Sunday, August 17, 2014

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.

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

1 comment:

Anonymous said...

I think having so many qualitative labels just confuses the average person. In fact, the use of any such labels is inherently less informative than just using numbers. Since the NWS has percentages assigned why not just use them - as we do for the probability of precipitation? I think people can much more easily make sense of a "15% chance of tornados" instead of trying to determine which is worse: "enhanced" or "moderate".