Mesoscale convective system passes through area overnight with a period of rain

Mid-Evening Update

The latest computer guidance shows a weaker storm complex overnight meaning the rainfall will be limited and flooding isn’t a concern. Early blog from this morning is below.

Over the past couple of winters the term “polar vortex” has gained a lot of favor within the media. Other meteorological phenomena such as derecho and microburst have also made their way into the public’s lexicon.

When I get to the section on thunderstorms in my college course, one of the more interesting topics is mesoscale convective systems (MCS) and mesoscale convective complexes (MCC’s). These clusters of organized thunderstorms are where much of the Midwest’s precipitation arises from during the summer.


Later today and overnight there will be an MCS passing just south of the area. These blobs of storms can take on several shapes and sizes the exact configuration of which is critical in determining if you are going to get rain or not.

What’s An MCS?
Think of these entities as organized clusters of thunderstorms acting together rather than the scattered nature of storms we often see in the summer. On a hot humid day when thunderstorms pop up they only last about an hour and often appear seemingly randomly. These storms don’t move together, but do generally travel in the flow of the atmosphere, often west to east or north to south in this area. Sometimes, the individual storms will also move southwest to northeast ahead of a cold front or other boundary.

scs explainer.jpg

In the MCS and MCC scenario, the storms act as a unit and move west to east or northwest to south east along some boundary. In tonight’s case, the boundary is well defined. The divide between the cooler and drier air over New England and the hot and humid air to the south will provide an ideal breeding ground for an MCS to form.


Very Common

There isn’t anything unique or unusual about this. I’m just using terminology which typically isn’t shared in the mainstream media. You’ll likely hear about a cluster or storms or an areas of rain, rather than an MCS.

Within the structure of the MCS you can have a squall line of storms with severe storms and torrential rain. The conceptual radar evolution below shows how these systems form over time. Notice the line of reds and oranges inside the larger blob of precipitation. That is the squall line. As one progresses through the letters the line dissipates and a new line forms. This process can continue for many hours.

radar image mcs.jpg

Radar Tells The Story
The radar loop below is predictive radar for the upcoming rain. Notice how similar this actual model is to the conceptual one above. Notice how over time the individual storms are organizing into a cluster and a squall line is forming to the south. This loop is courtesy of WeatherBell Analytics

mcs tonght 4315.gif

One of the reasons there is a flash flood watch posted overnight is a result of the prediction the placement of the heaviest rainfall within the MCS will be south of the Massachusetts Turnpike. The area in the red rectangle is along the predicted path of the heaviest rainfall from the upcoming MCS.

flood watch 4314.png

These systems have a way of turning out somewhat different than the models. I am confident this is going to happen, but the placement of the heavy rain is still somewhat in question. It would not surprise me if the flood watch is expanded further north and east based on data later today.


If you are really interested in reading and learning more about MCS’s and MCC’s you can click here. I put a link to a lesson on these phenomena there. It’s a fairly advanced explainer about them, but I think you can likely gain more knowledge about them.

If you have a question tweet me @growingwisdom. I’ll be watching for where this sets up later this afternoon as I would love an inch of rain tonight.

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