Just after sunrise on April 27th, the SPC elevated the thunderstorm forecast for southeastern Missouri to a moderate risk. A deep upper level trough was generating a strong area of low pressure in the Plains that was feasting on the chaos of colliding air masses. Cold and Canadian with tropical and maritime. The helicity in the atmosphere meant tornadoes were likely.

There were indeed several tornadoes from Missouri to Indiana, with a great deal of damage wrought by straight line winds. The tornadoes were embedded within a line, and at some point, it becomes a bit academic as to whether damage is caused by winds or tornadoes.

The changes to the SPC and the deeper set of information within the convective outlook are new features to the thunderstorm season. I wanted to take this specific example to look at what the SPC released, and see what factors best correlate to the outlook that is provided. First, here is the outlook, issued in the early hours on Monday.

As a bit of background, the change that was recently made to the storm hazard maps added a storm intensity rubric to go along with the probability guidelines that had been a part of those maps before. Take a look at the wind and hail maps to get the idea.

So, you can see there was a high probability of strong wind, and a lower probability of hail, but the hail in the forecast was expected to be larger. We can also see that neither the high probability of severe wind, or the expectation of large hail defined the moderate outlook boundaries. That’s not particularly surprising. That the significant hail in central Texas didn’t lead to a higher outlook than a marginal risk in central Texas surprised me, though.

Moderate and high risk events generally need a threat of tornadoes, so let’s look at the tornado outlook issued at the same time as the full outlook.

The threat for strong tornadoes is only designated on the charts as 1, 2 or 3, and the probability of a tornado within 25 miles of a spot is more well defined. Perhaps it is no surprise, then, that the moderate hugs the 15% chance of a tornado risk. I’m sure it didn’t hurt the case that the forecast was a level 2 on the intensity scale, either.

For what it’s worth, here is the break down of severe reports.