The U.S. National Weather Service has been issuing tornado and severe thunderstorm watches and warnings since 1965. More than 50 years later, however, people still fail to fully understand the difference between a watch and a warning. You probably know some of them, I certainly do.
As my contribution to the second day of Indiana’s Severe Weather Preparedness Week 2016, I’ve written this blog to share with your friends and family members who remain confused.
A watch is essentially a forecast
In fact, before 1965, the NWS called what we now know as “tornado watches” “tornado forecasts.” A tornado watch indicates the possibility of tornadoes forming, just as a rain forecast indicates the probability of rain. Similarly, a severe thunderstorm watch indicates the possibility of severe thunderstorms forming. A watch does not mean these things are already happening, it means they could happen.
Like a forecast, a watch covers a period of many hours and usually covers a large area of at least several counties, if not several states. We should watch for possible dangerous weather in the near future.
A warning is an indication of immediate danger
It’s a call to take shelter now, because the tornado or severe thunderstorm is already happening. Depending on where you are, you might only have couple of minutes to protect yourself and your family. Or, you might have 10 or 15 minutes, if you’re at the far edge of the “warning polygon.” If you want to survive a tornado or a severe thunderstorm, don’t waste time seeking more information. When a warning comes out, take shelter immediately.
Because dangerous weather has already formed and is on its way, a warning usually covers a period of less than an hour and a small area that’s sometimes smaller than a county. The time to watch is over. It’s now time to heed the warning and take shelter.
Want even more lead time?
If you’re really interested in weather, or want to know even earlier whether severe weather is possible, there are two other NWS products to check out.
The NWS Storm Prediction Center (SPC) usually issues a “mesoscale discussion” before it issues a watch. This product lets you know that the SPC is thinking about (or planning to) issue a watch, the geographical area of concern and the reasons. Some of the text of a mesoscale discussion can get pretty technical, but anyone can figure out from this product whether a watch is likely to be issued. If any mesoscale discussions are in effect, you can find them on the SPC website.
For even more lead time, the SPC issues “convective outlooks” that indicate the amount of risk of severe weather as much as eight days in advance. You can also find these products on the SPC website.