We've been lucky enough to see a lull in severe thunderstorms over the past couple of months, sparing countless towns from damage or destruction. Unfortunately, all good things have to come to an end. Here's a primer on how to use severe weather forecasts to keep you and your loved ones safe this spring.

Severe Weather

The term "severe weather" causes a bit of confusion for people. Severe weather is often synonymous with extreme weather—including thunderstorms, floods, blizzards, and droughts—but "severe weather" in the United States is almost exclusively used to talk about severe thunderstorms.

A severe thunderstorm is one that produces wind damage, recorded wind gusts of 58 MPH or stronger, and hail the size of quarters (1.00" in diameter) or larger.

Watch vs. Warning vs. Emergency

The difference between a watch, warning, and emergency tends to trip people up. Knowing the difference between a watch and a warning is a matter of life and death.

Watch

A tornado or severe thunderstorm watch is issued when weather conditions are favorable for the development of dangerous thunderstorms that could produce large hail, damaging winds, or a tornado. A watch doesn't necessarily mean that severe weather is occurring right now, but it means that you should stay alert for rapidly changing conditions.

Every once and a while, forecasters will issue a "Particularly Dangerous Situation (PDS)" tornado or severe thunderstorm watch, which is enhanced wording reserved for the most dangerous severe weather events. A PDS watch is issued when destructive tornadoes or a violent derecho is possible.

The above image shows one of the many tornado watches issued on April 27, 2011.

Warning

A tornado or severe thunderstorm warning is issued when Doppler weather radar (or storm spotters) detect a thunderstorm that is capable of producing large hail, damaging winds, or a tornado. A warning is usually issued between 15 and 30 minutes before severe weather is expected to affect your location. You need to take immediate action during a severe thunderstorm or tornado warning—even though a severe thunderstorm doesn't sound as dire as a tornado, large hail and damaging winds can cause serious damage to even a well-built building.

On radar imagery and other weather maps, severe thunderstorm warnings are typically shown in yellow, while tornado warnings are shown in red. The above radar image shows the various warnings in effect ahead of the supercell complex that produced the devastating EF-5 tornado that destroyed the southern half of Joplin, Missouri in May 2011.

A tornado emergency is wording reserved for the most serious situations where a thunderstorm is producing a large, destructive tornado that's heading towards a populated area. Tornado emergencies have only been issued a few dozen times since the term was coined by the NWS office in Oklahoma City during the Bridge Creek-Moore tornado back in May 1999.

Storm Prediction Center

The Storm Prediction Center (SPC) is an agency of the National Weather Service (NWS) that issues severe weather forecasts up to eight days out. The SPC is responsible for issuing tornado and severe thunderstorm watches, while local NWS offices are responsible for issuing smaller tornado warnings.

The SPC's site is a goldmine for severe weather information, including severe weather forecasts, current tornado and severe thunderstorm watches, localized short-term forecasts (called "mesoscale discussions"), and storm reports.

One of the most important products the SPC issues is their severe weather forecasts, called "convective outlooks."

Categorized Outlook

The categorized severe weather outlook shows the overall risk for severe weather on any given day. The newly-created scale runs from zero to five, with higher numbers corresponding to a higher risk level.

0 | Non-Severe Thunderstorms

1 | Marginal Risk for Severe Thunderstorms

2 | Slight Risk for Severe Thunderstorms

3 | Enhanced Risk for Severe Thunderstorms

4 | Moderate Risk for Severe Thunderstorms

5 | High Risk for Severe Thunderstorms

There is some ambiguity to the terms—for instance, many people I've spoken with say that "enhanced" sounds worse than "moderate"—but the numbers and associated colors are meant to help identify the various levels of risk.

Here's a severe weather forecast showing all six risk levels, using the historic April 27, 2011 tornado outbreak as an example:

The worst severe weather occurred across the high risk area from Mississippi to Tennessee, where hundreds of people lost their lives in the violent tornadoes that formed that afternoon. The severe weather outbreak wasn't limited to Mississippi and Alabama, as the map shows, and there was an enhanced risk for severe weather as far north as Buffalo, New York.

When possible, I will always create maps specifically for The Vane using SPC data, so the maps you see on this blog will always appear in the same format as you see above. In order to create consistency, whether it's my maps, the SPC's maps, or a news outlet's maps, the color scheme is always the same, following the SPC's green/yellow/orange/red/violet pattern.

Probabilistic Outlook

The SPC uses the probability of severe weather to arrive at their categorized outlooks. A higher probability of a certain type of severe weather warrants a higher risk level. These probability don't exactly work like the chance of rain, however. A 30% chance of tornadoes is much more dire than a 30% chance of rain.

Tornadoes

In the SPC's forecasts, the risk for tornadoes runs on a percentage scale from 2% to 60%. A 2% risk for tornadoes warrants concern, while anything above that is enough to trigger a slight risk for severe weather. Tornado probabilities generally don't tick above 10% unless it's a major outbreak like we would see in the Plains or southeastern U.S.

The above map shows the tornado risk during the tornado outbreak of March 2, 2012.

A 10% risk for tornadoes means that there's a 10% chance you'll see a tornado within 25 miles of any point within the forecast area. For example, if St. Louis is under a 5% risk for tornadoes on Wednesday, it means that there's a 5% chance for tornadoes within 25 miles of St. Louis.

These probabilities also relate to climatological averages. Say that a tornado occurred within 25 miles of St. Louis 0.10% of the time on March 25 between 1950 and 2013. This 5% chance of tornadoes means that the risk for tornadoes on Wednesday in St. Louis is 50 times higher than climatological normal.

Wind/Hail

The risk for wind and hail works similarly to tornadoes, with the percentage meaning that there's an x% chance of damaging winds/large hail within 25 miles of any point in the forecast area, as well as relating to how much higher the risk is on any given day compared to normal.

The percentage scale for damaging winds and hail is the same, running from 5% to 60%, with a 15% risk warranting a slight risk for severe weather. Typically, 45% and 60% are reserved for the most intense severe weather outbreaks.

The above map shows the risk for damaging winds on the afternoon of October 26, 2010, the day a major derecho tore through the middle of the United States, causing extensive damage from the Gulf of Mexico to the Great Lakes.

NOAA Weather Radio

Every home, business, and school should have a NOAA weather radio on hand to keep track of severe weather in the area. Almost all specially-designed receivers are equipped with a technology known as Specific Area Message Encoding, or SAME.

The annoying tone you hear at the beginning of the Emergency Alert System includes a code that tells receivers what type of alert is being sent out, and for which counties the alert is in effect. When you program a weather radio with your county's six-digit SAME code, your device can automatically sound a loud alarm when a severe weather alert (like a tornado warning) is issued. When set up properly, weather radios are highly effective and work like smoke detectors for the weather. Everyone should have one, because other alerting systems are extremely unreliable.

Weather Radar

It's important to keep tabs on the location of storms so you can have a little more heads up than when a warning is issued. There are countless sites, programs, and apps that offer weather radar, all of which have their strengths and their weaknesses.

If you're willing to shell out a couple of bucks, Gibson Ridge products are the best radar software you can buy. For those of you who are on the run or just prefer your phone, RadarScope has an excellent app for Android and Apple products (iPhone, iPad, iPod, and Macs). If you're frugal or just aren't into the weather enough to buy access to advanced radar, sites like Weather Underground have detailed weather radar imagery that allows you to track severe storms anywhere in the United States, Canada, and Australia.

Keep an Eye on the Forecasts

You can follow the latest watches, warnings, and forecasts by keeping up with the Storm Prediction Center and the National Weather Service. Local television news stations are always a great resource during severe weather outbreaks, as they're required to cut into scheduled programming when dangerous weather threatens their viewing area.

And, as always, I post weather coverage and analysis at least once per day here on The Vane—more frequently when the weather is active—so be sure to check in daily for in-depth weather geekery.

[top image: tlindenbaum on Flickr | tornado watch graphic: SPC | radar: Gibson Ridge | all other maps by the author]

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