Severe storm risks: colors and categories

Weather Blog

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.

Categories, labels and colors of severe weather risks

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 ONE Probability (%) to Category (Label/Color) Conversion

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.

DAY 2 Probability (%) to Category (Label/Color) Conversion
DAY 3 Probability (%) to Category (Label/Color) Conversion

“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.

EXAMPLE: black hatch marks indicate ‘significant’ risk of EF2 tornadoes, +75mph gusts and/or hail +2″ in diameter

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

EXAMPLE: Day 4-8 severe weather outlook

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.

Copyright 2020 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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