It’s officially spring, and that brings with it the potential of severe weather to Virginia and much of the United States. Meteorologists from the local level all the way up to the National Weather Service will be hard at work trying to make sure folks are prepared for any storms that pop up (we saw some already in Virginia earlier this month).
That work involves efforts to ensure warnings that go out to people in the days ahead of potential severe weather include the most information possible – an ever changing and ongoing mission for teams at the National Weather Service’s Storm Prediction Center.
The SPC analyzes conditions across the country that might be ripe for strong to severe thunderstorm development. The center then produces outlooks a couple of days ahead of when damaging wind gusts, hail and tornadoes may arrive. Those outlooks include a five-point scale on the risk of severe weather – one being the lowest, five being the highest – and a graphical representation of where that risk is across the country.
That outlook system recently got an upgrade according to Evan Bentley – the Warning Coordination Meteorologist at the Storm Prediction Center.
“In the past, we could only talk about intensity and whether or not we have the chance for greater than EF2 plus tornadoes, as well as hurricane force wind gusts or two plus inch hail – and we had to have a certain coverage of that significant to severe weather to include it.”
SPC has now added a new intensity metric to their outlooks – a four-point scale on what meteorologists there think the threat might be on any given day.
“So, we can say more information about the potential for strong, intense and even violent tornadoes or wind gusts maybe even up to 100 miles per hour or hail that might be much larger than two inch – to even the size of softballs,” Bentley says.
He adds that this has been a struggle for meteorologists in the past – the science and forecasts in general are quite accurate, but how do we best communicate intricacies – like different levels in intensity – in the forecast to the public?
“Because people were asking that – like, ‘In the past, you said EF2 plus tornadoes, so you’re not providing any information on whether or not it’s a ceiling of an EF2 or EF5.’ Which from an impacts perspective is a very different event.”
SPC has recognized the need for more nuanced intensity information for years.
“All the way back in 2019, we started in the Hazardous Weather Testbed testing these new products and new ideas,” Bentley says. “After a couple years of testing there and then several years of testing it internally, within the last year or two, we started showing that our verification data looked very good; that we were able to go public with this and that the science has gotten to the point where we do have the skill to add this intensity information.”
What is "severe weather" exactly?
As defined by the National Weather Service, a thunderstorm is considered "severe" when it produces one or more of the following:
- Winds of at least 58 miles per hour
- Hail that is at least one inch in diameter
- Any tornado
When it comes to producing severe weather outlooks with the new intensity information, Bentley walks me through the Storm Prediction Center’s process…
First, he says officials there look at what the environment is showing us using a variety of parameters – many of which were developed internally at the Storm Prediction Center. This includes certain metrics about the probability of significant tornadoes and others.
Bentley adds more is needed to really fill out what the risk is, however – storm mode information is vital:
“Is it what we call supercells or QLCS [quasi-linear convective system] tornadoes that are within a line? There are multiple different ways that you can have these events occur – and that’s going to have a big difference on what the intensity is going to be,” he says.
All of that information – environmental parameters, analysis and what the predominant storm mode will be – is combined to give a good picture of what overall intensity for tornadoes, damaging wind and hail will be.
Those details are vital for decision makers and the public to be best prepared for any situation, Bentley says.
“Making a decision on what you want to do [on a particular day] might be different if the message is, ‘Well, there could be a lot of storms around with 55-75 mile per hour wind gusts, but we don’t really expect any really strong, hurricane force wind gusts with these.’ That might require one protective action. In another scenario, you might say, ‘Hey, we don’t even know if there’s going to be a storm; the sky might remain blue for most of the day, but there might be one or two storms – and anyone who has those storms could see wind gusts up to 90 miles per hour.’ Those are two totally different preparedness pieces.”
The hope is that the additional nuance will be helpful to more people across the country, as severe weather threats may be more impactful to some communities than others.
“Some people are much more vulnerable to severe weather,” Bentley says. “To some people, 60 mile per hour wind gusts could be very impactful – if you’re in an area prone to tree falling [with a home near it], that’s a much greater impact than some others who might live in an open area that sees 60 mile per hour wind gusts quite frequently.”
This new intensity information has been in development for some time, so I asked Bentley if there are other enhancements on the horizon for the Storm Prediction Center. Right now, the center produces outlooks with detailed information for Day One and Day Two (essentially today and tomorrow) that breaks down the risk and intensity for hail, damaging wind gusts and tornadoes. For Day Three, just a generalized outlook outlining where severe weather could occur without the additional breakdown is available – and even less detailed information for Days Four through Eight.
That’s the current reality, but that may not always be the case.
“As the science grows, we’ll probably be expanding to have individual hazards into that Day Three timeframe,” Bentley said. He added that as forecasting ability with additional computing power and tools like artificial intelligence and machine learning continues to grow, there could one day be some level of information for the Day Nine and Ten outlooks – just increasing the amount of lead time for folks to be best prepared for the worst nature has to offer.
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