I’ve heard from a handful of folks asking about the current / recent weather pattern. It has been a bit cooler during the last few weeks with shots for rain nearly every day. And while daily afternoon storms aren’t uncommon, a lot of you have commented that it ‘just feels more rainy’ than normal.
Well, it looks like some of you may have a good gut feeling. Looking at some of the data, it appears as through the last four weeks have feature a rather anomalous weather pattern for August – particularly for the Southeast.
This has been a pattern that is cooler, more rainy, and more sheared than normal. We’ve held on to a southerly wind in the low levels of the atmosphere and that has helped to keep the atmosphere above us more humid. And, thus, the rain that has fallen, has fallen harder than normal, too.
The Explanation
I guess I can’t explain ‘why’ the atmosphere has decided to set up this way, necessarily. But I can tell you how the atmosphere has been constructed during the last 30 days. There has been more troughing in the east and more ridging in the west. This is highlighted in the map below by the blues and purples from Texas all the way up to Pennsylvania.
That means that up in the mid-to-upper levels of the atmosphere, the South has been in a prime area to see more storm systems glide through bringing a better chance for rain.
That extra troughing means more zonal wind (east-to-west) across the Gulf Coast. And a zonal wind up around 500mb (as shown below) means that storms are going to move from west to east, directed by the mid-to-upper level wind.
That added zonal wind may be playing a factor in storm motion. As a storm system swings through, these storms travel along the same (or similar) paths over and over again, leading to the potential for flooding.
Interestingly, underneath that zonal wind, we have a more positive meridional wind at 850mb. That is around 4,000ft up in the sky. The map is shown below. And a positive Meridional wind means that it is more likely to be south-to-north than normal for this time of year. Conversely, across Texas and Oklahoma it is more negative, meaning a more north-to-south type of wind.
This is likely allowing for more low-level moisture to be in place as systems move through the area. LEading to more rainfall. And perhaps even heavier rainfall.
On top of that, the wind has also been stronger than normal up at 850mb. Not by much but by about 10-20mph. And that is pretty meaningful. Especially when you consider this is averaged across an entire month.
So we are holding on to a more southerly wind up around 4,000ft and that wind is blowing harder than normal.
The cool thing about this is that – for free! – we actually end up with more wind shear in the region, too, because of the change in wind direction and speed with altitude. And looking at the map below, you can see the stripe of higher wind shear across the southern United States.
More wind shear also means more storms that are a bit better organized than they normally would be. And more orgnaized storms tend to be stronger, produce heavier rainfall, last longer, and also have the chance to produce wind gusts that are stronger and more prolonged lightning events.
This may all be sounding a bit too familiar.
So, when I tell you that the data suggest higher than normal rainfall rates, you are probably nodding your head, thinking, “makes sense!”
It isn’t by much, but it is enough, for sure. And if precipitation rates are higher than normal in an area where there is more zonal wind (west-to-east) it may also suggest a chance for more training storms in this area and thus higher rainfall totals and a higher chance for localized and flash flooding.
And where there is rain, there are cooler temperatures, too…
Ta Da! Just like that we have explained how this August has been a little weird.
