Changes to Storm Prediction Center (SPC) Day 1, 2, and 3 Convective Outlook Probabilities

Data from DaculaWeather.com

Overview

Additional intensity information will be added to SPC convective outlooks for Day 1, 2, and 3 on March 3 starting with the 1630Z Day 1 Convective Outlook. The outlook categories will not change and should happen in a similar frequency to the past. Intensity will now be conditional, meaning if a tornado/wind gust/hail report occurs, the expected strength/intensity/size will be represented. Conditional intensity is represented by 3 intensity levels for tornado and wind and 2 levels for hail on the individual hazard graphics (image on the right). The categorical outlook graphics will have a visual update, including text moving labels from the image to a legend, but will otherwise represent the same expected impacts from each category as before (see example categorical outlook graphic).

Conditional Intensity Groups (CIG)

The SPC will represent these intensity forecasts as "Conditional Intensity Groups" (CIG) or more simply, "Intensity Levels". These three intensity distributions (CIG1, CIG2, and CIG3) will be utilized to communicate increasing intensity of significant severe hazards, should severe hazards occur. These new Conditional Intensity Groups will be visible on the SPC Severe Weather Outlook Day 1, 2, and 3 graphics and disseminated through the SPC Outlook PTS product, replacing the current operational 'SIGN' label.
Visual Representation of these Intensity Groups will be as follows:

These Conditional Intensity Groups were derived using 20 years of SPC mesoanalysis data extracted from the time and location of observed tornadoes during that period. The target tornado distributions for tornado intensity can be seen in the chart below for each Conditional Intensity Group (CIG). Note that EF2+ tornadoes increase from 7% of the sample when no area is drawn to 20% at CIG1, 30% at CIG2, and ~40% at CIG3. Similarly, EF3+ tornadoes increase from ~1% with no areas to 6% at CIG1, 12% at CIG2, and 19% at CIG3.

New Probability to Category Conversion Tables

The probabilities and conditional intensity will be combined in the following way to create the SPC Categorical Outlook. These categorical outlook categories will not change and should happen in a similar frequency to the past. While additional intensity information has been added, the new Probability to Category conversion has been developed in a way to match historical SPC outlook creation practices.

The changes are best explained in this very short video.

This is a 40 minute video explaining the changes in greater detail