Blog Extra: How do low pressure systems develop?

Last Thursday's storm system was fairly impressive.  From Blizzard Warnings to Severe Thunderstorm Warnings across the Midwest, Thursday had it all.  But, just what exactly causes low pressure systems to form?  And more importantly, what causes them to strengthen?  To understand that, we first need to understand what exactly low pressure is. 

A low pressure system, or simply put a 'low', is an area in the atmosphere where the atmospheric pressure is lower than that of the area surrounding it.  Typically, low pressure systems are associated with rising air through the atmosphere and also produce clouds, precipitation and storms, such as thunderstorms and tropical cyclones.  In the Northern Hemisphere, winds around low pressure flow counterclockwise (due to the Earth's rotation) and spiral inward towards the center of the low.  This inward motion at the surface, also known as convergence, forces air to rise through the atmosphere.  In order for low pressure to strengthen and continue to move across the United States, there needs to be another process going on in the atmosphere.  For that, we have to look above.  Not only do lows, highs, cold & warm fronts form at the surface, but they also form above which allows the rising air from the surface to divergence once it reaches the top of the atmosphere.  Now, what is divergence?  It's the opposite of convergence; where air moves away, or spreads apart, from a center point.  This divergence occurs at the top of the atmosphere (where the jet stream is) and acts to remove the air from the surface, therefore lowering the pressure within the surface low allowing it to strengthen.  Think of it this way, the upper level low needs to be 'displaced' to the left of the surface low to keep the air within the atmosphere rising.   

So, what causes a low pressure system to lose it's strength?  When a cyclone loses it's energy, the upper level low will, instead of being displaced from the surface low, move on top of the surface system becoming 'stacked'.  Once a system is stacked, there isn't the continuous flow of rising air through the atmosphere and eventually the low will weaken.  Looking ahead through this week, the weather will be dominated primarily by high pressure, which is sinking air.  Late weekend, and early next week there will be a few systems we will be watching that could bring a little more active weather to the Midwest.