More storms in the forecast, here’s some tips on reading the Doppler radar

Mid-summer weather is hot and humid and often brings the chance of showers and storms to the region. Although we did have a steady rain for July 4th, most of the time all of our rain in the summer comes from scattered showers and not a widespread general rain.

So what can you do when the forecast says “chance of scattered showers and storms?” For over a decade, you the public have had access to weather radar in very similar fashion to what a meteorologist use to make our forecasts. Sites like this one and other media outlets all have links to Doppler radar. Many of the private television stations have their own Doppler radar and of course the government has their weather radar operated by the National Weather Service.


While you might not be a trained meteorologist, interpreting a radar loop isn’t very difficult. When I teach the lesson on radar to my college kids, I tell them looping the radar is the most advantageous thing one can do to interpret what’s going to happen.
Let’s look at a few pieces of the radar loop that can help you. Click here for the latest radar loop.

The warmer colors on the radar, such as orange and red, are an indication of heavy rain and often thunderstorms. In the winter, these colors often represent sleet, but that’s for anther entry. If you see red or orange colors moving towards your area you can expect a period of very heavy rain.

On the other end of the spectrum the blues are lighter rain and sometimes so light it might not reach the ground. I tend to like to use the first shade of green as my benchmark of steady rain. This doesn’t mean it doesn’t rain from a blue radar echo, it’s just light.

Most of the radar loops are over about a 30-50 minute period. It depends how the operators of the radar are letting the images pile up and also the software of the radar program. Sometimes radar loops last longer, but they use a lot of bandwidth and therefore you don’t see them as often. On the image loop below there is a time stamp in the grey bar running from 8:40PM until 9:09PM. The loop gives you a good idea how fast rain is moving. If a thunderstorm went from Worcester to past Framingham in this 30 minute period the storm would be moving about 60 miles per hour assuming it’s covering 30 miles. Most of the time, the speed at which storms move stays pretty constant for each weather event. This is because the pace of the air that carries the storms doesn’t change too much within a few hours this time of year.

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The National Weather Service uses box to indicate where a warning is in place. On the loop above you can see they use red boxes for tornado warnings, yellow for severe thunderstorm warnings, green for flood warnings and orange for warnings for marine interests. The boxes will appear and disappear as the warnings timeout and are reissued for new areas. The boxes are a better way to see where there storm might move than listening to county warnings which are too broad.

Coverage increase and decrease
The other aspect of storms to watch is how they are growing or dissipating and also note the shape of the storm cell. Showers and thunderstorms don’t maintain the shape and move neatly across the area, they will undergo all sorts of changes sometimes growing larger and other times shrinking. The intensity of the cells themselves also change and you can see the areas of red and orange grow or shrink depending on the conditions of the atmosphere where they are and where they are headed.

If you watch the loop several times you can see how new areas pop-up while at the same time others disappear.

If you watch the radar often enough you can start to get a feeling for how it works and you will begin to be able to take the forecasts you read and hear and do a bit of your own forecasting. It’s also a great skill to have if you have something happening outside and I am not tweeting about the storms at that moment.


With more chances of storms the next two days you’ll want to keep your favorite radar site handy because in summer these boomers can pop mighty fast.

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