They might sound straight forward, but watches and warnings can still confuse some people. Some of the misunderstandings are basic, but others do take a bit of explaining. Let’s start from the beginning…
Severe weather watches are designed to be the last line of defense before storms threaten an area. Often they are issued before any storms take shape. It’s the final chance for forecasters to warn an area of the threat of severe weather over the next several hours.
Severe Thunderstorm and Tornado Watches are issued by the Storm Prediction Center in Norman, OK. All other winter, wind, flood or other type of watch is issued by individual National Weather Service offices. For the sake of keeping things simple, we are going to focus on what the SPC does with the watches they are tasked with issuing.
I want to make it clear that this is NOT an easy job. No one knows for sure exactly when and where severe weather will strike. It’s still a very imperfect science and the Storm Prediction Center must make prediction for the entire country. That’s a LOT of ground to cover! They are human… and they make mistakes. But the science behind the forecasting is getting better and today, severe weather prediction is the best it’s ever been.
According to the Storm Prediction Center’s own figures, the average watch box covers 20,000-40,000 square miles and lasts for 6-7 hours. Over the course of a year, the SPC issues around 1,000 watch boxes. Their decision to issue a watch is based on the immediate threat for any given area. Although the Storm Prediction Center draws the "box" which you often see as the area under a watch, it’s up to each individual National Weather Service office to determine which counties fall into that watch. Usually the NWS will follow the box outline specifically, but sometimes, if they feel a certain area should or show not be included in the watch, they can extend or contract it as they see fit.
The National Weather Service is also responsible for eliminating areas they feel are no longer at risk for severe weather. This is a good way to take the "heat" off the Storm Prediction Center since they look at storm threats on a much larger scale than individual counties.
There are two types of watches that the Storm Prediction Center issues, Severe Thunderstorm Watches and Tornado Watches. They are somewhat deceptive. Tornadoes can occur in a Severe Thunderstorm Watch, and strong winds and large hail can also occur within a tornado watch.
So how does the Storm Prediction Center determine which watch should be posted? The simple answer is the tornado risk. If the primary threat from a storm system is strong winds and/or large hail and the tornado risk is somewhat limited, then the SPC will side with a Severe Thunderstorm Watch.
But if there’s a potential for strong or multiple tornadoes to form, the Storm Prediction Center will go with the tornado watch. Of course tornado watches are considered the most serious, but that’s not always the case. Strong winds and large hail can affect a much larger area and produce just as much damage if not more than a tornado or two.
Now while these are the two most common watches, there is a variant of each one that is not often seen. It is known as the Particularly Dangerous Situation Watch, or PDS Watch. A PDS Watch can take the form of either a Severe Thunderstorm or Tornado Watch. These are issued when EXTREME conditions, far beyond the normal threshold for severe weather are expected. This can include widespread wind gusts in excess of 90mph, 3" diameter hail or many violent tornadoes capable of serious damage and loss of life.
When storms develop and time has run out, warnings are the weapon of choice in the early warning system. ALL warnings are issued by your local National Weather Service office. In Iowa there are 5 offices that cover portions of the state. They are located in Des Moines, Sioux Falls, Omaha, Davenport and Lacrosse. These offices work together to coordinate forecasts. When severe weather strikes, they turn to NEXRAD radar and storm spotters to give them the most up to the second information in determining where severe weather will strike.
When a spotter reports severe weather, or radar detects the potential for severe weather in a matter of minutes, the National Weather Service will issue the appropriate warning. A Severe Thunderstorm Warning is issued when a storm is producing or is believed to be capable of producing winds in excess of 58mph or hail 1" in diameter or larger. A Tornado Warning is issued when a storm is producing a tornado or it is believed the cell is capable of producing a twister within minutes.
The advent of modern radar, and a growing network of storm chasers and spotters has made these warnings more accurate than ever before, saving countless lives along the way. Another way the warnings have become more accurate is the adoption of polygon based warnings. Instead of issuing warnings for counties, the National Weather Service now draws a box, similar to the way the SPC issues watches. This means parts of a county might not be included, and allows the public to better understand exactly which areas the storm will impact.
Severe Thunderstorm and Tornado warnings are the final warning issued before a storm strikes. Once things take a turn for the worst, it’s time for a lot of manpower and a little help from technology to get the best information out to the public as fast as possible. More on that in the final part of our series, Severe Weather 101.