drier air below causes evaporation and the formation of these bulbous features. While commonly observed under the anvil of severe storms, mammatus themselves do not indicate severe weather and can form under clouds other than cumulonimbus, as looks to be the case in this picture from over the UK in May.

Nicky Clarke
Location: Manchester, UK
Date: Late May 2014