With 2020’s first severe weather event set to unfold on Friday, it’s a good time to review the language and colors used to identify the risks.
THE BASICS
The Storm Prediction Center in Norman, Oklahoma issues outlooks for Day 1 (“today”), 2 (“tomorrow”), 3 (“next day”) and 4-8 (“extended forecast”) based on the probability of severe weather. These outlooks outline areas of organized thunderstorm activity – both non-severe and severe – across the contiguous U.S.
NUMBER / CATEGORY / CODE
Areas with potential to see severe weather on a given day are given a number, category and code to indicate the level of risk. The risk takes into consideration both forecast intensity and coverage of thunderstorm activity.
DAY ONE OUTLOOKS
An outlook issued for day 1 (or the current day) breaks down the individual probabilities of three primary threats: tornadoes, wind and hail. The probability (left column) represents the chance of severe weather occurring withing 25 miles of any point. Day one outlooks are issued at 12AM, 7AM, 10:30AM, 2PM and 7PM daily.
DAY 2 and DAY 3 OUTLOOKS
The outlooks for day 2 and 3 (next day and the day after from date of issue) combine the probability of all three types of severe weather (tornadoes, wind and hail). Day 2 outlooks are issued at 1AM and 11:30AM daily. Day 3 outlooks are issued at 2:30AM daily.
“SIGNIFICANT” RISK
Particular events can have a “significant” risk meaning a 10% or higher probability of one or all of the following threats:
— EF2 tornadoes or stronger
— 75mph wind gusts or stronger
— Hail 2″ in diameter or greater
This can be indicated by black hatch marks over the colored probability.
DAY 4-8 OUTLOOKS
The day 4-8 outlooks (four to eight days from date of issue) indicate the probability of severe weather – combining all threats (tornadoes, wind and hail) – both as individual days and other the 4-day period as a whole. The day 4-8 outlooks are issued at 4:00AM daily. Two probability thresholds are used — 15% and 30% — and are equivalent to a slight risk (yellow) and enhanced risk (orange) respectively, on the Day 1-3 outlooks.
If the risk of severe weather is <15%, two phrases will be used:
— “Predictability too low”: severe storms still possible but too much uncertainty on when/where and if the risk is still viable
— “Potential too low”: 15% risk of severe weather is highly unlikely on that given day
NOTE: It’s important to note that the severe weather does not follow the lines. It’s certainly possible to see an EF2 tornado in a slight risk… or pea-sized hail in an enhanced risk. The purpose of the outlooks is to highlight areas where the atmosphere has “all the right ingredients” for severe weather to occur on a particular day.
To find current severe weather outlooks, click here.
