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It has been said before that the spring season often becomes violate with severe weather, with little or no advance warning.  This is because the atmosphere is fighting between cold, polar air to the north and warm, tropical air to the south.  Now, we investigate some types of spring severe weather your local meteorologist may refer to from time-to-time, and the dynamics that go behind building them.

Mesoscale Convective System (MCS):  This is a thunderstorm complex, consisting of individual cells embedded within a broader area of precipitation.  A typical thunderstorm consists of several stages.  As a single cell continues to grow at an irate amount of speed, it spawns additional cells, usually along a line and to the south.  As these thunderstorms grow stronger, cells to the north weaken as a result, allowing the complex to grow to immense proportions.  An MCS can last in upwards of several hours and are often linked to flash floods, due to the heavy rains that accompany them.  Additionally, they are often nocturnal and can contain damaging winds, large hail, and isolated tornadoes.

(The following are attributes of an MCS, especially during the late spring months):

• Squall Line:  A type of MCS that forms along or ahead of a cold front.  During the spring months, cold and dry air sweeps across the Plains, interacting with warm, moist air aloft.  As it does, it causes the less-dense air ahead of it to rise, where the moisture condenses into single cell thunderstorms.  As mentioned above, as a thunderstorm nears the end of it's life-cycle, it becomes "choked-off" by rain-cooled air within the thunderstorm column, known as the downdraft.  As this happens, it sparks additional thunderstorms within the warm sector, parallel to the line of initial development.

• Bow Echo:  A type of squall line that is associated with the appearance of an archer's bow.  During the spring months, cold and dry air often lies near a surface, which is in return layered by warm, moist air aloft.  These two components help to enhance mid-level flow between them (similar to wind being channeled between two pieces of paper).  In return, it helps to strengthen the rear-inflow jet (portion of the jet stream that rapidly descends to the surface), producing strong, straight-line winds within the the squall line.

Furthermore, it is important to note that wind shear plays an important role in any thunderstorm development.  Wind shear is attributed to a change in wind speed and direction over a relatively short distance in the atmosphere (similar to a plastic bag that is propelled upward between two vehicles moving opposite directions on a busy freeway).

Because of this, hail is a big component of wind shear, which can tap into colder air near the top of a thunderstorm.  As a result, droplets of supercooled water freeze to pieces of dust or debris and fall towards Earth.  Given the strength of the updraft, they can be fed back into the thunderstorm, where they under-go this process a series of times, until becoming too-heavy and falling to the ground (this is why hail has several layers within a single stone).

Tornadoes form as a component of wind shear as well.  In short...  horizontal winds can be fed into a strong updraft, where they enhance upward vertical motion.  The resulting drop in surface pressure causes a column of air to descend towards the surface, resulting in a funnel cloud.  ONLY If the funnel cloud makes contact with a surface, it is deemed a tornado.

JB