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3.1.1 What is a Squall Line?


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Probably the most frequently observed form of mesoscale convective organization is the squall line. A squall line may be thought of as any line of continuous or nearly continuous convective cells. There is NO strict size definition for squall lines, although for investigative purposes some authors have proposed 50 kilometers long, and 10 kilometers wide. A significant distinction from other type of convective storms is that squall lines have a larger length-to-width ratio. They are most commonly composed of mostly ordinary cells; however, when the environment exhibits strong deep vertical wind shear, squall lines may also initially be composed of primarily supercells, as is the case in this radar loop. During the early stages of such systems, supercells may exist along the entire extent of the line. However, over time the supercells are often disrupted as their varying storm motions cause them to interact with other nearby cells.

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3.1.2 Supercell-Favored Locations


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Even for the strongest, most favorable shear profiles, supercell structures within the center portion of a squall line usually evolve into more linear convective structures like bow echoes. However, the cells at the ends of the lines are more able to maintain their independence and can remain supercellular for long periods of time. It is fairly common to find supercells only at the southern end of mature, north-south oriented squall lines. It is also common to find supercells at breaks within otherwise solid squall lines. Both of these locations need to be monitored carefully for an increased potential of severe weather.

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3.1.3 Squall Line Classification

Squall lines form in a variety of ways. They often originate as a scattered line of convective cells, with new cells eventually filling in the holes in the line, but they also may be triggered as a nearly solid line to begin with. As mentioned before, this latter scenario is especially likely when there is a strong linear forcing mechanism present, as with a cold front or dryline. In some cases, squall lines are also observed to form from more scattered regions of convective cells or embedded within a more uniform region of stratiform precipitation. Bleustein and Jain’s study of the modes of severe squall line formation in Oklahoma classified them into four categories: “broken line,” “back building,” “broken areal,” and “embedded areal,” with broken line and back building cases being the most common.

A 2003 study considering both satellite and radar observations of warm season MCSs between the Rockies and the Appalachians determined that greater than 70% of the systems evolved from the merger of multiple convective clusters. They also noted that MCSs that were linear at the time of formation resulted in larger, longer-lived, more severe and effective precipitation producers than those that formed from initially scattered storms. These results contain useful operational information and are undoubtedly often related to the presence of a stronger linear forcing mechanism for the more intense systems.

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3.1.4 Review Question: Identify Form of MCS

As you look at this next review question, recall the scenario at the start of this module. In the radar loop from 2100 UTC on 24 June through 0800 UTC on 25 June, Sergeant Slade observed the formation of a long squall line over the Central Plains and upper Midwest.


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Basing your answer on the radar loop above, answer the following question: What form or forms of MCS development do you see?

Please select all that apply:

a) Back building
b) Broken line
c) Broken areal
d) Embedded areal

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Expert Answer: The correct answers are b) Broken line and c) Broken areal. Slade witnessed at least two types of formation in this developing system. Cells from southwest Nebraska to southeast South Dakota became a broken line that matured by 0100 UTC. There was evidence of an expanding cold pool from the new cell development northwest of the line and an eastward-moving bulge in the line. Broken areal formation is represented by the larger area of broken cells and clusters from eastern South Dakota to extreme northwest Wisconsin. It should be noted that this larger area of convection did not take on the characteristics of a mature squall line until several hours later.


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End of Section 3.1: Squall Lines - Overview