Radar images are just snapshots in time

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As I prepared for work this morning, I glanced at the radar and saw a number of signs in a single image that let me know I could be in for a rough commute. The screen, a snapshot of which is to the right, showed that a gust front had just passed my part of North Raleigh, a heavy rain shower was moving toward my house, and a large area of rain was heading into the Triangle.

radar image

A screenshot of the radar image at 6:19 A.M. on Tuesday, March 28, 2017.

I set my phone with its radar app back down and rushed to get out the door before that heavy rainshower hit, but it never did. The shower dissapated, or rained itself out, before it made it to my neighborhood. The large blob of rain behind it developed holes and weakened into lighter showers. The next time I looked at the radar, about 20 minutes after the first, I saw a very different situation. That’s how it goes with the radar.

Any time someone asks me if it’s raining, I follow my answer with the obligatory “right now,” because a radar image is just a snapshot in time. Just like any photograph, it shows the situation at that moment whether that be rain, hail, high winds, rotating winds, or nothing at all.

Doppler radars take four to six minutes to complete a full scan, so the image takes four to six minutes to update. A lot can happen between updates. Tornadoes can form, hail can fall, rain can begin, snow can end, etc. What you see at that moment is what the radar saw in the last sweep.

Radar is a tool for nowcasting and very short-term forecasting at best — a highly important and effective tool. Without radar, we would be utterly dependent upon people on the ground reporting tornadoes, and by the time those reports were disseminated and the public was alerted, the damage would already be done. It wasn’t long ago when that was the case –merely decades.

With Doppler radar, we can see velocity signatures that indicate rotation in storms before a funnel even develops. Our warning time for most storms has greatly improved, and tornado warnings for long-lived, slow-moving storms may even be 45 minutes to an hour in advance of the storm’s arrival at a point on the map. That’s pretty impressive when you think about it!

What about those moments when storms are forming faster than the radar can keep up? Those are the times when it is extremely important to take it upon yourself to pay attention to weather watches and warnings and to look at the sky. Your own eyes on the horizon are invaluable to your safety. If you see a rotating wall cloud, green clouds, lightning, or the effects of strong winds, don’t wait for a warning to be issued. Take shelter.

A rotating wall cloud is a lowering of the cloud base that is rotating around a vertical axis, and it is a sign that a storm could produce a tornado at any moment. Greenish-tinted clouds tend to signal that there could be hail or very large raindrops inside. Lightning is a danger if thunder is close enough to be heard. Strong winds can kick up debris and do as much damage as a weak tornado. All of these things could develop quickly, and being weather aware could be the one thing that saves your life. It sounds rather dramatic, but it’s true.

As we head into one of our busiest storm seasons of the year, practice safe weather watching. Watch the radar and look at the sky to stay truly weather aware.