An Early 90° day has no correlation with a hot summer

I have gotten this question a lot since we have this mini heat wave this weekend. We hit our 1st 90° day of the year today. Which was a bit early compared to average. Our average 1st date for 90° is around May 20th. So today was about 9 days earlier than average.

The question then gets asked does this heat mean it’s going to be hot a Summer? The simple answer is no because there just isn’t any correlation between an early 90° and a hot summer. The pattern we have this Spring will have little to do with what developments this summer for the most part. The other thing is this Spring has actually been cooler than average so far. So let’s remember that first. People tend to have recency bias when it comes to weather. They think the current or recent weather has been going on longer than it has and they believe it will keep going on like this into the future. The thing is the atmosphere just doesn’t work like that. So let’s look at the numbers.

Hottest Summers on Record in Charlotte:

Here’s a look at the top 15 hottest summers in Charlotte 1878-2017. I also included the 1st 90° date for those years in the right column.

If you notice the 1st 90° days are all over the place but 10/15 of these years, the 1st 90° occurred before June 1st. Though if we know the average date of 90° is May 20th, then only 9/15 had an early 90° day. Seems like a maybe a correlation but………

Coolest Summers on record in Charlotte 1878-2017

Similar to above I took the top 15 coolest summers on record for Charlotte. I also show the 1st 90° of each year on the right.

So even in the years, we had a cool summer the 1st 90° day dates were all over the place. Remember 10/15 of the hottest summers had a 90° before June 1st, but even the coolest summer we still had 8/15 years with a 90° before June 1st.  If we look just at the pre-May 20th date which is on average even in the coolest summers, we hit 90° 6 times out of those 15 years early.

There just doesn’t seem to be any correlation to an early 90° day and a hot summer.

So what does correlate to a hot summer?

The biggest correlation to hot summers was actually drought or below average rainfall. The biggest driver of a hot or cool summer is cloud cover and rainfall. When we have drought we get hot and that heat then dries us out more when then makes it even hotter, Drought begets drought is the saying for this reason. Show me a dry winter, spring and I’ll show you a dry hot summer. So if we want hints on our summer weather look at the rainfall this spring and into the summer, not the temperatures we had during the winter.