What Happened to the Heavy Rain? - KMSP-TV

What Happened to the Heavy Rain?

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At the beginning of this week, it looked like many of us were in for one really wet week with an inch of rain or more looking pretty likely. Even though it has been a pretty dark and damp couple of days, it didn't exactly work out that way. So what gives??

Well, the storm system wasn't as compact or as strong as many of the forecasting models were suggesting. It doesn't help that the placement of the strongest low was also off by quite a bit. So let me show you.

First off, here is how much rain has fallen across the area since this storm started Monday night.

Not much right? Widespread amounts of a tenth or so, with a quarter inch pockets in some isolated areas… about a quarter of what we were originally expecting. So here is the main reason. Below are is an image of what we meteorologists call vorticity. Basically, it's a big fancy word for spin… with the brighter colors circled showing more spin in the atmosphere… these locations are where you would find your low pressure centers. The one below is the status of the atmosphere Tuesday morning.

A few things to take note. The first are the two circled areas showing large amounts of vorticity which is where the low pressure areas sit. Next is the red line that I drew down the middle of the graphic. This is where the center of the trough axis is located. Notice that the trough axis is quite long and is very spread out. With an elongated trough axis… the storm system's energy is spread out, which explains why there are 2 areas of low pressure and not just one. With things spread out, the storm is less potent to those it impacts... especially those on the northern end which is far removed from the moisture source… which more often than not the Gulf of Mexico. Also, the stronger of the two lows is the southern one which is of course, is further away from Minnesota. In order to get rainfall on what we would call the "backside" of the storm system, or basically the northwest side, you need a strong area of low pressure. That is low is what we would call a "closed low" or a low that is strong enough to morph into one area of vorticity and create its own lift pocket on the backside of the storm which would then create precipitation. With an elongated trough and therefore, 2 weaker low pressures, you will get a long line of showers and storms (or snow, depending on temperature and time of year) that will impact more people, but will often be less intense on the northern end. This is what has occurred with our storm.

You can see in the additional vorticity picture below that as this storm continues northeastward through the Great Lakes and southern Canada that it continues to strengthen and forms into one big area of vorticity.

This creates a lift pocket on the northern side of the storm which produces precipitation and can be seen in the following image below that shows model interpretation of what the radar will look like early Friday morning across much of the United States. The circled area is the precipitation that I am talking about.

So, if this low had strengthened and moved to our southeast like it originally looked, then our "rain shield" would have looked a lot like southern Canada will be getting Friday morning, but no such luck this time around. Oh well, we might have another chance for some heavier precipitation next week.

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