Roll Cloud Our Precursor to the Major Hail Event Tuesday Night - KMSP-TV

Roll Cloud Our Precursor to the Major Hail Event Tuesday Night

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The storm Tuesday evening that pushed through parts of the metro has been the talk of the town the last couple of days. Hail up to the size of tennis balls coated the ground across many parts of the southern metro. But this storm started all the way in far western Minnesota dumping truckloads of hail across a 200 mile long path from northwest of the Morris area all the way to the Minnesota Wisconsin border. The following image shows the radar indicated hail tracks on the left, or the areas that received thunderstorms with hail production in them. On the right it shows the 75+ hail reports from across the area. Notice nearly straight line of reports across central Minnesota associated with the supercell thunderstorm that caused the vast majority of destruction.

But as the storm was rolling into the west metro, I was able to position myself on the southwest side of the storm in the spot that a typical supercell may have a tornado attached. To my relief there was no tornado, or even precursor to one like a wall cloud, but I saw something that might be just as scary if you came across it and had no idea what it was. It's what we call a roll cloud. I was able to capture some video of it below. If you watch closely, you can actually see the cloud rolling.


 

VIDEO: Roll cloud near Watertown, Minn. http://on.fb.me/14ohRmZ


A roll cloud is a low tube shape cloud that has horizontal spin to make it look like it's actually "rolling." In this particular case it's more likely it's a mix of a roll cloud and a shelf cloud. A shelf cloud can be very similar to a roll cloud with a very low shelf like appearance. The biggest difference between these two clouds though is that a shelf cloud is connected to the base of a storm like the picture below. The roll cloud is not connected to a storm and stands alone as its own feature.

Because of my view, it was really tough to see whether or not the cloud was actually connected to the storm. In reality, it likely was but you can definitely see the rolling motion to this cloud in the video above.

Shelf clouds and roll clouds associated with thunderstorms form in a very specific way. The diagram above is a rough estimate on how they form. These clouds are associated with the outflow of a storm. Outflow is the rain cooled dense air that rushes out to the leading edge of a storm and can often push these storms along at a much higher rate of speed than it would otherwise. The shelf cloud forms on the leading edge of this outflow as the warm moist air out ahead of this outflow is forced upwards. It is forced upwards so quickly that it cools rapidly forming condensation which we know as cloud cover. This gives the very ominous shelf like appearance as the storm moves.

Do you think these shelf clouds (or roll clouds) are a bit ominous looking? I hope so, because these are often the visible signs that the storm is quite strong. It may not necessarily have a severe weather warning associated with it; however it would be a storm that means business with gusty winds, heavy rain, lightning, and possibly hail. The shelf clouds hover very low to the ground making their visibility from the ground very limited. So if you see one of these coming at you, the storm is likely only a few minutes away or less so take substantial shelter immediately… don't wait! Many of you have seen firsthand what these types of storms can unleash.

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