Jonathan Belles
Published: April 9,2020
Severe thunderstorm forecasts are issued daily by NOAA's Storm Prediction Center with threat levels ranging from "marginal risk" on the low end to the rarely used "high risk." But what do those terms mean, exactly?
The SPC, a branch of the National Weather Service located in Norman, Oklahoma, issues forecasts for thunderstorms over the next eight days.
The forecasts show areas that have a threat of severe thunderstorms and how high the threat is, by categories based on the probability that a severe weather event will occur within 25 miles of a given location.
Example of a convective outlook from NOAA's Storm Prediction Center. Not a current forecast.
These forecasts are based on current trends in satellite and radar imagery, weather model output, pattern recognition, forecaster expertise and how confident the forecaster is.
According to Dr. Patrick Marsh, warning coordination meteorologist at NOAA's Storm Prediction Center, the "SPC works very hard to internally collaborate the forecast. This allows for the expertise from numerous forecasters to be conveyed in each outlook." Although one or two forecasters write a particular forecast update at the SPC, many great minds enter their thoughts into each forecast.
Severe weather is defined as a thunderstorm that produces one of the following: measured wind gusts to at least 58 mph, storms capable of producing wind damage (trees, structures, power lines), hail at least one inch in diameter (the size of a quarter) and/or a tornado.
These forecast categories do not include the chance for excessive rainfall or flooding. Those outlooks can be found at NOAA's Weather Prediction Center.
Outlooks also do not explicitly forecast for lightning, but the risk is implied if thunderstorms are forecast.
Lightning and flooding are just as deadly as tornadoes, large hail and damaging winds, if not moreso.
Here is a description of the thunderstorm outlook scale.
Marginal Risk - Category 1
Isolated severe thunderstorms are possible.
Expect strong to occasionally severe thunderstorms that are generally short-lived. These storms generally lack organization, but can still drop hail up to quarter-sized, produce gusty winds and frequent, deadly lightning.
This outlook is very common and occurs throughout the year.
Slight Risk - Category 2
Scattered severe thunderstorms possible.
Short-lived, isolated severe storms won't be widespread, but may still produce isolated tornadoes, spotty wind damage and hail in excess of one inch in diameter.
This outlook is common, especially in the warmer months.
Even though this outlook level is called "slight," weather that can occur in this risk area is no less deadly than the weather that occurs in a high risk. Isolated severe storms may produce significant tornadoes, very large hail and damaging winds.
If the SPC believes that the severe weather may occur from a single thunderstorm or has a low confidence in a severe thunderstorm event occurring, then forecast probabilities and thus the category will be lowered, according to Marsh.
Enhanced Risk - Category 3
Numerous severe thunderstorms possible.
More persistent and widespread storms are expected. Thunderstorms may produce a few tornadoes and pockets of wind and hail damage.
Not all storms will be severe, but a few storms could be intense.
Moderate Risk - Category 4
Widespread severe thunderstorms likely.
Most storms that form will reach the severe criteria listed above within this risk area. Several tornadoes and numerous thunderstorms containing large hail and damaging winds are likely.
This risk is uncommon, and is generally used only when supercells are capable of strong tornadoes or long-lived squall lines are expected to produce widespread damaging winds.
These risks are typically issued a few times a year.
High Risk - Category 5
Widespread severe thunderstorms expected.
A severe weather outbreak is expected with multiple tornadoes and/or a destructive long-lived derecho. Damage is expected.
Tornadoes that do occur in and around this area are expected to become strong (EF2+) or violent (EF4+) and/or long-tracked.
According to the SPC, "This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe."
Not everyone in a high-risk area will see severe weather, but the chances that communities in the high-risk area will see severe storms are generally around 30% or higher.
Sample convective outlook from April 2, 2017, with preliminary storm reports.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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