There were several microbursts across southern New England Friday evening. There could be one or two more Saturday afternoon and evening.
So that begs the question…”what is a microburst?!?” You hear that word a lot in the summer, which prompted a gentlemen to ask me to explain it just the other day. I figured he wasn’t the only one. Here’s what you need to know…
A downburst is a generic term for a strong wind gust generated by a thunderstorm. Downbursts happen when large rain drops and/or hailstones in a thunderstorm drag very cold air from higher up in the atmosphere down to the ground. Cold air is very dense and very heavy, so that cold air hits the ground with a lot of force. Think of it like a giant brick that was suspended in the atmosphere suddenly falling to the ground. Once it hits the ground, it spreads out in all directions.
Downbursts are subdivided into microbursts and macrobursts. Microbursts impact an area of 2.5 miles (4 km) or less in diameter. Macrobursts are larger than that. Despite their small size, microbursts can pack a punch! Often the wind speeds in a microburst exceed 80 mph or even 100 mph and produce wind damage similar to a EF-1 tornado. In fact, many victims of a microburst assume it was a tornado. Microbursts can even produce winds as high as 168 mph according to the National Weather Service.
The strongest microbursts generally occur when you have hot, humid air in the lower levels of the atmosphere (near the surface), and cold, dry air above that along with strong winds higher up in the atmosphere. Why is dry air important? If the air is dry, some of the rain drops will evaporate on the way down. The process of evaporation causes rapid cooling. You know this if you’ve ever felt chilly after getting out of a pool on a dry, breezy day. So, air that is already cold, gets even colder, and even heavier, and hits the ground with even more force.
Microbursts can be particularly dangerous to airplanes during takeoffs and landings.
Part of the danger results from the fact that microbursts can be tough to detect. Doppler radar can determine the speed and direction of precipitation particles relative to the radar (either coming or going). But, it takes about 5-6 minutes for a complete radar scan and microbursts only last 5 minutes or less. So, microbursts can occur in between radar scans and go undetected. When you do catch one on radar, the National Weather Service says offers this as a perfect example…
The image above on the left shows where it is raining. Red indicates heavy rain as you probably know. The image above on the right shows the velocity of those rain drops. In this case, red indicates the winds are blowing away from the radar and green indicates the winds are blowing toward the radar. Compare that to what we saw on our radar just before 8 PM Friday evening…
The bright green color indicates strong winds around Flanders Road blowing toward the radar located in Taunton. That’s an obviously strong microburst! Wind speeds were estimated at 50-70 mph in that case. By comparison, if this were a tornado, you would see bright red colors right next to the bright green colors.
Why are we concerned about microbursts again today? Well, as you have probably guessed, we have many of the same ingredients. Saturday will be almost as hot as Friday, maybe a degree or two cooler if we are lucky. However, Saturday won’t be quite as humid as Friday. So, there is slightly less fuel for strong thunderstorms in the lower part of the atmosphere. Nonetheless, the air above us is unusually cool and there are some strong winds up there as well. While most of the damage on Friday occurred roughly near and south of the Pike, the best ingredients for microbursts on Saturday will be along and north of the Pike. It also appears that the storms on Saturday will start and finish a bit earlier than they did on Friday. The best chance for storms will be between 2-9 pm Saturday.
Sunday will be dry. Heat AND humidity are back in a big way on Monday. So is the chance for strong storms. Monday is looking very similar to Friday. Once again we could be in for some storms with some strong wind gusts.
Tuesday and Wednesday will be dry. But, a few isolated storms will be back in the forecast for Thursday and Friday afternoon.
Bottom-line, its that time of year when you need to keep an eye on the weather if you have outdoor plans. And now you know what to look for!