New severe weather outlook categories roll out

Beginning up to eight days ahead of potential severe weather, the Storm Prediction Center (SPC), a part of NOAA/NWS and the worldwide leaders in severe weather prediction, begins issuing convective outlooks (a.k.a. "risk areas") for the possibility of severe storms.  For many years, these outlooks have included a "general thunderstorm" area, as well as Slight, Moderate, and High Risk areas.

Background

In recent years, especially with the widespread use of social media and availability of information around the clock and at our fingertips, the general public has become familiar with these outlooks and the terms used.  There has also been a call to update the outlooks to provide better delineation of the risk that the public faces from severe storms. In particular, "Slight Risk days" seem to cover a wide gamut of severe weather possibilities, from a small threat for a damaging wind gust or large hail to something just short of a tornado outbreak.

Responding to the call to better define the risk, take into consideration the research and advice of social scientists who specialize in communicating risk or threat to the general public, and to provide better consistency with other NWS products, SPC is using a new classification system for their severe weather outlooks.  The new system can be found in the tables below (pay particular attention to the first table).  The probability of tornadoes, 1"+ hail, and/or 58+ mph wind in a particular area defines the risk outlook category issued by SPC. These probability-to-outlook category conversion tables are shown below the new outlook classification system.  NOTE: There will be NO changes to the watches or warnings issued as a result of this modification.

SPC outlook categories prior to October 22, 2014 ("Old") and since October 22 ("New"), as well as the numeric scale that will accompany the categories in SPC outlooks and MWN postings.

Probability matrix SPC uses for determining outlook category on the Day 1 outlooks. For instance, a 10% risk of a tornado or 30% risk of severe wind or hail warrants an Enhanced risk. "Significant Severe" means EF-2 tornadoes, 74 mph wind, or 2 inch hail.

Probability matrix SPC uses for determining outlook category on the Day 2 outlooks

Probability matrix SPC uses for determining outlook category on the Day 3 outlooks. High Risks are not issued on Day 3.

Details

The biggest change users will see will be the addition of two new risk areas - "Marginal" and "Enhanced."  The Marginal Risk indicates that the chance of severe weather is very low but not non-existent (or marginal). The Marginal Risk would replace the current "See Text" areas in the outlooks. According to first table above, areas under a marginal risk of severe weather have a low chance of severe storms - less than the slight risk of previous years.  Enhanced Risk indicates a more significant chance of severe storms and it will be used for "high-end" slight risk areas.  In other words, the current slight risk category will be split between "slight" and "enhanced."  Enhanced risk indicates a higher chance of severe weather than slight risk, but not quite up to a moderate risk.  There will be no changes to the moderate or high risk areas.

In addition, each Severe Weather Outlook text bulletin that accompanies the maps will contain a "public discussion" section that describes the weather risks for that day in non-meteorological jargon, so that the general public can understand the threat. An example of a day which had a high risk of severe weather (May 24, 2011) is shown below, followed by what the outlook areas would look like under the new classification system (click each for a larger image).

Example showing the classification system for severe weather outlooks as used on May 24, 2011

How the convective outlook would look for May 24, 2011 using the new classification system

How MWN will handle the change

We understand that this change seems to make things more complicated. We will employ a supplemental numerical scale (1 to 5), in addition to the new categories, to help better define the risk. These numerical categories are similar to the Saffir-Simpson Hurricane Wind Scale and the Enhanced Fujita Scale for tornadoes with a higher number indicating a greater risk of severe storms. (We have learned since deciding to use the numerical scale that SPC actually intends to do the same thing. They must've thought we were on to something there! :-)  The scales will be identical.)

More important than the number or name of an outlooked area are the threats posed. As a Weather-Ready Nation Ambassador that promotes information and education, we'll be sure that potential impacts are the key ideas in blog and social media posts, just as we have always done!  If you have any comments or questions, feel free to send them to us via our social media feeds or as a comment on this blog.

Erik Proseus
MWN Meteorologist

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Storm Prediction Center revises severe weather outlook categories

Beginning up to eight days ahead of potential severe weather, the Storm Prediction Center (SPC), a part of NOAA/NWS and the worldwide leaders in severe weather prediction, begins issuing outlooks (sometimes called risk areas) for the probability of severe storms.  For many years, these outlooks have included a "general thunderstorm" area, as well as Slight, Moderate, and High Risk areas.

In recent years, especially with the widespread use of social media and availability of information around the clock and at our fingertips, many more people have become familiar with these outlooks and the terms used.  There has also been a call to update the outlooks to provide better delineation of the risk that the public faces from severe storms.  In particular, "Slight Risk days" seem to cover a wide gamut of severe weather possibilities, from a small threat for a damaging wind gust or large hail to something just short of a tornado outbreak.

Responding to the call to better define the risk, take into consideration the research and advice of social scientists who specialize in communicating risk or threat to the general public, and to provide better consistency with other NWS products, SPC will use a new classification system for their severe weather outlooks starting October 22, 2014.  The new system can be found in the tables below (pay particular attention to the first table).  The probability of tornadoes, 1"+ hail, and/or 58+ mph wind in a particular area defines the risk outlook category issued by SPC.  NOTE: There will be NO changes to the watches or warnings issued as a result of this modification.  

Top: New classification system to be used by the NWS for severe weather outlooks beginning October 22, compared to the current scheme. Bottom: probability of each type of severe weather that corresponds to each risk category. For instance, an area on day 1 that has a 15% probability of a tornado will be in an Enhanced risk. "Sig" (significant severe) indicates that there is a 10% or higher probability of  EF-2 or stronger tornadoes, 74+ mph wind, or 2"+ hail.

The biggest change we will see will be the addition of two new risk areas - "Marginal" and "Enhanced."  The Marginal Risk indicates that the chance of severe weather is very low but not non-existent (or marginal). The Marginal Risk would replace the current "See Text" areas in the outlooks. According to table above, areas under a marginal risk of severe weather have a low chance of severe storms - less than the slight risk of previous years.  Enhanced Risk indicates a more significant chance of severe storms and it will be used for "high-end" slight risk areas.  In other words, the current slight risk category will be split between "slight" and "enhanced."  Enhanced risk indicates a higher chance of severe weather than slight risk, but not quite up to a moderate risk.  There will be no changes to the moderate or high risk areas.

In addition, each Severe Weather Outlook text bulletin that accompanies the maps will contain a "public" section that describes the weather risks for that day in non-meteorological jargon, so that the general public can understand the threat. An example of a day which had a high risk of severe weather (May 24, 2011) is shown below, followed by what the outlook areas would look like under the new classification system (click each for a larger image).

Example showing the classification system for severe weather outlooks as used on May 24, 2011

How the convective outlook would look for May 24, 2011 using the new classification system

We understand that this change seems to make things more complicated. While the changes will be fairly well understood by meteorologists and NWS partners, education will be necessary for those who do not look at these categories on a daily basis. We at MWN are considering using a supplemental scale, in addition to the new categories, to help better define the risk - perhaps a numerical scale (1 to 5) similar to the Saffir-Simpson Hurricane Wind Scale and the Enhanced Fujita Scale for tornadoes.

We'd love your feedback! Let us know how we can make the transition easier for you, as ultimately the decision to prepare and take action in the face of severe weather lies in our ability to communicate the threat and your willingness to react.

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SPC Outlooks: What does Slight, Moderate, High Risk mean?

Many times when severe weather is expected, meteorologists will refer to an area having a certain "risk" of severe weather.  When severe weather outbreaks are possible, the risk may be Slight, Moderate, or rarely High. So what do these terms mean?  

As an overview, the Storm Prediction Center (SPC), which is the NOAA/NWS organization responsible for severe weather forecasting in the U.S., issues convective outlooks covering the following 8 days.  On days 1-3, maps are drawn that may contain one of 4 colored areas: General Thunderstorm, Slight, Moderate, and High Risk.  General thunderstorm areas indicate the indicated zone will have a likelihood of thunderstorms which, for the most part, will be sub-severe.  (Remember a thunderstorm must produce 1" hail, 58 mph wind, or tornadoes to be considered severe.)  Slight, Moderate, and High risk areas represent progressively larger threat for organized severe storm episodes.

SLIGHT RISK:  Issued frequently during the peak severe weather season and implies that well-organized severe thunderstorms are expected but in relatively small numbers/coverage, or there is a small chance of a more significant severe event. Typically, Slight Risk areas produce scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes.  Significant severe events (75 mph wind, EF-2 tornadoes, or 2" hail) typically do not occur on Slight Risk days.

MODERATE RISK:  Issued much less frequently than Slight Risk areas (a couple of times a month during a typical severe weather season) and is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak.  A Moderate Risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a Slight Risk. Typical Moderate Risk days include multiple tornadic supercells with very large hail, intense squall lines with widespread damaging winds, or in some cases, land-falling tropical storm systems.

HIGH RISK: A High Risk area implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe storms (i.e., violent tornadoes or very damaging convective wind events).  On most High Risk days, a major tornado outbreak is expected.  The High Risk category is reserved for the most extreme events with the least forecast uncertainty, and is generally only used a few times each year.  In fact, 51 High Risk days have occurred in the past 12 years (an average of about 4 a year).

The forecast of Slight, Moderate, and High Risk is based on percentages indicating the probability of severe weather in the forecast area.  The tables below show the conversion from outlook probabilities to risk areas for Day 1 (today) and Days 2-3 (tomorrow, next day).  As an example, if there is a 15% probability of a tornado within 25 miles of a point on Day 1, a Moderate Risk area will be issued.

Graphic courtesy Wikipedia. 60% probability High Risk not used on Day 3.

Hopefully this helps explain the risk terms used frequently by MWN and many other meteorologists!  Learn more on the SPC website.

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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring a fresh new interface and StormWatch+ severe weather alerts!