Squall lines
A squall line is a type of multicell storm that consists of a line of storms with a continuous, well developed gust front at the leading edge of the line. The band of precipitation must be at least partly convective in nature and may stretch for hundreds of even thousands of kilometers and last for many hours. Squall lines can produce strong, "squally" winds with heavy rain and some hail. They generally do not produce tornadoes and may pass overhead in less than half an hour.  
A squall line over the south-east UK on the 10th August 2014. This line of storms produced locally torrential rain and strong winds, rapidly increasing rainfall totals. 
From: https://weatheraction.wordpress.com/2014/08/12/watch-squall-line-passing-through-london-on-youtube/
Squall lines can form in a variety of ways:
  • Can Originate as a scattered line of convective storms, with new cells eventually filling in the holes in this line. 
  • May be triggered as a nearly solid line to begin with.

The latter scenario is more likely when there is a strong large-scale or mesoscale forcing present, such as with a cold front or dryline. â€‹They have also been observed to form from more scattered regions of convective cells or embedded within a more uniform region of stratiform precipitation. Generally, the most severe squall lines tend to from in highly unstably environments, away from forcing mechanisms. Squall lines do occur in a wide range of instability and vertical wind shear, although for a given CAPE, strength and longevity of the system increase with the vertical wind shear. As will be shown, the strength of the wind shear at the lowest levels (0 - 1km) is fundamental in determining squall line evolution. 
     Once formed, squall lines can display a characteristic lifecycle, starting as a narrow band of convective cells and evolving into a broader, weaker system over time. As low-level wind shear increases, the squall line will likely last longer (sometimes over 12 hours) and produce more severe weather elements, such as bow echoes.
The typical evolution of a squall line when forming in environments with different wind shear, represented in the form of radar reflectivity. The weak-moderate wind shear environment produces a squall line that dissipates quickly, with the mature stage producing a linear line of storms. The green area of reflectivity indicates a moderate intensity precipitation shield - known as the trailing stratiform region, and is indicative of a mature squall line. The system dissipates when the cold pool (generated by the downdraft cooled air from the storms) surges ahead of the storm cells, and so deprives the squall line of its forcing mechanism for cell regeneration. 
In an environment with strong wind shear (and especially high CAPE too), some of the initial cells that form may be supercellular, bringing the highest risk of tornadoes. The key is that the system develops much slower than with the weaker case and the mature stage is characterised by a much narrower line. This is becasue the stronger shear prevents the cold pool from racing ahead of the system, creating a more vertically-erect structure. Bow echoes may form at this stage, which have the potential to produce damaging straight-line winds at the surface. 
For both cases, as the cold pool moves away from the decaying cells in the later stage, a new line of cells may be triggered if it encounters a more favourable environment. 
From: https://www.meted.ucar.edu/training_module.php?id=18#.Vc88_nhcVUR
A large squall line of the central USA, stretching around 1000 kilometres from Oklahoma to Indiana, in spring 2001. Notice how the initial band of thunderstorms is very thin, and there is another thicker band of moderate precipitation behind - indicated by the yellow colours. This is the trailing stratiform region of rain. A squall line with a radar signature like this is called a leading-line, trailing-stratiform system. 
From: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/squall_lines.html  
A classical cross-section of a mature, well-developed squall line. The storm moves in the direction of the mean wind, from left to right. The primary storm cells are located at the front of the system, with the trailing stratiform region behind. The rear inflow jet (RIJ) is typical of a mature squall line, and it is when this descends to the surface that really destructive winds can occur. 
From Markowski and Richardson textbook: Mesoscale Meteorology in Midlatitudes, 2010). 
The maintenance of a squall line is very similar to the multicell storm. Rotunno, Klemp and Weisman, (1988) proposed that the optimal condition for the generation of new convective cells is when there is a balance between the vorticity generated by the cold pool and the opposite vorticity generated by the low-level wind shear. 
Schematic showing how an updraft may be influenced by environmental wind shear and a cold pool. (Wind is blowing from the left and is increasing away from the surface.) Arrows indicate shear vectors. 
From Markowski and Richardson textbook: Mesoscale Meteorology in Midlatitudes, 2010). 

(a) - When there is no shear and no cold pool, the updraft is vertically titled due to the symmetric vorticity distribution (created at the edges of the updraft). 
(b) - With a cold pool, the vorticity is biased towards the negative vorticity generated by the cold pool, causing the updraft to tilt back over it.
(c) - With shear and no cold pool, the distribution is biased towards the positive vorticity generated by the shear, and the updraft tilts forwards.
(d) - With both a cold pool and shear, the two effects can negate each other and promote the formation of a vertical updraft. 
From this theory, it becomes apparent that low-level shear is required to balance the vorticity produced by the cold pool, and produce a healthy, mature squall line with a vertical updraft. As the squall line continues to develop, the cold pool is constantly being reinforced and strengthened from continuous downdraft cooled air (and so the negative vorticity generated is constantly increasing). This helps to explain why squall lines in a low wind shear environment reach maturity and dissipation quickly - the wind shear is not sufficient to negate the negative vorticity generated by the cold pool for very long and so the thunderstorms tilt backwards and weaken quickly. With stronger environmental wind shear, the thunderstorms can remain tilted vertically for longer and the whole squall line can take longer to go through its lifecycle. In general, the deepest and strongest updrafts occur when the vortictiy generated along the cold pool is balanced by the environmental wind shear, promoting a vertical updraft.