On October 22 the NWS Storm Prediction Center (SPC) will revise its severe weather outlooks by including two new risk categories.
“There are no changes in how we forecast severe weather,” says Greg Carbin, the warning coordination meteorologist at the SPC in Norman, Okla. “These are not replacements they are enhancements. We’ve been doing probabilities for over a decade.”
“Severe weather” refers to thunderstorms that produce winds of 58 mph or faster, hail stones one inch in diameter or more, or a tornado. A tornado that doesn’t touch the ground, which is technically a “funnel cloud,” makes a thunderstorm severe.
Forecasters are unable to pinpoint exactly where and when a thunderstorm will form, much less whether it will become severe until it produces damaging winds, large hail, or a tornado.
In recent years, however, SPC forecasters have become quite skilled at predicting — at least three days ahead of time — which parts of the country tornadoes and other severe thunderstorms are most likely to hit.
The outlooks that are being updated reflect the SPC’s growing skill at predicting which areas severe storms are likely to threaten.
The current maps of severe weather threats show areas facing “general thunderstorms” with the very lowest risk, and areas with sight, moderate, and high risks of severe thunderstorms.
The new outlooks will include two new risk categories: “Marginal” which replaces the current “SEE TEXT” in areas marked for “general thunderstorms) and “Enhanced,” which is an additional category to delineate areas of risk in the high end of the current SLIGHT risk, but below MODERATE risk.
The risk categories are complicated, as Carbin explains:
The Moderate Risk category is reached at Day 3 and Day 2 with the forecast of 45% probability of hail, wind, tornado (combined) and a probability that some of those events will be considered “significant.” Significant, meaning, EF2+ tornadoes and/or 2 inch or greater hail, and/or thunderstorm winds in excess of hurricane force (faster than 75 mph).
“A High Risk cannot be issued at Day 3 but to get a High Risk at Day 2, the probability has to be 60% (for all severe types) along with the same conditions mentioned above for significant severe weather.”
Examples of these Outlooks using historical data and additional information are online at: www.spc.noaa.gov/exper/dy1-3example/.
The October 22 changes expand the risk categories from four to five. They also clarify the risk previously labeled as “See Text,” which will be replaced by a line outlining the category and the term “Marginal” to denote areas with a 5 percent probability of severe weather.
The upper end of the “Slight Risk” category is being renamed “Enhanced,” which is short for “Enhanced Slight.” This category shows an area with at least a 30 percent probability of severe wind or hail or a 10 percent chance of a tornado during the Day 1 period.
For Days 2 and 3, the “Enhanced” risk category will denote a 30 percent total severe probability. The Moderate and High-risk thresholds will remain essentially unchanged.
Carbin thinks the enhanced category will show up more on the East Coast because this region rarely sees the higher-end probabilities. “Any day with above zero probabilities is a big day in the Mid-Atlantic,” he says.
He says that in all cases “the risk is high if you’re in the wrong place, but the likelihood you’ll be in that small area will be low. It’s important to keep abreast of the latest weather when there is any chance the a watch might be issued.”
This is because you might not realize you are in the wrong place until it’s too late to escape, by taking shelter when alerted by sirens, a cell phone alarm, the approach of a tornado, or a cloud or heavy rain that could hide a tornado.
Carbin says, “outlooks can help you do that in the Day 1 time frame. As for the more extended range outlooks, I would say that those provide an opportunity for individuals, families, and other interested parties from school districts to government agencies to review their hazardous weather response plans and be ready in the event severe weather activity increases in the days ahead.”
Professionals, such as emergency managers, are comfortable working with outlook probabilities.
For an individual, if you learn in the morning that your area faces a risk of severe thunderstorms or tornadoes that day you should ensure you stay close enough to a safe shelter to quickly get there if your cell phone buzzes and vibrates with a Wireless Emergency Alert of a tornado or severe thunderstorm warning.
The kind of planning you’re most likely to do based on an outlook for two or three days ahead is to consider changing your schedule. For example, what would you do if you were leading a youth group on a hike in Shenandoah National Park when your cell phone buzzed and vibrated with a tornado warning? (This assumes you’re close enough to a cell tower to receive the warning).
While the odds are tiny that a tornado will hit the particular place you and your group are at, why take the chance when an SPC alert could have prompted you to change your plans for the hike a day or two ahead.
The new outlooks will also include “close-up” maps designed for the news and social media.