Playing with Data

Personal Views Expressed in Data

SPC Day 2 High Risks

Today, the Storm Prediction Center has issued a Day 2 High Risk for only the second time in it’s history. When was the previous one? 06 April 2006 for the vicinity affected by the 27 April 2011 outbreak. For those interested, the SPC DID NOT issue a Day 2 High Risk for the 27 April 2011 outbreak.

Below is a figure that plots the SPC forecast probabilities for the Day 2 High Risk. (The area contained in the 60% area is what was defined to be the High Risk.) In addition to plotting the forecast probabilities, the verifying storm reports from the National Weather Service publication Storm Data are overlaid. You may notice in the report legend (lower right), the total number of reports of that type and the total number of significant reports of the given type as included. In case you do not know, a significant report is:

  • Wind Gust great than or equal to 64 kts
  • Hail greater than or equal to 2 inches in diameter
  • Tornadoes greater than or equal to F/EF2 strength

So, since the SPC did not issue a Day 2 High Risk for 27 April 2011, does this mean that the SPC is expecting an even bigger severe weather event across the central US tomorrow? Not necessarily. On 26 April 2011, most meteorologists knew a significant tornado outbreak was going to occur the following day. In fact, most everyone I know expected the SPC Day 1 outlook to have a High Risk on 27 April 2011. The reason the 26 April 2011 SPC Day 2 outlook did not include a High Risk is because there were lingering uncertainties as to where the greatest concentration of severe weather would occur. So, even though today’s Day 2 outlook contains a High Risk, it does not necessarily mean that SPC is expecting an even bigger outbreak than 27 April 2011 tomorrow. It just means that SPC has a higher confidence on the region to be affected.