Storm Prediction Center Convective Outlook

I’m sure you read Landon’s post this morning and saw this image:

So what exactly does this graphic tell us? That’s going to be the topic for today’s Weather 101 post. This is an example of the Storm Prediction Center Convective Outlook. It is basically a map depiction of where the Storm Prediction Center, a sector within the National Weather Service, believes severe weather is possible. The map indicates severe risk based on three categories; slight, moderate, and high.

A slight risk day typically will mean the threat exists for scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes. During the peak severe weather season, most days will have a slight risk somewhere in the US. Isolated significant severe events are possible in some circumstances, but are generally not widespread.

A moderate risk day indicates that more widespread and/or more dangerous severe weather is possible (sometimes with major hurricanes), with significant severe weather often more likely. Numerous tornadoes (including some strong tornadoes), more widespread or severe wind damage and very large/destructive hail could occur.

Major events, such as large tornado outbreaks, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not uncommon and typically occur several times a year, especially during peak season. A slight risk area typically surrounds a moderate risk area, where the threat is lower.

A high risk day indicates a considerable likelihood of a major tornado outbreak or (much less often) an extreme wind event. On these days, the potential exists for extremely severe and life-threatening weather, including widespread strong or violent tornadoes and/or very destructive straight-line winds (Hail cannot verify or produce a high risk on its own, although such a day usually involves a threat for widespread very large hail as well).

Many of the most prolific severe weather days were high risk days. Such days are quite rare; a high risk is typically issued only a few times each year. High risk areas are usually surrounded by a larger moderate risk area, where uncertainty is greater or the threat is somewhat lower.

So those are the three categories of risk, but the map also displays risks as a percentage. For example, in Landon’s picture there is a 30% risk of severe weather within 25 miles of a given point in places like Paducah, Memphis, and Little Rock. There is a 15% risk of severe weather within 25 miles of a given point in Bowling Green for Thursday.

As you get closer to the severe weather event, outlooks will become more detailed. When it is the day of the severe weather event, the Storm Prediction Center will begin issuing individual percentage risk for different threats. There is a percentage risk for large hail, damaging winds, and tornadoes.

For example, here is the Storm Prediction Center (SPC)  tornado probabilities for November 17, 2013. The SPC issued a 30% risk for a tornado occurring within 25 miles of places such as Indiana and central Illinois. Meanwhile, a 15% risk area was issued for Bowling Green. The hatched region indicates as least a 10% risk of seeing a tornado of EF-2 or greater strength within 25 miles of a given point.

As we head more into spring, you will likely begin seeing more and more of these images floating around the web. Thus, it is good to know what they mean for your area. Be sure to follow @WxOrNotBG and @WarrenCountyWX for the latest severe weather information for Bowling Green.