Here we present a succinct comparison of the existing, and proposed new, risk classification system for severe weather from the National Weather Service’s Storm Prediction Center (SPC). These were introduced at the 2014 Weather and Climate Summit being held at Breckenridge, Colorado this week. We also discuss the pros and cons of the changes.

What is SPC?

The Storm Prediction Center is one of several weather and climate specialty centers run by NOAA’s National Weather Service. Located in Norman, Oklahoma, SPC’s forecast experts monitor severe weather across the United States. Among other responsibilities, including issuing severe thunderstorm watches, it prepares convective outlooks, which show specific geographic regions having a potential for severe thunderstorms up to three days in advance (and in more general terms, out to 8 days).

What are the criteria used in current convective outlooks?

The current convective outlooks describe risk in four categories or three risk levels: General Thunderstorms (green), Slight Risk (yellow), Moderate Risk (red) and High Risk (magenta).

An example of the how these categories are used in the context of geographical coverage and a specific weather event is shown below.



SPC Day 1 Convective Outlook issued at 8 a.m., November 13, 2013. Note the use of rare “High Risk” category (magenta.)

Changes In 2014

SPC is proposing to shift from three risk levels (four overall categories) to five (six overall categories) for Day 1-3 Outlooks (except no high risk on Day 3).

In short, the risk spectrum will evolve from slight to moderate to high, to marginal to slight to enhanced to moderate to high.

As shown in the tables below, the changes will take place at the level of General Thunderstorm and Slight Risk.

General Thunderstorms will be split into two new categories: General Thunderstorms (blue) and Marginal Risk (green). Slight Risk will be divided into Slight Risk (yellow) and Enhanced Risk (orange). The Moderate and High Risk categories will remain unchanged.

SPC is doing this to better delineate low-end severe weather threats, in response to numerous requests for more specific guidance. In the existing system, areas highlighted “green” can convey either garden-variety storms, or strong thunderstorms (albeit sub-severe i.e. wind gusts below 58 mph, hail less than ¾” diameter).

Strong storms just below severe criteria are still capable of minor damage e.g. small limbs broken, crops damaged from wind-blown, small hail. But to divine this information currently, one has to jump to the text portion below the graphic (the “See Text” sometimes embedded in SPC’s risk graphics). There, you must wade through meteorological jargon to elucidate the likelihood of strong thunderstorms in your region. So why not assign a new risk category instead, called “Marginal Risk”, with its own color discriminator? Hence the rationale for this added “marginal” category.

The second new category, “Enhanced Risk” gets a new color (orange) and is designed to reduce some of the uncertainty range (15-30%) associated with the existing Slight Risk category.

Will the new scheme do a better job?

In essence, we have more colors that convey greater risk specificity, across the spectrum of severe weather threats. One can argue, however, that this will only confuse users with the introduction of additional terminology (“Marginal” and “Enhanced”). Sometimes, simpler is better, with fewer categories to interpret. Without looking at the table of probabilities, are you able to divine any difference between “Marginal” and “Slight” when it comes to understanding the risk? What does the term “Enhanced” in and of itself actually convey, in terms of relative risk?

These changes may be an easier “sell” to residents of the Midwest and Mid-South, regions that are frequented by severe weather. There, citizens are more familiar with nomenclature that is used on a weekly basis for several months of the year.

The National Weather Service now recognizes the need to better convey the threat of high impact weather, based more on the science of risk perception than the nuts and bolts of the actual meteorology. Is the rollout of the new SPC Convective Outlook a step in the right direction?

What do you think?

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Some additional notes (referring to the table above)….

Day 1 Outlooks (updated five times daily) provide specific guidance based on type of threat: Tornado, Damaging Wind and Hail. (Note that the tornado threat carries a reduced probability threshold compared to wind and hail, in each risk category, see table below).

For Day 3 and Day 2 Outlooks, probabilities for specific storm impacts are not provided.

The High Risk category is only used in Day 2 and Day 1 outlooks, and is issued sparingly (some years have had no High Risk days). High Risk Outlooks are reserved for the most extreme events, including tornado outbreaks (a large number of tornadoes and/or high likelihood of strong-violent tornadoes) and significant derechos. A High Risk may be combined with a special informational bulletin, called a Particularly Dangerous Situation (PDS), when widespread threat to life is anticipated.