TLDR: The National Weather Service Storm Prediction Center issued an outlook early this morning that indicates a probability of all forms of severe weather throughout Indiana Sunday, including tornadoes; damaging straight-line thunderstorm winds; and large hail. People should pay attention to reliable sources of weather information as Sunday nears, especially people who are planning outdoor activities Sunday afternoon and evening.
How we know this is possible
The Storm Prediction Center (SPC) is a branch of the U.S. National Weather Service (NWS) that’s responsible for providing timely and accurate forecasts and watches for severe thunderstorms and tornadoes over the contiguous United States. Every day, it issues severe weather outlooks for the current day (Day 1) and for days two (tomorrow), three, four, five, six, seven and eight.
Accurately predicting severe weather more than two days in advance is difficult. So, if the SPC indicates a possibility of severe weather in an outlook for three days from now, it’s wise to pay attention.
In the outlook for Sunday that SPC issued this morning (Aug. 3), it indicated a 15% probability of severe weather, including tornados, damaging straight-line thunderstorm winds and large hail, within 25 miles of any point in Indiana as well as parts of neighboring states.
Tornadoes, damaging straight-line winds and large hail are all possible in Indiana Sunday.
Why is 15% a big deal?
As percentages go, 15% looks pretty small. But that’s because severe storms don’t happen all the time. The SPC, in collaboration with another branch of the NWS, the NOAA National Severe Storms Laboratory (NSSL), analyzes historical weather data to determine the normal probability of severe weather on any given date (based on how many times it happened on that date in the past). That analysis indicates that in Indiana, the normal probability of severe weather on any August 6 is 2% to 3% (depending on where in the state, see graphic below).
That means Sunday’s 15% probability of severe storms is 5 to 7.5 times what’s normal for that date.
What’s most likely, tornadoes or severe thunderstorms?
Day 4 is too far away for SPC to provide probabilities for specific severe weather hazards, like tornados. Nonetheless, the text of today’s Day 4 outlook implies that tornadoes, damaging straight-line winds and large hail are all possible in Indiana Sunday.
In addition. local NWS offices in Indiana issued their own outlooks today that indicate the possibility of severe weather Sunday, especially in the afternoon and evening. These local outlooks also indicate the NWS might need reports from trained, volunteer storm spotters Sunday.
Tomorrow, SPC will issue a Day 3 outlook for Sunday that will provide a categorical risk level from one to five (five being the greatest risk) and possibly modify the target area based on the latest available weather data.
Saturday, SPC will issue a Day 2 outlook for Sunday that will provide individual probabilities of a tornado, damaging straight-line thunderstorm winds (58 mph or stronger), and large hail (one inch or larger). By then, we’ll have a much better idea of what parts of the state have the highest risk and which hazards are most likely.
What should you do now?
No matter who you are, if you’ll be in Indiana Sunday afternoon and evening, and especially if you plan to be involved in an outdoor activity, pay close attention to a trusted source of weather information as Sunday approaches, such as your local NWS office or professional broadcast meteorologists. If you don’t have a weather alert radio in your home, this would be a good time to buy one.
If you’re a trained Skywarn storm spotter like me, prepare for possible activation Sunday, including charging all your battery-powered devices and reviewing what to look for, what to report, and how to report it. The NWS will likely need your eyewitness reports so they can issue the best warnings and keep people safe.