Thunderstorm intensity can be communicated in a variety of ways. Strong, Severe, Strong/Severe, strong to severe. Isolated severe? A few rumbles of thunder? What exactly does Wisconsin Weather mean when we use these terms? Why don't we just use Storm Prediction Center Convective Outlooks?
We love to do things differently. We are one of the few weather forecasting services who create their own outlook maps.
Five years ago during the Aug 6-7, 2013 tornado outbreak in northeast Wisconsin we found the SPC Convective Outlook to be inadequate when it mattered most. It sent the wrong message about storm severity and impact despite the parameters coming together across Wisconsin. Our philosophy is that by not sharing SPC Convective Outlooks we can better serve Wisconsin through creating our own outlook maps and improving our own outlook strategy/knowledge over time. While SPC outlooks have improved in accuracy recently, making our own outlooks keeps our team ahead of everyone else.
SPC convective outlooks were not designed for public dissemination.
Since Wisconsin Weather will be expanding our outlook map services on WISCONSINWX.COM over the next few months, we thought now would be a good time to explain what we really mean.
Wisconsin Thunderstorm Outlook Categories
Most thunderstorms are harmless. Even thunderstorms in a Severe Thunderstorm Warning are not always severe. We are trying to cut through the clutter to get the message through and improving the problem of complacency at the same time.
Our system has tough requirements. We want you to know when it's time to take the threat seriously, when that top level of intensity is going to be reached. We want that message to be clear.
We recognize this and want to motivate you towards action when it matters most. For starters, we assume that all thunderstorms are capable of Heavy Rain and lightning. We will never waste precise time on that. When we say "Heavy Rain" it's usually due to slow movement or training of thunderstorms that warrants a response other than huddling in your basement. Separate problems. Separate messaging.
Wisconsin Thunderstorm Outlook categories are simple. You should always be aware for lower severity levels such as general and strong. We want people to say "If you see Wisconsin Weather issue a "Severe" or "Significant" outlook, it's time to look out. They don't do that often."
Weak thunderstorms with no threat of severe weather. Thunderstorms produce rain which fall toward the earth's surface and pushing air out of the way. Therefore all thunderstorms can produce gusty winds, even the weakest ones. Of course it wouldn't be a thunderstorm without lightning, perhaps even deadly cloud to ground bolts? We usually use this simply to highlight where thunderstorms are possible.
Not only is a strong t-storm everything a general t-storm is, it's distinctly more intense with the potential to briefly reach severe criteria of wind gusts >=58MPH or hail >=1.0". Sometimes we say "storms could be strong" many days ahead of time to highlight that thunderstorms could produce severe weather. You could consider "strong" an open ended statement suggesting anything could happen. We think of it as the space between general storms and severe storms, which could very well be the majority of storms one location sees any given year.
This comes from the direct observation of hundreds of Wisconsin storms where intensity is greater than the average. The storm might be producing wind gusts, small hail, and a wall cloud but not producing severe criteria hail wind or tornadoes. The question we usually have is will it become severe? Strong storms can knock small leaves and twigs off trees but usually the property damage is minor or non-existent.
Strong storms can produce weak tornadoes. This would cover the situation of high-shear low-cape setups where a tornado occurs but no hail or damaging winds are observed. Unless you were impacted by the 50 yard wide 1 mi long tornado, odds are that it wasn't too bad.
Thunderstorms capable of severe criteria weather. Damaging wind gusts >= 58MPH, large hail >=1.0, or even a tornado. This is aligned with the National Weather Service criteria of a Severe Thunderstorm. Many times warnings will be issued based on a radar appearance despite a lack of storm reports confirming that these conditions are actually occurring. This is one reason why false alarm rate is high. The warning philosophy is "better safe than sorry" so things tend to get over-warned.
Severe thunderstorms will cause property damage. Strong vs. severe is a category line that gets blurred all the time which is one reason why these two words are often merged together.
When we do add a severe region to a our outlook maps, there needs to be a clear & consistent forecast signal because the visuals are so powerful. It needs to be distinct to the point we are highly confident it will occur within the area we highlight. If the potential cannot be separated confidently we will keep it at "strong" and use our words to explain the threat. If storm coverage is isolated, then "Strong/Severe" can be a good messaging compromise.
The "Severe" category will be best used for squall line or thunderstorm complexes where storm coverage is relativity widespread. Another good use case would be discrete supercells.
When severe thunderstorms are about to flip the switch and hit a level beyond what we are used to seeing. Storm events such as Aug 6-7, 2013. The May 31 1998. June 7 2007, Oakfield 1996 would all fit this category.
Extreme events often happen by accident, something that slips through the cracks but becomes obvious in the final hours before the event. To forecast significant severe weather we need to be thinking worst of the worst. Unfortunately this can be hard to see it more than a few hours in advance. Confidence level needs to be very high for this category plus a little "gut feeling". The area impacted will need to be clear, consistent, and constrained.
Furthermore, it needs to be widespread impact. Severe thunderstorms produce damaging gusts greater than >70 mph multiple times a year in Wisconsin. Large hail exceeding 2.0" happens more often than you think. This is mostly about coverage (widespread) and organization. If we see winds at least that up to or exceeding 100 mph over a large area, it's time to sound the alarm. An outbreak of strong fast moving tornadoes would also be enough. A strong bow echo with mesovortex tornadoes. The damage could be significant on many levels. We want to get the information out so you can take appropriate action.
Lastly, we recognize not every forecast is perfect. Ours certainly have room for improvement. We expect to be wrong from time to time. While our accuracy relative to the SPC will be debatable, at least you multiple opinions. We recommend you look at both. This will give you the confidence to make the best decision for you, your family and community!
Other Notes - If you do go outside to look at approaching severe storms, there many not be visual distinction between strong, severe, and significant. Some of the worst thunderstorms have high cloud bases and fast moving rain walls. On the contrary, I've seen many strong thunderstorms with wall clouds and impressive shelf clouds which might make you think a tornado is imminent but actually is not.