Our location in New England places us in an ideal spot to see the passage of mid-latitude cyclones with great frequency throughout the year. Not to be confused with tropical cyclones, storms that develop and form over warm ocean waters of the tropics, mid-latitude cyclones all have similar tendencies with respect to their structure and evolution. All of our storms during the past couple of weeks can be classified as mid-latitude cyclones, and we’ll take a look at some common patterns and tendencies leading to their development.

 

Today, we’ll take a look forward to model projections of a storm incoming Monday morning, meaning more snow in New England next week. In my previous post last Thursday, I discussed the influence of large scale patterns in the upper atmosphere. Today, we’ll take a closer look at the interactions of the upper and lower troposphere, the layer of the atmosphere closest to the surface where all weather occurs.

 

The fundamentals of strengthening cyclones are governed by pressure patterns and the movement of air according to these patterns. Cyclones are characterized by air converging on a low pressure center at the surface, but diverging from high pressure aloft. Keeping in mind that pressure always moves from high to low pressure, two examples are shown below.

 

 

 In this image above, we see that the air diverging aloft exceeds the converging air below, represented by the arrows in the diagram. This means that the flow of rising air in the column will increase, and so the pressure continues to drop at the surface, allowing the storm to strengthen.

 

 

 

The counter example is where convergence at the surface exceeds the divergence below causing a net decrease in rising air in the column, and the pressure rises at the surface, weakening the storm.

 

Now that we know a little more about the interaction between the upper and lower levels of the troposphere, we can see how this plays out in the model runs for next week. Let’s take a look at the 500mb level, or the upper troposphere. We can see the shortwave trough, the disturbance of low pressure, marked by the black line. The circled divergence area is in advance of the trough, recognizable by the widening isobars (lines of equal pressure). This is the projection for 7AM Monday morning:

 

 

 

 Later in the model run, we see the shortwave trough reach the New England 7PM Monday night:

 

 

 Let’s see how the surface model responds to this. Here are the corresponding maps. First on Monday morning…

 

 

 And then on Monday night, we see the low pressure system strengthen and intensify as it settles in the Gulf of Maine.