What the Storm Prediction Center's Convective Outlook Means

April 9,2017
You may have heard the terms "convective outlook" or "moderate risk" on social media regarding forecasts of severe thunderstorms, but do you know what they mean?
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The Storm Prediction Center, 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 the 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.
Severe weather is defined as a thunderstorm that produces one of the following: measured wind gusts to at least 58 mph, wind damage (trees, structures, power lines), hail at least one inch in diameter (the size of a quarter), and/or a tornado.
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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 convective outlook scale:

Marginal Risk

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

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.
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Enhanced Risk

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

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.
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High Risk

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 Storm Prediction Center, "This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe."
Since 2010, only 21 days – less than one percent of days – have been outlooked as high risk days.
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 50 percent or higher.

Sample convective outlook from April 2, 2017 with preliminary storm reports.
(Storm Prediction Center)

For more on the Storm Prediction Center's convective outlooks, see below:

MORE: Severe Weather, Early April 2017

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