Interesting weather tidbits from Monday's wintry mix - WNEM TV 5

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Interesting weather tidbits from Monday's wintry mix

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Atmospheric Profile From Weather Balloon Launch Atmospheric Profile From Weather Balloon Launch
Atmospheric Profile From Weather Balloon Launch Atmospheric Profile From Weather Balloon Launch

Last weekend and into Monday morning, the big forecast focus in the weather department was on the possibility of ice Monday night. After seeing the impacts from that system that are even lasting into Wednesday, it's no question why it was a high priority. 

But before that event got underway, you may have noticed something interesting.

We all have our methods for getting weather information and a lot of us check the radar using our phones or other electronics. Outside of the office, those of us in the First Warn 5 Weather Center do the same thing. 

As the storm system moved closer to Mid-Michigan from the southwest, areas of blue, pink, and green started showing up on the radar. You may have seen some of those colors over your town at some point during the morning and afternoon. then prepared for rain or snow as you headed out the door. 

Pretty typical, right? Not necessarily, especially if you looked during the morning and early afternoon. You may have stepped outside shortly after checking, realizing that there likely wasn't anything actually falling outside, or what was falling was not nearly as heavy as it may have looked on radar.

The culprit? A very dry air mass that was already in place over Mid-Michigan from the beautiful weather we experienced on Sunday. 

If you watch us on the air, you've likely heard us refer to a "layer of dry air" during the forecast. While we try to give perspective, it's not always easy to explain it thoroughly in the limited time we have on the air. After watching this system evolve, this seemed like a perfect opportunity to explain it a little more.

Examining The Atmosphere Near The Ground & Above

First we have to understand how our atmosphere looked that day. 

Attached in the article you'll find three graphs. Those three graphs are from three weather balloon launches at the National Weather Service office in Detroit, one from 7 PM on Sunday, one from 7 AM on Monday, and one from 7 PM on Monday. 

At first glance, the graph looks like a mess. But what's important here are the red and green lines. The red line indicates temperature, the green line indicates dew point. The bottom of the graph where the two lines start, indicates temperature and dew point at the surface. As you follow the line to the top, it represents what the readings are at higher levels of the atmosphere. 

The farther the red and green lines are apart, the drier the atmosphere. The closer together, the closer our atmosphere is to saturation. Think of our atmosphere like a towel or sponge. It can only hold so much moisture. Once it becomes saturated, precipitation occurs (like ringing out a towel or sponge).

When those lines have a very minimal distance between each other from top to bottom, precipitation is likely falling.

As you can tell from the launch at 7 PM Sunday and 7 AM Monday, there was plenty of distance between the two lines, showing we were still very dry. But at 7 AM Monday, we saw those lines closer together around 18,000 feet and higher. 

From 7 AM onward as the system moved in, we'd gradually start adding moisture, and by the 7 PM launch at the NWS office in Detroit, we had a very minimal distance between the two lines, indicating a sufficient atmosphere for precip. We can confirm this with observations indicating light freezing rain was falling at that time. 

How Can This Dry Air Fool Us On Radar?

To show how we may be deceived by radar, we need to look back at the graph from 7 AM on Monday, where we had some moisture starting to fill in around 18,000 feet.

When our weather radars send out their beams, they do so at an angle, reaching higher into the atmosphere the farther the beam travels away from the radar location. So the closer to the radar site, we're able to see what's occurring close to the surface, while the farther away from the radar we see things that are happening high up in the atmosphere.

With only so many radar sites around the Lower Peninsula, we're only able to see so much. The radar sites in Detroit and Grand Rapids were the first to have the view of the incoming storm. However, with both radars at a distance, the beam was already very high in the atmosphere.

Therefore, the precipitation it was seeing and sending back to our phones and electronic devices was very high up in the atmosphere, and with a very deep layer of dry air near the surface, it was evaporating before it even had a chance to reach the ground. 

This lasted through a good chunk of the afternoon in Mid-Michigan, before freezing rain became more efficient during the evening hours. 

Dry Air Also Allowed Temperatures To Cool

One final interesting note was the effect this dry air had on our temperatures on Monday. Through most of the afternoon, we saw plenty of locations get above the freezing mark. This led to a lot of questions about whether or not we would see freezing rain. 

Once again, that layer of dry air became involved. As that precipitation falls through the dry layer of the atmosphere, it evaporates. Evaporation is a cooling process.

This allowed temperatures across the area to fall, leading to some of us seeing a few snow flakes before a warm layer of air moved in above the surface to melt that frozen precip to rain. With sub-freezing temperatures at the surface, we were left with freezing rain and the messy morning commute that followed. 

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