Robert's Post....

A few passing showers this morning with temperatures eventually reaching the upper 70s this afternoon. I will say there is a chance for storms this afternoon but I would hold off any really significant chance until Saturday and into Sunday.

With such a prolific storm system making its way through the Plains and into the Southeast I figured I would blog about the recent history of SPC convective forecasting.

Including yesterday we are set to have at least 3 moderate risk days in a row for the US. In the last 3 years we have only had 3 events where we saw 3 moderate risk days in a row, with none in 2009. It is not out of the question for Sunday to see a moderate risk as well, east of the I 85 corridor. This would make for four days of Moderate risk outlooks for the same storm system. From the research I have done I have been unable to find an event where this occurred.

The Convective outlook is issued 5 times a day by the Storm Prediction Center for up to 3 days and has three main criteria of Slight, Moderate, and High risk outlook. SPC also issues a thunderstorm chance line and regional text box which are both below slight risk, so they are not consider risk outlooks. These convective outlooks are based on the probabilities of the occurrence of three factors: tornadoes, hail, and wind. This basically means if there is a 30% chance or higher for tornadoes on day 1 then a moderate risk will be issued. Wind and hail probabilities must be higher for a moderate risk to be issued since they are consider less harmful to human life. For day 2 and 3 probabilities must be significantly higher than day 1 to issue a moderate risk. That is why you don't regularly see Moderate risk days on day 2 or even if ever on day 3.

High Risk days are even rarer; in the last 25 years the US averages about 3 high risk days a year. There has only been 3 consecutive high risk days once and only 1 high risk ever issued on day 2.

For this weekend I expect a high risk issued later on for today and then one for tomorrow as well. Should be interesting to see if a moderate risk is issued for NC on Sunday. As of right now this system is strikingly similar to April 8 1998 southeast outbreak. So we definitely need to keep an eye on this system.

Robert Elvington
robert.elvington@gmail.com