GRAND RAPIDS, Mich. (April 29, 2014) — After two straight deadly days of severe weather across much of the South, it’s important that we pay close attention to the possibility of strong storms that continues across much of the country over the next couple of days.
With that in mind, here are some things to remember as we discuss forecasts for severe thunderstorms:
What makes a thunderstorm “severe”?
The official guidelines from the National Weather Service say that a severe thunderstorm must produce one or more of the following:
- Hail of one inch or greater in diameter
- Wind gusts of 58 mph or higher
- A tornado
NO amount of heavy rain or lightning can trigger a Severe Thunderstorm Warning.
What do the risk areas mean on the outlook maps we see on possible severe weather days?
The Storm Prediction Center is the agency responsible for issuing the severe weather outlooks for the U.S., covering the next eight days’ worth of weather. These outlooks are meant to increase the awareness that the ingredients for severe weather will likely be present. The outlooks break down into three levels: slight, moderate, and high risk.
Being included in a risk area doesn’t mean severe weather is happening now, or that it is guaranteed to happen. In fact, the technical definition of a slight risk region is the area where the forecast contains “at least a 15 percent chance of severe weather within 25 miles.” Even the highest of high risk areas usually only have forecast probabilities around 60 percent.
That said, the accuracy of these outlooks continues to improve over the last several years, especially when it comes to the biggest tornado outbreaks. Check out this article about Sunday’s forecasts for tornadoes in Arkansas.
In short-term forecasts for the next 24 hours, the SPC breaks down the threat by category for gusty winds, large hail, and tornadoes.
In the tornado outlook, you can see a great variation in the potential threat of tornadoes even within regions characterized as a “slight risk.” In lower Michigan, there is only a 2% risk of a tornado within 25 miles of any point; that increases to 10% around Nashville and Atlanta. The moderate risk areas of Mississippi and Alabama are more than seven times MORE likely to see a tornado than anywhere in West Michigan.
What does it mean that “75 million people are under a threat of severe weather”?
Recently, the Storm Prediction Center has started including the total population of the outlook areas in their products. As mentioned above, the actual risk of severe weather varies GREATLY within those areas. And Tuesday, only about 2.5 million of the 75 million total people are located within the moderate risk area. So the idea that 75 million people will actually experience severe storms, much less tornadoes, is a huge exaggeration. But those people should all make a point to remain aware of the weather situation at all times when included in the severe weather outlook.
So we’re in the “slight risk” area. What happens next?
Well, the first step is to watch as the conditions continue to evolve during the day. It’s likely we’ll see at least some scattered storms Tuesday afternoon and Tuesday night, but their strength will depend on timing and changing weather throughout the day. If conditions warrant, the Storm Prediction Center may issue a watch as storms start to pop.
OK, I forget this every year. What’s the difference between watches and warnings?
No problem, that’s why we’re going through this. Here is the info straight from the National Weather Service:
“Tornado Watch—NWS meteorologists have determined that tornadoes are possible in your area. Remain alert for approaching storms. Know if your location is in the watch area by listening to NOAA Weather Radio, visiting
http://www.weather.gov or by tuning into your favorite radio or television weather information broadcast stations.
Severe Thunderstorm Watch—NWS meteorologists have determined that severe thunderstorms are likely to occur in your area. Watch the sky and stay tuned for NWS warnings.
Tornado Warning—NWS meteorologists have determined that a tornado is occurring, or likely to occur within minutes, in the specified area. Warnings indicate imminent danger to life and property.
Severe Thunderstorm Warning—NWS meteorologists have determined that a severe thunderstorm is occurring or likely to occur. Warnings indicate imminent danger to life and property.”
Click this link for the full preparedness guide from the National Weather Service.